Academic team

Taklimakan Desert

(China; 105,000 mi2) The Taklamakan is an extremely cold, sandy desert known for splitting the Silk Road into branches running north and south of it. It is bounded by the Kunlun, Pamir, and Tian Shan mountain ranges.

King Arthur

"The Once and Future King," was the son of Uther Pendragon and Lady Igraine. Uther disguised himself as Igraine's husband Gorlois to sleep with her. Arthur wields the legendary sword Excalibur and rules the Britons from the castle of Camelot beside his wife, Queen Guinevere. The stories of Arthur and his Knights of the Round Table are recounted by Geoffrey of Monmouth in Historia Regnum Britanniae, works by Chrétien de Troyes, and Thomas Mallory's Le Morte d'Arthur, among others. The different sources disagree on various details; for instance, some sources state that Arthur received Excalibur from the Lady of the Lake, while in others he pulls the sword from a stone. After the Battle of Camlann, King Arthur gives Excalibur to his marshal, Sir Bedivere, and is taken to the isle of Avalon to die.

Cnidaria

(10,000 species): Also called Coelenterata see-LEN-tur-AH-tuh, the cnidarians develop from a diploblastic (two-layered) embryo, and have two separate tissue layers and radial body symmetry. Many cnidarians have two life stages, the mobile, usually bell-like medusa and the sessile polyp. All cnidarians have nematocysts, or stinging cells, for capturing prey, and some can inflict painful stings on swimmers. Examples include the hydras, sea anemones, corals, jellyfishes, and Portuguese man-o-war (which is actually an aggregation of colonial cnidarians).

Cicero

(106 BC-43 BC) Though he is better remembered today for his role in the political life of the Roman Republic, Marcus Tullius Cicero (sometimes known as "Tully") was also a significant philosopher. He described the ideal state in such dialogues as On the Republic and On the Laws, while he discussed Epicurean and Stoic views on religion in On the Nature of the Gods. Through the Middle Ages and Renaissance, Cicero was considered one of the most important of ancient philosophers. Indeed, Saint Augustine asserted that he turned to philosophy as a result of reading a now-lost work by Cicero known as the Hortensius

Annelida

(11,500 species): The annelids are segmented worms and represent the first lineage of truly acoelomate animals, meaning their body cavities are lined with tissue derived from the embryonic mesoderm. Annelid classes include the marine Polychaeta, as well as the mostly terrestrial Oligochaeta (including the earthworms, Lumbricus) and the mostly aquatic Hirudinea, or leeches. Characteristics of annelids include nephridia (kidney-like structures), blood vessels, and, in some classes, hermaphroditism.

Platyhelminthes

(15,000 species): The flatworms are the most primitive phylum to develop from a triploblastic (three-layered) embryo. They have bilateral body symmetry, and are acoelomate (lacking a true body cavity), so that the space between the digestive tract and the body wall is filled with tissue. As the name implies, they are generally flat-bodied. They have a true head and brain, but the digestive system has only one opening, which functions as both mouth and anus. Most are hermaphroditic. This phylum includes parasites such as the tapeworms and flukes, as well as free-living (i.e., non-parasitic) organisms such as the planarians.

Nematoda

(15,000 species): The roundworms are unsegmented worms that live in a variety of habitats. They are pseudocoelomate; the three tissue layers are concentric, but the body cavity is not lined with tissue derived from the mesoderm (middle embryonic layer). They include both free-living and parasitic species; human parasites include hookworms and the causative agents of elephantiasis, trichinosis, and river blindness. Soil nematodes may be crop pests, while others are beneficial predators on other plant pests. The nematode species Caenorhabdis elegans is a common subject in genetics and developmental-biology labs.

Pierre de Fermat

(1601-1665, French) is remembered for his contributions to number theory including his little theorem, which states that if p is a prime number and a is any number at all, then ap - a will be divisible by p. He studied Fermat primes, which are prime numbers that can be written as 22n + 1 for some integer n, but is probably most famous for his "last theorem," which he wrote in the margin of Arithmetica by the ancient Greek mathematician Diophantus with a note that "I have discovered a marvelous proof of this theorem that this margin is too small to contain." The theorem states that there is no combination of positive integers x, y, z, and n, with n > 2, such that xn + yn = zn, and mathematicians struggled for over 300 years to find a proof until Andrew Wiles completed one in 1995. (It is generally believed that Fermat did not actually have a valid proof.) Fermat and Blaise Pascal corresponded about probability theory.

Isaac newton

(1643-1727, English)His work in pure math includes generalizing the binomial theorem to non-integer exponents, doing the first rigorous manipulation with power series, and creating Newton's method for finding roots of differentiable functions. He is best known, however, for a lengthy feud between British and Continental mathematicians over whether he or Gottfried Leibniz invented calculus (whose differential aspect Newton called the method of fluxions). It is now generally accepted that they both did, independently.

Gottfried Leibniz

(1646-1716, German) is known for his independent invention of calculus and the ensuing priority dispute with Isaac Newton. Most modern calculus notation, including the integral sign and the use of d to indicate a differential, originated with Leibniz. He also did work with the binary number system and did fundamental work in establishing boolean algebra and symbolic logic.

Leonard Euler

(1707-1783, Swiss) is known for his prolific output and the fact that he continued to produce seminal results even after going blind. He invented graph theory by solving the Seven Bridges of Königsberg problem, which asked whether there was a way to travel a particular arrangement of bridges so that you would cross each bridge exactly once. (He proved that it was impsosible to do so.) Euler introduced the modern notation for e, an irrational number about equal to 2.718, which is now called Euler's number in his honor (but don't confuse it for Euler's constant, which is different); he also introduced modern notation for i, a square root of -1, and for trigonometric functions. He proved Euler's formula, which relates complex numbers and trigonometric functions: ei x = cos x + i sin x, of which a special case is the fact that ei π = -1, which Richard Feynman called "the most beautiful equation in mathematics" because it links four of math's most important constants.

Carl Friedrich Gauss

(1777-1855, German) is considered the "Prince of Mathematicians" for his extraordinary contributions to every major branch of mathematics. His Disquisitiones Arithmeticae systematized number theory and stated the fundamental theorem of arithmetic (every integer greater than 1 has a prime factorization that is unique nonwithstanding the order of the factors). In his doctoral dissertation, he proved the fundamental theorem of algebra (every non-constant polynomial has at least one root in the complex numbers), though that proof is not considered rigorous enough for modern standards. He later proved the law of quadratic reciprocity, and the prime number theorem (that the number of primes less than n is is approximately n divided by the natural logarithm of n). Gauss may be most famous for the (possibly apocryphal) story of intuiting the formula for the summation of an arithmetic sequence when his primary-school teacher gave him the task — designed to waste his time — of adding the first 100 positive integers.

William Rowan Hamilton

(1805-1865, Irish) is known for a four-dimensional extension of complex numbers, with six square roots of -1 (±i, ±j, and ±k), called the quaternions.

James Frazer

(1854-1941) Frazer was a Scottish anthropologist who primarily studied mythology and comparative religion. His magnum opus, The Golden Bough, analyzed a wide range of myths that center on the death and rebirth of a solar deity; the original publication controversially discussed the crucifixion of Jesus as one such myth. The work's title refers to a gift given to Proserpine (Persephone) by Aeneas so that he could enter the underworld in the Aeneid.

Franz Boas

(1858-1942) Often called the founder of modern anthropology, this first professor of anthropology at Columbia University trained Margaret Mead, Ruth Benedict, Alfred Kroeber, author Zora Neale Hurston, and many others. He conducted fieldwork on the Inuits of Baffin Island and the Kwakiutl (now referred to as Kwakwaka'wakw) on Vancouver Island. His publications include 1911's The Mind of Primitive Man, which describes a gift-giving ceremony known as the "potlatch."

David Ricardo

(English, 1772-1823): Ricardo is best known for Principles of Political Economy and Taxation, which introduced more-or-less modern notions of comparative advantage and its theoretical justification for unfettered international trade. He also put forth the so-called iron law of wages.

Max Planck

(1858-1947) Planck allowed quantum theory to move forward in the early 20th century by correctly modeling how an object radiates heat, solving the ultraviolet catastrophe, which was a predicted unbounded increase in the amount of radiation emitted at high frequencies. Planck's Law of Radiation superseded the Rayleigh-Jeans Law. He suggested that electromagnetic energy could only be emitted in specific packages, called quanta (singular quantum, from the Latin for "how much"), positing that the energy of this photon was equal to its frequency times a fixed value h, now known as Planck's constant.

Gregor mendel's experiments with pea plants

(1860s) pioneered the studio of genetics. Mendel, an Austrian monk, proposed the law of segregation, which holds that each organism has two alleles for each trait, which are segregated into gametes so that each gamete inherits one copy, as well as the law of independent assortment, which says that genes for individual traits are inherited independently. He supported both of those laws with experimental evidence by growing and counting pea plants. He worked with seven characteristics including plant height, seed shape, and color. His experiments primarily consisted of hybridizing plants with certain characteristics, and observing what fraction of the next generation had certain traits. His results were remarkably close to the values that would be predicted from modern genetics—close enough, in fact, that Mendel has been accused of manipulating his data.

Marie and Pierre Curie

(1867-1945) and (1859-1906) rigorously isolated and experimented on radioactive materials, forming the basis for early nuclear and particle physics.

Robert Millikan

(1868-1953; not to be confused with Robert Mullikan, a chemist) determined the charge of the electron by meticulously observing oil droplets in an electric field and noting the time it took them to fall a certain distance.

Ernest Rutherford

(1871-1937) Rutherford's gold foil experiment provided the first evidence that each atom is made up of a large, positively-charged nucleus, surrounded by a cloud of negatively-charged electrons. Rutherford won the 1908 Nobel Prize in Chemistry for this work. Rutherford was also an early leader in nuclear fission techniques, having discovered the decay of carbon-14 and providing the impetus for modern carbon dating. As part of this research, he discovered the proton and neutron, the latter in cooperation with James Chadwick. He is also the only native New Zealander with an element named after him (Rutherfordium, atomic number 104).

Albert Einstein

(1879-1955) In one year — 1905, called his annus mirabilis, or "miracle year" — Albert Einstein authored four papers that revolutionized modern physics. The first explained the photoelectric effect in terms of quantized electromagnetic radiation. The second formed the foundation for modern statistical physics by explaining the seemingly-random motion of particles in a fluid, a behavior called Brownian motion. The third reconciled Maxwellian electrodynamics with classical mechanics by positing a finite, constant speed of light, a theory now known as special relativity. The fourth paper contained his statement that the energy of a body is equal to its mass times the speed of light squared (that is, E = mc2). Ten years later, in 1915, Einstein published his theory of general relativity, which generalized special relativity to account for gravitational fields.

Alfred Radcliffe-Brown

(1881-1955) Radcliffe-Brown is considered the founder of a school of anthropology known as structural functionalism, which focuses on identifying the groups within a society and the rules and customs that define the relationships between people. His own early fieldwork was conducted in the Andaman Islands and Western Australia, where he studied the social organization of Australian tribes. After teaching in Australia, South Africa, and at the University of Chicago, he returned to England, where he founded the Institute of Social and Cultural Anthropology at Oxford.

Bronisław Malinowski [BROH-nee-swaf mah-lee-NAWF-skee]

(1884-1942) The Polish-born Malinowski studied at the London School of Economics, where he would later spend most of his career. He described the "kula ring" gift exchanges found in the Trobriand Islands in Argonauts of the Western Pacific, and the use of magic in agriculture in Coral Gardens and Their Magic. He also argued, in opposition to Sigmund Freud, that the Oedipus complex was not a universal element of human culture in his book on Sex and Repression in Savage Society.

Niels Bohr

(1885-1962) Bohr reconciled Rutherford's results from the gold foil experiment with Max Planck's quantum theory to create the Bohr model in which electrons resided in specific energy levels at specific stable radii. This model was the basis for Johann Balmer's work with spectroscopy and Johannes Rydberg's energy formula, which explicitly stated the frequency of light that an electron would emit if it went from a higher energy to a lower energy. Bohr and his son fled to the U.S. in World War II under the pseudonym "Baker," and contributed to the Manhattan Project.

Michelson-Morley Experiment

(1887), conducted by Albert Michelson and Edward Morley at what is now Case Western Reserve University, disproved the existence of the luminiferous aether, a hypothetical medium through which light waves supposedly moved. (The aether is sometimes called simply "ether," but is not to be confused with ethers from organic chemistry.) The experiment used an interferometer, a device that splits a beam of light and aims it using mirrors to allow the beam to interfere with itself; the interferometer was mounted on a slab of marble floating in a pool of mercury so that it could turn without friction, to eliminate the possibility that the interferometer was misoriented. While Michelson and Morley expected to find a shift in the interference pattern's fringes as a result of the ether, the experiment showed that the Earth had no motion relative to the ether, suggesting that the ether did not exist. Often called the most famous failed experiment in science, the Michelson-Morley experiment is a fundamental test of special relativity.

Ruth Benedict

(1887-1948) A colleague and friend of Margaret Mead, Benedict studied the Zuni, Dobu, and Kwakiutl cultures in Patterns of Culture, using them to illustrate the idea of a society's culture as "personality writ large." She also described Japanese culture in The Chrysanthemum and the Sword, a work written during World War II at the request of the U.S. government.

Thor Heyerdahl

(1914-2002) In 1947, Heyerdahl and five companions sailed across the Pacific Ocean — going from Peru to the Tuamotu Islands — on a balsa-wood raft named Kon-Tiki, after the Incan sun god Kon-Tiki Viracocha. He later built two boats from papyrus (Ra, which failed in 1969, and Ra II, which succeeded in 1970) to sail across the Atlantic Ocean. These voyages demonstrated the possibility that ancient people could have migrated around the globe using only primitive rafts.

Erwin Schrödinger

(1887-1961) Schrödinger contributed to the early formulations of quantum theory as a foil to Werner Heisenberg, Niels Bohr, and Paul Dirac, criticizing their Copenhagen interpretation of quantum mechanics with thought experiments like his famous Schrödinger's Cat argument. He formulated both the time-independent and time-dependent Schrödinger equations, which are partial differential equations that describe how quantum systems behave. Schrödinger's work was the basis for Heisenberg's matrix formalism, Feynman's path-integral formalism, and quantum mechanical perturbation theory, which considers the effects of a small disturbance to a quantum system.

Louis de Broglie

(1892-1987) De Broglie's work quantifying the wave-particle duality of quantum mechanics earned him the 1929 Nobel Prize in Physics. His doctoral thesis, which proposed that all particles have a characteristic wavelength dependent on their momentum, was so groundbreaking that the reviewers passed it directly to Albert Einstein, who endorsed it. In opposition to the probabilistic interpretation of quantum mechanics, de Broglie later worked to define a purely causal interpretation, but his work remained unfinished until David Bohm refined it in the 1950s.

Wolfgang Pauli

(1900-1958) namesake exclusion principle prohibits most types of particles from occupying the same state, and forms the basis for chemical bonds.

Enrico Fermi

(1901-1954) Fermi is best known to the public as a main contributor to the Manhattan Project. His work with statistical physics laid the groundwork for modern electronics and solid-state technologies. He applied the Pauli exclusion principle to subatomic particles to create Fermi-Dirac statistics, which accurately predicted the low-temperature behavior of electrons. Particles that obey Fermi-Dirac statistics are called fermions in his honor. Fermi also suggested the existence of the neutrino in order to balance nuclear beta-decay chains.

Werner Heisenberg

(1901-1976) Heisenberg is most known for his matrix interpretation of quantum theory, which constructs observable quantities as operators that act on a system. His famous uncertainty principle (better translated, however, as "indeterminacy principle") states that the more accurately an object's position can be observed, the less accurately its momentum can. This is because shorter wavelengths of light (used as a sort of measuring-stick) have higher energies, and disrupt a particle's momentum more strongly. Heisenberg earned the 1932 Nobel Prize in Physics for discovering the allotropic forms of hydrogen.

Margaret Mead

(1901-1978) For her best-known work, Coming of Age in Samoa, Mead interviewed young girls on the island Ta'u, which led her to conclude that adolescence in Samoan society was much less stressful than in the United States; in The Fateful Hoaxing of Margaret Mead, Derek Freeman claimed that she was lied to in those interviews. She also studied three tribes in New Guinea — the Arapesh, Mundugumor, and Tchambuli — for her book on Sex and Temperament in Three Primitive Societies.

Paul Dirac

(1902-1984) was one of the first to attempt a generalization of quantum theory to relativistic speeds, the result of which was the Dirac equation.

J. Robert Oppenheimer

(1904-1967) oversaw much of the Manhattan Project, but was later stripped of his security clearance during the McCarthy-era Red Scare, as a result of his acquaintance with communists and his enmity with Edward Teller.

George Gamow

(1904-1968) Gamow was one of the first to explain the implications of the Big Bang theory of cosmology. He correctly predicted the abundance of hydrogen and helium in the early universe, nicknamed Alpher-Bethe-Gamow theory (an intentional pun on the first three letters of the Greek alphabet, alpha, beta, and gamma, for which the otherwise unrelated physicist Hans Bethe was included), and also theorized that the the heat from the Big Bang would still be visible as the cosmic microwave background radiation. Although Gamow received no Nobel for this prediction, the CMB's discoverers, Arno Penzias and Robert Wilson, as well as two later observers, John Mather and George Smoot, did receive Nobels.

Kurt Godel

(1906-1978, Austrian) was a logician best known for his two incompleteness theorems, which state that if a formal logical system is powerful enough to express ordinary arithmetic, it must contain statements that are true yet unprovable. Gödel developed paranoia late in life and eventually refused to eat because he feared his food had been poisoned; he died of starvation.

Rutherford's Gold Foil Experiment

(1908-1913), sometimes named after Rutherford's assistants Hans Geiger and Ernest Marsden, discovered the positively-charged nucleus of the atom; as a result, it disproved J. J. Thomson's plum pudding model. The experimenters fired alpha particles (helium nuclei) at a sheet of gold foil. (They also used other elements, including silver.) The scattered particles were detected by a screen containing zinc sulfide, which fluoresced when the alpha particles hit it. While most of the alpha particles went straight through the foil with minimal scattering, a small fraction of alpha particles were reflected back at the source. This result was unexpected, as backscattering could only occur if the alpha particles were colliding with a particle massive enough to reverse their momentum. Rutherford said "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Claude Levi-Strauss

(1908-2009) In the 1930s, Lévi-Strauss did fieldwork with the Nambikwara people of Brazil, which formed the basis for his thesis on "The Elementary Structures of Kinship." He held the chair in social anthropology at the Collèege de France from 1959 to 1982, during which time he published such books as The Savage Mind and a tetralogy about world mythology whose volumes include The Raw and the Cooked. He pioneered in applying the structuralist methods of Ferdinand de Saussure to anthropology, which led him to study cultures as sets of binary oppositions.

Millikan's Oil Drop Experiment

(1909), performed by Robert A. Millikan and Harvey Fletcher to measure the charge of the electron. In the first step, the terminal velocity of an oil drop was measured, which means that the drag force (which can be calculated using Stokes' law) equals the force of gravity. From this, the mass can be calculated. Then, by turning on an electric field, the particle starts to move upward with a terminal velocity when the electric force balances out the forces of gravity and drag. Using this, and the mass of the drop, the total charge on the drop can be calculated. Millikan and Fletcher found that the total charge on the drops were always quantized—that is, always an integer multiple of some constant; specifically, the constant they found is about 1.59 × 10-19 coulombs, within 1% of the currently accepted value.

Stern-Gerlach experiment

(1922), conducted by Otto Stern and Walther Gerlach, demonstrated that the angular momentum of an atom is quantized. A beam of silver atoms was fired through an inhomogeneous magnetic field (one that varies through space). Instead of hitting a screen in a continuous distribution, they hit at discrete points, demonstrating the quantized nature of angular momentum. This experiment was actually performed several years before the concept of electron spin was even proposed. A variant on this experiment was used to create an energy source for the first hydrogen maser.

Davisson-Germer Experiment

(1923-1927), performed by Clinton Davisson and Lester Germer, confirmed the de Broglie hypothesis by showing that electrons can exhibit wave-like behavior. The experimenters fired electrons at a nickel crystal, and measured the diffraction patterns using an electron counter called a Faraday box (or Faraday cup) mounted on an arc so that it could detect electrons emitted at various angles. The peak intensity was observed at 50 degrees and 54 electronvolts, corresponding to the diffraction predicted for X-rays by Bragg's law. (Note that diffraction is a property of waves, not particles, and thus could only be observed if electrons can act as waves.)

Clifford Geertz

(1926-2006) Geertz is best known for his work in symbolic anthropology, a view that he expounded in his book The Interpretation of Cultures. In that book, he introduced the term "thick description" to describe his method of analyzing behavior within its social context. One such "thick description" appears in his essay "Deep Play: Notes on the Balinese Cockfight," in which Geertz discusses cockfighting as a symbolic display of a certain kind of masculinity.

Murray Gell-Mann

(1929-2019) predicted the existence of quarks, which compose protons, neutrons, and other, heavier particles.

Miller-Urey Experiment

(1952) was an attempt to demonstrate a possible mechanism—proposed by John Haldane and Alexander Oparin—for how life could form from inorganic chemicals. Stanley Miller and Harold Urey modeled Earth's prebiotic atmosphere as a mixture of water, methane (CH4), ammonia (NH3), and hydrogen. They allowed those four substances to react in an apparatus over a one-week period; the apparatus included a heater to convert the water to water vapor and an electrode to simulate lightning strikes. The resulting mixture contained more than 20 distinct amino acids that formed spontaneously; a more modern "volcanic" version of the experiment produced even more amino acids by including sulfur compounds.

Hershey-Chase Experiment

(1952), by Alfred Hershey and Martha Chase, demonstrated that the material responsible for inheritance of traits was DNA rather than protein. The experiment was carried out by creating radiolabeled T2 bacteriophages (viruses that infect bacteria). In one population, the phages' DNA contained phosphorus-32 in its backbone; in the other population, the phages' proteins contained sulfur-35. The phages were then allowed to infect E. coli. After using a centrifuge to remove the viral coats from the bacteria, Hershey and Chase found that the viruses labeled with sulfur did not transfer their radioactivity to the cells, while the viruses labeled with phosphorus did. This result, combined with other experiments, demonstrated that the genetic material was DNA, not protein.

Andrew Wiles

(1953-present, British) is best known for proving the Taniyama-Shimura conjecture that all rational semi-stable elliptic curves are modular forms. When combined with work already done by other mathematicians, this immediately implied Fermat's last theorem

Meseleson-Stahl experiment

(1958) proved that DNA replication is semiconservative, meaning that when a double-helix strand of DNA is duplicated, the result is two double-helix strands, each of which has one helix from the parent molecule and one newly-synthesized helix. Matthew Meselson and Franklin Stahl used E. coli grown in a medium containing only nitrogen-15; they were then allowed to synthesize DNA in an environment containing only nitrogen-14 over two generations. The net result was that in the second generation, half the DNA molecules contained nitrogen-15 in one strand and nitrogen-14 in the other, while the other half of the molecules contained only nitrogen-14.

Archimedes

(287-212 BC, Syracusan Greek) is best known for his "eureka" moment, in which he realized he could use density considerations to determine the purity of a gold crown; nonetheless, he was the preeminent mathematician of ancient Greece. He found the ratios between the surface areas and volumes of a sphere and a circumscribed cylinder, accurately estimated pi, and developed a calculus-like technique to find the area of a circle, his method of exhaustion.

Epicurus

(341 BC-270 BC) Epicurus's namesake school, Epicureanism, believed that pleasure was the highest (or only) good, and that the absence of pain (aponia) was the highest pleasure. They also believed that human happiness consisted of a kind of tranquillity known as ataraxia. Critics of Epicureanism accused his school of promoting hedonism and making selfishness into a good, though Epicureans did not believe themselves to be hedonists.

Porifera

(5,000 species): The sponges are all water-dwellers (98% marine, 2% freshwater), and are sometimes classified separately from other animals because of their asymmetric bodies and lack of distinct tissues. They are sessile (immobile) except in early dispersing stages, and collect food particles via the sweeping motions of flagellated cells called choanocytes

Antartica

(5.4 million mi2) Because it is covered with (solid) water, it is somewhat surprising that Antarctica is considered a desert, but it is classified as such due to its lack of precipitation. Players should be familiar with its tallest mountain, Vinson Massif in the Ellsworth Mountains; its active volcano Mount Erebus; the surrounding Ross and Weddell Seas; and the Ross Ice Shelf. The Norwegian explorer Roald Amundsen was the first to reach the South Pole (1911), while the Englishman Robert Scott died trying to reach it. Ernest Shackleton had to abandon his ship, the Endurance, during an attempt to cross Antarctica on foot.

Mollusca

(50,000 species): The molluscs are second in diversity only to the arthropods. Body plans within this phylum are diverse, but general characteristics include a soft body covered by a thin mantle, with a muscular foot and an internal visceral mass. There are two fluid-filled body cavities derived from mesodermal tissue: a small coelom and a large hemocoel that functions as an open circulatory system. Many molluscs have a shell composed of calcium carbonate and proteins, secreted by the mantle. Familiar groups within the Mollusca include the classes Gastropoda (slugs, snails), Bivalvia (clams, oysters, scallops), and Cephalopoda (nautilus, squids, octopi).

Echinodermata

(6,500 species): Characteristics of this phylum include an endoskeleton composed of many ossicles of calcium and magnesium carbonate, a water vascular system, a ring canal around the esophagus, and locomotion by tube feet connected to the water vascular system. Unique to echinoderms is the five-fold radial symmetry obvious in sea stars (starfish), sea urchins, and sea lilies. Others, like sea cucumbers, have varying degrees of bilateral symmetry. In the echinoderm body plan, a true head is absent; the anatomical terms oral (mouth-bearing) and aboral (away from the mouth) are used to describe orientation of the body surfaces. Feeding adaptations include particle feeding through the water vascular system, everting the stomach to engulf prey (sea stars), and a scraping device called Aristotle's lantern (sea urchins).

Confucius (or Kong Fu Zi)

(6th century BC) A pivotal thinker from China's Spring and Autumn period, Confucius's views on proper conduct and filial piety influence China to this day. Many sayings attributed to Confucius were compiled by his disciples following his death in a text known as the Analects. Confucius put much importance on ren, the inner state which allows one to behave compassionately toward others, and on a concept called li, which can help individuals attain ren.

Thorstein Veblen

(American, 1857-1929): Veblen is primarily remembered for his The Theory of the Leisure Class (1899), which introduced phrases like "conspicuous consumption." He is remembered for likening the ostentation of the rich to the Darwinian proofs of virility found in the animal kingdom.

John B. Watson

(American, 1878-1958): John Watson was the first prominent exponent of behaviorism; he codified its tenets in Behavior: An Introduction to Comparative Psychology, arguing that psychology could be completely grounded in objective measurements of events and physical human reactions. His most famous experiment involved conditioning an eleven-month-old boy to be apprehensive of all furry objects by striking a loud bell whenever a furry object was placed in his lap.

B.F. Skinner

(American, 1904-1990): B. F. Skinner was one of the leading proponents of behaviorism in works like Walden II and Beyond Freedom and Dignity. He argued that all human actions could be understood in terms of physical stimuli and learned responses, and that there was no need to study — or even believe in — internal mental states or motivations; in fact, doing so could be harmful. Guided by his ideas, he trained animals to perform complicated tasks, including teaching pigeons to play table tennis.

Abraham Maslow

(American, 1908-1970): Abraham Maslow is principally known for two works, Motivation and Personality and Toward a Psychology of Being, that introduced his theory of the hierarchy of needs (food, shelter, love, esteem, etc.) and its pinnacle, the need for self-actualization. Self-actualized people are those who understand their individual needs and abilities and who have families, friends, and colleagues that support them and allow them to accomplish things on which they place value. The lowest unmet need on the hierarchy tends to dominate conscious thought.

Milton Friedman

(American, 1912-2006): Conservative thinker famous for his advocacy of monetarism (a revision of the quantity theory of money) in works like A Monetary History of the United States, 1867-1960 (1963). He is strongly associated with the ideals of laissez-faire government policy.

Stanley Milgram

(American, 1933-1984): Though he did the work that created the idea of "six degrees of separation" and the "lost-letter" technique, he is mainly remembered for his experiments on obedience to authority that he performed at Yale in 1961-1962. Milgram found that two thirds of his subjects were willing to administer terrible electric shocks to innocent, protesting human beings simply because a researcher told them the experimental protocol demanded it.

Rub al Khali Desert

(Arabian Peninsula; 250,000 mi2) Its name means "Empty Quarter" in English, is sometimes considered the most inhospitable place on earth. It is known for the world's largest oil field, the Ghawar, and for once being part of the frankincense trade.

Sigmund Freud

(Austrian, 1856-1939): Sigmund Freud founded the extremely influential discipline psychoanalysis, which used the free-association technique to identify fears and repressed memories. He argued that many problems were caused by mental states rather than by biochemical dysfunction — a purely materialist viewpoint then in vogue. He separated the psyche into the id (illogical passion), ego (rational thought), and superego (moral and social conscience). His best-known works are The Interpretation of Dreams and The Psychopathology of Everyday Life.

Alfred Adler

(Austrian, 1870-1937): Alfred Adler was another close associate of Freud who split with him over Freud's insistence that sexual issues were at the root of neuroses and most psychological problems. Adler argued in The Neurotic Constitution that neuroses resulted from people's inability to achieve self-realization; in failing to achieve this sense of completeness, they developed "inferiority complexes" that inhibited their relations with successful people and dominated their relations with fellow unsuccessful people, a theory given the general name "individual psychology."

Kalahari Desert

(Botswana, Namibia, South Africa; 360,000 mi2) The Kalahari is a large region, not all of which is arid enough to qualify as a desert. It is known for its red sand, large game reserves (meerkats, gemsbok, springbok, steenbok), and mineral deposits (notably uranium). Its San Bushmen speak a click language.

John Kenneth Galbraith

(Canadian, 1908-2006): His liberal popular writings like The Affluent Society and The New Industrial State (with their emphasis on public service and the limitations of the marketplace) ensure his coming up again and again.

Atacama Desert

(Chile; 70,000 mi2) The Atacama's chief claim to fame is the rain shadow of the Andes, which makes it the driest hot desert in the world. The desert was the primary bone of contention in the War of the Pacific (1879 1883; Chile defeated Peru and Bolivia) that sought to control its nitrate resources, which were necessary for the production of explosives.

Gobi Desert

(China and Mongolia; 500,000 mi2) The Gobi, Asia's second largest desert (after the Arabian Desert), is bounded on the north by the Altai Mountains. It is known for its role in the Silk Road trading route and the Nemegt Basin, where fossilized dinosaur eggs and human artifacts have been found.

John Stuart Mill

(English, 1806-1873): Also a social philosopher, Mill is mainly known in economic circles today for his work extending the ideas of Ricardo in Essays on Some Unsettled Questions of Political Economy (1844) (for example, the relationship between profits and wages), and also for exhaustively examining the necessity of private property in his Principles of Political Economy (1848).

Alfred Marshall

(English, 1842-1924): Marshall's magnum opus, 1890's Principles of Economics, introduced the notions of consumer surplus, quasi-rent, demand curves, and elasticity, all fundamental concepts in introductory macro- and microeconomics.

John Maynard Keynes

(English, 1883-1946): He is most famous for The General Theory of Employment, Interest and Money (1936), which judged most of classical economic analysis to be a special case (hence "General Theory") and argued that the best way to deal with prolonged recessions was deficit spending.

Francois Quesnay

(French, 1694-1774): Quesnay was the undisputed leader of the Physiocrats, the first systematic school of economic thought. Among its tenets were the economic and moral righteousness of laissez-faire policies and the notion that land is the ultimate source of all wealth.

Karl Marx

(German, 1818-1883): Also a historian and social philosopher, Marx's principal contribution to economic thought was extending the labor theory of value to its logical conclusion, his theory of surplus value. This theory, along with his defense of economic materialism, appeared in Das Kapital (1867, 1885, 1894).

Erik Erikson

(German-born American, 1902-1994): Erik Erikson is best known for his theories on how social institutions reflect the universal features of psychosocial development; in particular, how different societies create different traditions and ideas to accommodate the same biological needs. He created a notable eight-stage development process and wrote several "psychohistories" explaining how people like Martin Luther and Mahatma Gandhi were able to think and act the way they did.

Negev Desert

(Israel; 4,700 mi2) The triangular Negev covers the southern half of Israel.

Namib Desert

(Namibia and Angola; 30,000 mi2) The Namib, a coastal desert, is known for its bizarre Welwitschia and medicinal Hoodia plants. It is thought to be the oldest desert in the world.

Sahara Desert

(Northern Africa; 3.5 million mi2) The Sahara is the world's second largest desert, but its largest hot desert. The Atlas Mountains bound the western Sahara on the north, and the Sahel — a savannah-like strip — bounds it on the south. It is dominated by rocky regions (hamada), sand seas (ergs), and salt flats (shatt) and dry river valleys (wadi) that are subject to flash floods. The Berber and Tuareg peoples are native to the Sahara.

Painted Desert

(Northern Arizona) The Painted Desert, which is shared by Grand Canyon and Petrified Forest National Parks, is known for its colorful, banded rock formations.

Ivan Pavlov

(Russian 1849-1936): Ivan Pavlov was more of a physiologist than a psychologist, but questions about him are more often classified as psychology than biology by question writers. He is largely remembered for his idea of the conditioned reflex, for example, the salivation of a dog at the sound of the bell that presages dinner, even though the bell itself is inedible and has no intrinsic connection with food. He won the Nobel Prize in 1904 for Physiology or Medicine for unrelated work on digestive secretions.

Adam smith

(Scottish, 1723-1790): Though he wrote on nearly every subject of moral and social philosophy, he is basically remembered as the author of An Inquiry into the nature and causes of the Wealth of Nations (1776) and as the creator of the metaphor of the "invisible hand." This work more-or-less single-handedly founded the Classical school of economics.

Carl Jung

(Swiss, 1875-1961): Carl Jung was a close associate of Freud's who split with him over the degree to which neuroses had a sexual basis. He went on to create the analytic psychology movement and introduced the controversial notion of the collective unconscious — a socially shared area of the mind. Quiz bowlers should be familiar with anima, animus, introversion, extroversion, and archetypes, all terms that occur frequently in questions on Jung. (Of course, being familiar with them entails knowing what specifically they mean, and how they relate to Jung's work.)

Jean Piaget

(Swiss, 1896-1980): Jean Piaget is generally considered the greatest figure of 20th-century developmental psychology; he was the first to perform rigorous studies of the way in which children learn and come to understand and respond to the world around them. He is most famous for his theory of four stages of development: sensorimotor, preoperational, concrete operational, and formal operational. His most famous works are The Language and Thought of a Child and The Origins of Intelligence in Children.

Great Sandy Desert

(Western Australia; 140,000 mi2) Part of the Western Desert, and the ninth largest in the world.

Jane Goodall

(born 1934) Goodall is a British primatologist who is best known for her work with chimpanzees in Gombe Stream National Park in Tanzania. Her first research was carried out with Louis Leakey at Olduvai Gorge. In her pioneering work with primates, which is detailed in such books as In the Shadow of Man, she discovered that chimpanzees have the ability to use tools, such as inserting grass into termite holes to "fish" for termites.

Euclid

(c. 300 BC, Alexandrian Greek) is principally known for the Elements, a textbook on geometry and number theory, that has been used for over 2,000 years and which grounds essentially all of what is taught in modern high school geometry classes. The Elements includes five postulates that describe what is now called Euclidean space (the usual geometric space we work in); the fifth postulate — also called the parallel postulate — can be broken to create spherical and hyperbolic geometries, which are collectively called non-Euclidean geometries. The Elements also includes a proof that there are infinitely many prime numbers.

Vacoules

Found mainly in plants and protists, vacuoles are liquid-filled cavities enclosed by a single membrane. They serve as storage bins for food and waste products. Contractile vacuoles are important for freshwater protists to rid their cells of excess water that accumulates because of salt imbalance with the environment.

Aristotle

(c. 384 BC-322 BC) Aristotle was a student of Plato; in turn, Aristotle was a tutor to Alexander the Great. Many of his works come to us in the form of lectures he gave at his school, known as the Lyceum. His philosophical output includes the Nicomachean Ethics, which argues that virtues consist in a "golden mean" between two extremes; the Physics, which describes motion and change in terms of "four causes" that make a given thing what it is; and the Metaphysics, which describes the structure of reality. Aristotle's Poetics discusses the types of drama and considers an effect of tragedies known as catharsis, or the purging of bad feelings.

Diogenes

(c. 410s BC-323 BC) A student of Antisthenes, who founded the ancient school of philosophy known as Cynicism. (The term "cynic" comes from the Greek for "dog-like," and is thought to have originated as an insult to the school's members.) The Cynics rejected conventional social norms in search of a truly virtuous life. Diogenes himself was something of an eccentric—according to legend, he lived in a tub or a barrel on the street, and wandered Athens holding a lamp in his futile search for an honest man.

Plato

(c. 429 BC-347 BC) Plato's Socratic dialogues are our main source both for Socrates's philosophy and his own; Plato often put his own thoughts in Socrates' mouth. Plato's dialogues include the Republic (about justice and the ideal city-state), the Symposium (about the nature of love), and the Meno (about whether virtue can be taught). Plato believed in a world of "forms"—or ideal versions of real things that lie beyond the human senses—which he discussed in such works as the Phaedo. Plato founded a school called the Academy, from which we get the common word.

Socrates

(c. 469 BC-399 BC) We have no writings from Socrates own hand, and know about him largely from the dialogues of his student Plato. Proclaiming his own ignorance of all things, Socrates went around Athens engaging in question-and-answer sessions to search for truths or draw out contradictions (the "Socratic method"). The Athenian state disapproved of his conduct; he was put on trial for corrupting the city's youth, and sentenced to death by drinking hemlock. Socrates' trial, imprisonment, and death are recounted in Plato's Apology, Crito, and Phaedo, respectively.

Zeno of Elea

(c. 490 BC-430 BC) Zeno of Elea was a student of Parmenides, who founded the Eleatic school in a Greek colony of the Italian peninsula. He is most famous today for "Zeno's paradoxes," the best-known of which involve an arrow in flight and a race between Achilles and a tortoise. Zeno's paradoxes purport to show that physical movement is impossible, since any attempt to travel a distance must be preceded by moving half that distance, which must be preceded by moving half of half that distance, and so on. (Zeno of Elea is not to be confused with Zeno of Citium, who founded the philosophical school of Stoicism two centuries later.)

Thales

(c. 620 BC-546 BC) Thales was a pre-Socratic thinker from the Greek colony of Miletus who many consider to be the "first philosopher." Rejecting mythical explanations of the universe's nature, he believed that the first principle of all existence, the natural element from which all things emerged, was water. Thales was also a civil engineer and mathematician, and is credited with discovering that if a circle goes through all three vertices of a triangle and one side of the triangle is a diameter of the circle, then the triangle is a right triangle. He is sometimes thought of as the founder of a "Milesian school" of philosophy, whose other members include Anaximander and Anaximenes.

Thomas Young's double-slit experiment

(circa 1800), which predated the development of quantum mechanics by over a century, demonstrated that light can behave as either a wave or a particle. After passing a beam of light through two narrowly spaced slits, Young observed the characteristic light and dark fringes of interference seen when light acts as a wave. More modern versions that add detectors showing which slit the light passes through, however, show that the light passes through one slit or the other, acting as photons; moreover, the interference pattern disappears. A modern variant, called a quantum eraser, demonstrates quantum entanglement (the ability to exchange information over large distances instantaneously).

Lao Tzu (also given as Lao Tse or Laozi)

(dates unknown, 6th century BC) is a quasi-mythical thinker of the Taoist tradition, to whom the pivotal Tao te Ching is attributed. Concepts associated with him include that of the Tao, or "the way," and wu wei, or a life of non-action in accordance with the Tao. In later centuries, Lao Tzu was accorded godlike status as one of the Three Pure Ones of Taoism, and is frequently depicted as an old man with a donkey. To Lao Tzu is attributed the quote "The journey of a thousand miles begins with a single step."

Mojave Desert

(mostly California, and some of Arizona, Utah, and Nevada; 25,000 mi2) The Mojave is bounded by the San Gabriel and San Bernardino mountain ranges along the San Andreas and Garlock Faults. It is between the Great Basin and the Sonoran Desert, and it contains the lowest and driest point of North America, Death Valley. It is most strongly associated with the Joshua tree (Yucca brevifolia).

Arthropoda

(over 800,000 species described; estimates of actual diversity vary but go as high as 9 million species): The most diverse and successful animal phylum on earth (incorporating about 75% of all described animal species), the Arthropoda are characterized by jointed legs and a chitinous exoskeleton. Like annelids, they are segmented, but unlike annelids, their segments are usually fused into larger body parts with specialized functions (such as the head, thorax, and abdomen of an insect). Arthropods are often divided into four subphyla: Uniramia (insects, centipedes, millipedes); Chelicerata (arachnids, sea spiders, horseshoe crabs); Crustacea (shrimps, lobsters, crabs, crayfish, barnacles, pillbugs), and Trilobitomorpha (the trilobites, now extinct).

Ronald ross Nobel

(won in 1902) was the first British recipient of the Nobel Prize, recognized for his work related to the transmission of malaria. While studying in India, he dissected a mosquito of the species Anopheles (which he termed "dappled-winged" due to its strange posture), and discovered the presence of the malarial parasite (from the genus Plasmodium). He then used birds to demonstrate that the parasite is stored in the salivary gland of mosquitoes and is released when a host is bitten. Ross received the Nobel prize individually in 1902, after fellow bacteriologist Robert Koch voiced his support for him over Italian scientist Giovanni Battista Grassi, who had reported similar findings on the malaria lifecycle.

Ivan Pavlov Nobel

(won in 1904) performed detailed research on the process of digestion, which he published in 1897 in The Work of the Digestive System. By surgically exposing selective sections of a dog's digestive system, Pavlov was able to study in detail how material moves through the stomach and intestines; he identified the timing of various gastric and pancreatic secretions. While he won the Nobel purely for his contributions to physiology, he's more well-known for performing the first psychological studies of classical conditioning. After observing the behavior of dogs upon repeatedly bringing them food, Pavlov noticed that the dogs would begin to salivate prior to food even arriving, providing the basis for the concepts of a conditioned response and conditioned stimulus.

Robert Koch Nobel

(won in 1905) pioneered the field of modern bacteriology, and established his theory of four postulates that must be satisfied in order to determine that a disease is caused by a particular microbe. He used these postulates to discover the bacterial agent that causes tuberculosis, contradicting the popular opinion that the disease was inherited. Koch also determined the causative agents of cholera and anthrax and examined the concept of acquired immunity while on exhibition in German New Guinea.

Paul Ehrlich Nobel

(won in 1908) performed early work in the field of immunology. He discovered that, after exposing mice to a small dose of ricin and gradually increasing the dose given, the animals developed an immunity to it. In addition to his Nobel-winning work, he also postulated the concept of a "magic bullet," a compound that could be used to selectively target and eliminate agents of disease. He made his own "magic bullet" in 1909 when he discovered that the compound arsphenamine (now called Salvarsan) could be used as an effective cure for syphilis. In the early phase of his career, Ehrlich developed a close friendship with Robert Koch after presenting him with an improved technique for staining bacteria, which would become the precursor to the technique of Gram staining.

Willem Einthoven Nobel

(won in 1924) performed groundbreaking work related to electrocardiography. He invented a device called the string galvanometer to measure the electric current of the heart, which until then had been impossible without applying electrodes directly to the heart. This galvanometer would form the basis of the modern ECG (EKG) machine. His method of assigning three leads on human limbs, forming an inverted equilateral triangle centered on the heart, is still used in EKGs today, and is known as Einthoven's triangle. He also assigned the letters 'P', 'Q', 'R', 'S', and 'T' to the various deflections seen on an EKG; that notation, which describes the familiar QRS complex pattern, is also still used today.

Karl Landsteiner Nobel

(won in 1930), known as the "father of transfusions," made important discoveries in the field of serology in the early 1900s, including the identification of the three major blood groups (A, B, and O). He determined that mixing blood from two individuals with incompatible blood types can lead to a dangerous form of blood clumping called agglutination. In 1937 (after he received the Nobel), he also identified the Rh factor (the "plus" or "minus" in one's blood type) in the blood of the rhesus monkey, a discovery which further increased the safety of blood transfusions. While studying in Vienna in 1908, Landsteiner isolated the polio virus, providing the basis for Jonas Salk's vaccine against the disease.

Thomas Hunt Morgan Nobel

(won in 1933) discovered the basic mechanisms of genetic inheritance via his experiments on fruit flies, specifically the species Drosophila melanogaster. By conducting a series of experiments in his renowned "Fly Room" at Columbia University, Morgan and his students tracked mutations, noting how various traits were inherited across successive generations of the flies. His data supported the idea of genetic linkage by noting that the probability of two genes being inherited together correlates inversely with their physical distance on a given chromosome, leading Morgan to postulate the mechanism of crossing over. Because of his work, the unit centimorgan is now used to measure the physical distance between genes.

Alexander Fleming Nobel

(won in 1945) made the accidental discovery of the antibiotic penicillin after haphazardly stacking Petri dishes in the corner of his notoriously untidy laboratory. He noticed that one culture of the Staphylococcus that he was studying had developed a fungus that destroyed all the nearby colonies of bacteria. After developing the mold in a culture of its own, he discovered that it produced a "mold juice," later called penicillin, that killed a number of disease-causing bacteria. Although Fleming originally believed penicillin to be too slow-acting to have any positive therapeutic effect, it formed the basis for beta-lactam antibiotics, which are still effective in many medical cases today.

Barbara McClintock Nobel

(won in 1983) described the existence of transposons, also called "jumping genes," from her observations that certain DNA sequences on the maize she was studying could change position on a chromosome. While studying generations of maize at Cold Spring Harbor Laboratory, she hypothesized that these "controlling elements" were regulatory in nature, and that their motion around the genome could be the driving factor behind how two identical genetic sequences can exert different functions. McClintock is the only non-shared female recipient of the Nobel Prize in Physiology or Medicine.

Ymir

A primordial giant who formed in the void Ginnungagap from fire and ice. He gave birth to the frost giants and created the primordial cow Audhumla. He was killed by Odin and his brothers, who used his body to construct most of the universe.

Hubble Space Telescope Servicing Missions SM3

A replaced failed gyroscope systems. B replaced the Faint Object Camera (FOC) with the Advanced Camera for Surveys (ACS), and also repaired NICMOS, which was installed during SM2.

Corundum

a mineral composed of aluminum oxide. Common gem forms of corundum include rubies (turned red by inclusions of chromium) and sapphires (which can take a variety of colors from inclusions of iron, titanium, or vanadium). Corundum defines hardness 9 on the Mohs scale.

Inductor

A solenoid is a coil of wire. When electric current through the wire changes, a magnetic field is generated in the core (interior) of the solenoid. When a solenoid is used in a circuit, it is called an inductor and serves to oppose changes in the current in the circuit. The strength of an inductor is its inductance, symbolized L and measured in henries (after Joseph Henry, who discovered the principles of self-inductance and mutual inductance, though Michael Faraday published his findings on electromagnetic induction first). The voltage across an inductor only depends on the change in current, so inductors are particularly used with alternating current. For standard (sinusoidal) alternating current, an inductor only changes the phase of the current through it. Inductors function the same way as resistors when arranged in series or parallel.

Thor

A son of Odin and the giantess Jord, he is the god of thunder, weather, and crops. One of the most popular of the Norse gods, he travels in a chariot pulled by two goats, and wields the hammer Mjölnir. He is married to Sif, and his special nemesis is the Midgard Serpent.

Patroclus

Achilles' foster brother and closest friend. Although Patroclus is a formidable hero, he is valued for his kind and gentle nature. Patroclus is killed by Hector while wearing the armor of Achilles.

Lake victoria

Africa's largest and the world's second-largest freshwater lake by area, Lake Victoria lies along the Equator and is shared among Uganda, Kenya, and Tanzania. Located on a plateau between two rift valleys, its lone outlet is the Victoria Nile, a precursor of the White Nile. Named by the British explorer John Hanning Speke after Queen Victoria, the introduction of the predatory Nile perch in the 1950s has caused environmental degradation, sending many native cichlid species into extinction.

Lake Tanganyika

Africa's second-largest lake by area, it is also the second-deepest in the world, surpassed only by Lake Baikal. Due its extreme depth (over 4,700 feet), Lake Tanganyika contains seven times as much water as Lake Victoria. A source of the Lualaba River, it is shared by Tanzania, the Democratic Republic of the Congo, Burundi, and Zambia. On its Tanzanian shore is the town of Ujiji, at which Henry Morton Stanley "found" Dr. David Livingstone in 1871.

Congo River

Africa's second-longest river, it flows in a counterclockwise arc some 2,900 miles to the Atlantic Ocean. The Upper Congo's principal sources are the Lualaba, which rises in the Democratic Republic of the Congo's Katanga province, and Zambia's Chambeshi River. Boyoma Falls (formerly Stanley Falls), a section of seven cataracts near Kisangani, marks the beginning of the Congo River proper. Forming the Malebo Pool near the world capitals of Kinshasa and Brazzaville, the Lower Congo flows past Angola's Cabinda exclave as it enters the ocean. Joseph Conrad's Heart of Darkness depicts the often cruel conditions the Congo basin endured as a Belgian colony.

Lake Malawi

Africa's third-largest lake by area and the southernmost of the Great Rift Valley lakes, it is wedged between the nations of Malawi, Tanzania, and Mozambique. Fed by the Ruhuhu River, its lone outlet is the Shire River, a tributary of the Zambezi. Lake Malawi contains hundreds of species of endemic fish, especially cichlids.

Niger river

Africa's third-longest, it flows in a great clockwise arc through Guinea, Mali, Niger, and Nigeria before entering the Gulf of Guinea. The medieval Mali and Songhai Empires were centered on the Niger, whose course was mapped by the Scottish explorer Mungo Park in the 1790s. In Nigeria, it receives the Benue River, its main tributary. The massive Niger Delta, known for its fisheries, wildlife, and petroleum, is an area of increasing social unrest.

Chloroplast

Found only in plants and certain protists, the chloroplast contains the green pigment chlorophyll and is the site of photosynthesis. Like the mitochondrion, a chloroplast is a double-membrane-bound organelle, and it has its own DNA and ribosomes in the stroma. Chloroplasts contain grana, which are stacks of single membrane structures called thylakoids on which the reactions of photosynthesis occur.

Oder River

Along with the Neisse, the Oder forms the Germany-Poland border, as dictated at the Potsdam Conference in July and August of 1945. One of the largest rivers to enter the Baltic, it has been a major transport route for centuries. Ostrava in the Czech Republic and Breslau in Poland are on the river. Near its mouth is Stettin, which Churchill used as the northern terminus of his "Iron Curtain" (Trieste, in the South, is an Adriatic port not near a major river). At the mouth of the Oder are Usedom Island, Swinemuende, and Peenemuende, which were primary test sites for the German V-2 rocket in the 1940s.

Amon

Amon began as a local god of Thebes, governing the air, fertility, and reproduction. His wife was Mut, and his son was Khon. Later, Amon became linked with the sun god Ra, and the two combined as Amon-Ra. In this form, he became worshipped beyond Egypt, and identified with Zeus and Jupiter. His appearance in art was as a man in a loincloth, with a headdress topped by feathers, but other appearances show him with the head of a ram. The temple of Amon-Ra at Karnak was the largest ever built.

Shannon River

At 230 miles, the Shannon is Ireland's longest river. It flows from Lough Allen, and Loughs Ree and Derg are also on its course. At Limerick, the river widens into its namesake estuary and runs for 50 more miles before it enters the Atlantic. Peat bogs and marshes line the river for much of its course, and the Shannon is considered a dividing line between Ireland's more cultivated east and wild west. A chief tributary is the Suck River. The Shannon does not pass through Dublin, although the Liffey does.

phloem

Carries nutrients like sucrose from their origin of synthesis or absorption to all parts of the plant. Cell types in phloem include companion cells, fibers, and sclereids. In trees, it is usually the innermost layer of the bark.

Set

Created in opposition to the forces of Ma'at, Set (termed Typhon by Plutarch) fought the demon Apopis each day, emerging victorious, symbolic of the struggle of forces that brought harmony. In later times, this struggle led Set to be associated with the serpent itself, and Set became the personification of violence and disorder, and the cause of all disasters. Having killed his brother Osiris, Set did battle with Osiris' son Horus, being emasculated in the fight. His cult was diminished over time, due to reaction against violence. His effigies were destroyed by some, while others were changed into representations of Amon, by replacing the ears with horns.

Diamond

a mineral composed of carbon atoms in a "diamond cubic" crystal structure. Among natural materials, diamond has one of the highest thermal conductivities and highest hardnesses; it defines hardness 10 on the Mohs scale. Natural diamonds are almost always found in igneous rocks called kimberlites.

The Matterhorn

Edward Compton and John Ruskin are among the artists inspired by the Matterhorn, which is renowned for its almost perfectly pyramidal shape. The mountain is on the border between Switzerland and Italy, near the Swiss town Zermatt. It was first climbed in 1865 by Edward Whymper; four people died in that ascent.

Lake chad

Formerly Africa's fourth-largest lake, its surface area has been reduced by over 90% since the 1960s due to droughts and diversion of water from such sources as the Chari River. The lake is at the intersection of Chad, Cameroon, Niger, and Nigeria, but most of the remaining water is in Chad and Cameroon. Lake Chad is very shallow and has no outlet, so seasonal rainfall causes large fluctuations in its area.

Denali

Formerly and often called Mount McKinley, Denali is the highest mountain in North America. It is in south-central Alaska, in Denali National Park. The West Buttress route is considered the best path to ascend Denali. Frederick Cook, a man notorious for having faked the discovery of the North Pole, is now believed to have also faked his ascent of the mountain in 1906 as well, leaving a climbing party seven years later with the honor.

Loire River

France's longest river, the Loire begins in the Cevennes range of southern France, flows north to the center of the country, then flows due west to the Bay of Biscay. Many notable cities are on the river, including Nevers, Orleans, Blois, Tours, and Nantes. The Loire is sometimes called the "last wild river in Western Europe," and many proposed dams on the river have not been built because of opposition to the flooding of land and to interference with Atlantic salmon. The Loire Valley is particularly known for its vineyards and for its châteaux, a collection of over 300 castles dating to the 16th and 17th centuries.

Hathor

Hathor (or Athor or Athyr) was the patron of women. Hathor was the daughter of Ra and the wife of Horus. She fulfilled many functions as goddess of the sky, goddess of fertility, protector of marriage, and goddess of love and beauty. In that final role she became equated with Aphrodite and Venus. Pictures of Hathor show the goddess with the head of a cow.

Loki

He's actually giant-kin, but lives with the Aesir and is Odin's blood-brother. The god of fire and trickery, his many pranks include duping Hoder into killing Balder. His children include the wolf Fenrir, the Midgard Serpent Jormungandr, Hel (the ruler of the underworld), and Sleipnir. After killing Balder he was chained to three boulders with snakes dripping poison onto him.

Osiris

Husband of Isis, father of Horus, and brother of Set, Osiris served as god of the underworld and protector of the dead. In addition to his role as the chief and judge of the underworld (as a result of the above-mentioned murder by Set), Osiris also served as a god of vegetation and renewal; festivals honoring his death occurred around the time of the Nile flood's retreat. Statues representing him were made of clay and grain, which would then germinate. Osiris was represented either as a green mummy, or wearing the Atef, a plumed crown.

Peduncle

If multiple flowers bloom from a peduncle, the stems supporting each flower are called pedicels. The torus is the swelling at the top of the pedicel or peduncle, just below the calyx.

Diomedes

In his day of glory, Diomedes kills Pandarus and wounds Aeneas before taking on the gods. He stabs Aphrodite in the wrist and, with Athena as his charioteer, wounds Ares in the stomach. Along with Odysseus, he also conducts a successful night raid against King Rhesus.

Mohs scale

Invented by Friedrich Mohs in 1812, the Mohs scale is based on the abilities of minerals to scratch one another. The original scale assigned a value of 1 to talc, which can be scratched by essentially every solid known, and a value of 10 to diamond, which (among naturally occurring minerals) can only be scratched by other diamonds. The steps are of arbitrary size, with a 1-point difference corresponding to anywhere from a 1.5-fold to a 4-fold increase in hardness; diamond is about 1600 times harder than talc. Following the discovery of more ultra-hard minerals, scientists have proposed extending the scale so that diamond is 15 instead of 10.)

Isis

Isis, daughter of Geb and Nut, protected love, motherhood, and fate in the Egyptian mythos. Many of her roles are similar to those of the goddess Hathor, but she is often equated with the Greek Demeter. Her powers were gained through tricking the god Ra. By placing a snake in his path, which poisoned him, she forced him to give some power to her before she would cure him

Po river

Italy's longest river at 405 miles, it passes through Piedmont and Lombardy before entering the Adriatic 30 miles south of Venice. It flows through Turin and Cremona, and it passes near Milan, Padua, Pavia, and Mantua. The river has a long history of floods, and the manmade levees called argini which protect towns and crops can exacerbate the floods. Pollution, especially from Milan, is becoming a major environmental concern.

Temperature scales

Kelvin and Celsius scales. The Celsius scale, developed by Swedish scientist Anders Celsius in the early 1700s, assigns a value of 0°C to the freezing point of water (at a pressure of 1 atmosphere), and 100°C to the boiling point of water (at the same pressure). The Kelvin scale is based on the triple point of water, the point at which water's solid, liquid, and gaseous phases can coexist in equilibrium: 1 K is defined as 1/273.16 of the temperature of water at its triple point. Kelvins are treated like all other SI units: a temperature of 100 K is read as "one hundred kelvins," not "one hundred degrees kelvin." Differences on the Celsius scale have the same magnitude as differences on the Kelvin scale: a gap of 1°C between temperatures is the same as a gap of 1 K. Therefore the lowest possible temperature, absolute zero (0 K) is equal to -273.15°C, and (at a pressure of 1 atmosphere) water freezes at 273.15 K and boils at 373.15 K.

Mount Kilimanjaro

Kibo, Mawenzi, and Shira are the three summits of Mount Kilimanjaro, the tallest mountain in Africa. It is notable for also being the tallest mountain that is not part of a mountain range, having been formed by a now-extinct volcano. A corpse of a leopard is found on top of the mountain in "The Snows of Kilimanjaro," a short story by Ernest Hemingway that uses the mountain as the backdrop for the memories, and ultimately the death, of a writer suffering from gangrene.

Morded

King Arthur's illegitimate son by his half-sister Morgause (they were unaware of their shared parentage), possibly making him the rightful heir to Camelot. Mordred is best known as a traitorous figure who crowns himself King of the Britons while King Arthur is in Gaul fighting the mythical Emperor Lucius of Rome. Mordred is also frequently linked with Queen Guinevere: some accounts say that he reported the queen's affair with Lancelot to Arthur, some say that Mordred took Guinevere as a concubine during his usurpation of Arthur's throne, and some say that Mordred's wife was Guinevere's sister Gwenhwyfach. Arthur killed Mordred at the Battle of Camlann.

Sputnik 1, the first artifical satellite

Launch by the USSR kicked off the so-called "Space Race." The Space Race was manifestation of the Cold War where successes in launching objects and people into space was seen as a proxy for capacity to build intercontinental ballistic missiles. Sputnik 1 was originally intended to carry many scientific instruments, but those instruments were descoped, in favor of a simple radio transmitter. That transmitter broadcasted a "beep" at a specified interval, allowing scientists to map its deceleration as a result of atmospheric drag. Sputnik 1 was followed by Sputnik 2, which carried a dog named Laika (meaning "barker") into low-earth orbit, and was destroyed upon re-entry.

Hudson River

Missouri River

Danube River

Most of the Danube is in Eastern Europe, but it begins in Germany's Black Forest (Schwarzwald) near Freiburg, crossing Bavaria before it enters Austria. In all, it passes through (or touches the borders of) 10 nations on its 1,785-mile course ending at the Black Sea. Chief tributaries include the Drava and Sava, and it passes through four national capitals: Vienna, Bratislava, Budapest, and Belgrade. Formerly known as the Ister, the Danube was often used to define a northern border for the Roman Empire.

The family quarrel of osiris and set

Osiris took Isis, his sister, for his wife, and ruled over the earth. Set grew jealous of his brother and killed him, afterwards cutting his body into 14 pieces and hiding them in various places around Egypt. He then claimed kingship over the land. Isis searched the breadth of the land until she had recovered all of the pieces and, with the help of Anubis, embalmed the body. She conceived a son, Horus, by the (still dead) Osiris, then resurrected him. Horus defeated Set to regain the kingship, and all subsequent pharaohs were said to be aspects of him.

Centrioles

Not found in plant cells, centrioles are paired organelles with nine sets of microtubule triplets in cross section. They are important in organizing the microtubule spindle needed to move the chromosomes during mitosis.

K2

Qogir, Ketu, and Mount Godwin-Austen are other names for K2, whose most common names come from the fact that it is the second-tallest mountain in the world. The "K" in K2 stands for Karakoram, the mountain range in Pakistan and China in which the peak is found. The House's Chimney and the Black Pyramid are features of K2, which also possesses a different second-place record, behind the Annapurna Massif: it boasts the second-highest fatality rate among attempted climbers of any mountains above 8,000 meters.

Rhone river

One of Europe's few major rivers to flow directly into the Mediterranean (via the Gulf of Lion), the Rhone originates in the Swiss Alps and flows into Lake Geneva. It emerges from Geneva and flows south, passes through Lyon, Avignon, and Arles, and enters the sea just west of Marseille. At Arles, the river splits into "grand" and "petit" branches that encircle the island Camargue. The river's valley is famous for its red wine, and because it is navigable for 300 miles, the Rhone is the key access route of southern France.

Deimos

One-seventh the mass of Phobos and further away from the Martian surface, Deimos was found by Asaph Hall at the U.S. Naval Observatory six days before he discovered Phobos. Its largest and only named craters are Swift and Voltaire; Deimos's surface doesn't appear as rough as Phobos's because regolith has filled in some of the craters. A still-controversial and unproven hypothesis holds that Deimos (and possibly Phobos as well) were asteroids perturbed out of their orbit by Jupiter and then captured by the gravity of Mars.

Ra

Personification of the midday sun, he was also venerated as Atum (setting sun) and Khepri (rising sun), which were later combined with him. He traveled across the sky each day and then each night, the monster Apep would attempt to prevent his return. Other myths held that Ra spent the night in the underworld consoling the dead. The god of the pharaohs, from the fourth dynasty onward all pharaohs termed themselves "sons of Ra," and after death they joined his entourage. He was portrayed with the head of a falcon, and crowned with the sun disk.

Ptah

Principal god of the city of Memphis, he was portrayed as a mummy, or wearing the beard of the gods on his chin. His godhood was achieved by himself, much like his creation power, done merely by act of will. A patron of craftsmen, he also was seen as a healer, in the form of a dwarf. In the death trilogy (Anubis, Osiris, Ptah), he was seen as the god of embalming. His wife was the cat-headed Sekhmet, and his son was the lotus god Nefertem.

Limpopo River

Rising as the Crocodile (or Krokodil) River in South Africa's Witwatersrand region, it forms the Transvaal's border with Botswana and Zimbabwe, then crosses through Mozambique. Deforestation in Mozambique contributed to massive flooding of the Limpopo in 2000. Perhaps the most famous description of the Limpopo comes from Rudyard Kipling, who in "The Elephant's Child" referred to it as "the great grey-green, greasy Limpopo River, all set about with fever-trees."

Elbe river

Rising in the Krkonose Mountains of the Sudetenland in the Czech Republic, the river flows near Prague, then enters eastern Germany and flows northwest to the North Sea. It receives the Vltava (or Moldau), the Saale, and the Havel/Spree, and the many large cities on its course include Dresden, Dessau, Magdeburg, Wittenberg, and Hamburg. Like the Oder, the Elbe has been a key transportation route for many centuries.

Thames River

Running from Thames Head near Cirencester to an estuary near Southend in Essex, the Thames is the principal river of England and flows through central London. The Houses of Parliament and the London Eye overlook the Thames, as does Big Ben. Flowing through Reading, Oxford, and Swindon, the Thames is prevented from flooding London by its namesake Barrier near the Isle of Dogs. Though it is the longest river entirely in England, the Thames trails the Severn (which also flows into Wales) as the longest river in the United Kingdom.`

Lapetus

Saturn's third-largest moon after Titan and Rhea and, like Rhea, was discovered by Giovanni Cassini in 1671. It was named based upon a suggestion from John Herschel (son of the discoverer of Uranus, William Herschel) for the Titans of Greek mythology, the brothers and sisters of Cronos (Saturn). Iapetus has a distinctive two-tone coloration; part of it is red-brown, while part is bright gray. Features on Iapetus are named for people and places from the French Song of Roland, including Charlemagne Crater and the bright northern region Roncevaux Terra. In 2004 the Cassini orbiter found an equatorial ridge running over 800 miles long and 10 miles wide that gives Iapetus some of the highest peaks in the solar system; its existence has not yet been explained.

Thoth

Serving the gods as the supreme scribe, the ibis-headed Thoth was known as the "tongue of Ptah" for his knowledge of hieroglyphics, and as the "Heart of Re" for his creative powers. His knowledge of science and calculation made him the creator of the calendar, and his symbol was the moon due to his knowledge of how to calculate its path. His knowledge of magic led to his association with the Greek Hermes. Thoth was consulted by Isis when attempting to resurrect Osiris, and was again consulted when the young Horus was stung by a scorpion.

Pythagorean Triples

Sets of small integers that satisfy the equation of the Pythagorean theorem, a2 + b2 = c2, and could therefore be the side lengths of a right triangle. The simplest ones are {3, 4, 5}, {5, 12, 13}, {7, 24, 25}, and {8, 15, 17}. Note that any multiple of a Pythagorean triple is also a Pythagorean triple, so (for instance) {6, 8, 10}, {15, 20, 25}, and {300, 400, 500} are also ones by virtue of {3, 4, 5} being one.

Anubis

Son of Osiris and Nepthys, and god of embalming to the Egyptians, he was typically pictured with the head of a jackal. He also served as the god of the desert and the watcher of the tombs. He also served to introduce the dead to the afterlife, and as their judge. To decide the fate of the dead, Anubis would weigh the heart of the dead against the feather of truth. Anubis is sometimes identified with Hermes or Mercury.

Nepthys

Termed the "lady of the castle," for her role as guardian of tombs, she sided against her own husband, Set, in his battle against Osiris, but when Set was destroyed, she collected the bits of his body and brought him back to life, much as Isis had done for Osiris. In addition to being Isis' sister, she was also said to be Osiris' mistress, leading to much complaint from Isis. Due to her close ties to all the other gods, she was rarely associated with a cult of her own.

Odin

The "All-Father," he is the leader of the Aesir, the principal group of Norse gods. He is a god of war, death, wisdom, poetry, and knowledge, and rides the eight-legged horse Sleipnir. He hung himself for nine days on the world-tree Yggsdrasil, pierced by his own spear, to gain knowledge, and traded one of his eyes for a drink from Mimir's well to gain wisdom.

Mount Mitchell

The Black Mountain subrange of the Appalachians is the location of Mount Mitchell, the tallest peak in the United States east of the Mississippi. This mountain, in North Carolina, was the subject of a debate over its altitude between its namesake and Thomas Clingman, leading Elisha Mitchell to attempt another ascent in which he fell to his death.

Egyptian afterlife

The Egyptians believed that the soul had three components, the ba, ka, and akh, each of which had different roles after death. The ka remained near or within the body (which is why mummification was required). The ba went to the underworld where it merged with aspects of Osiris, but was allowed to periodically return (which is why Egyptian tombs often contained narrow doors). The akh could temporarily assume different physical forms and wander the world as a ghost of sorts. In the underworld, the ba was subjected to the Judgment of Osiris in the Hall of Double Justice, where the heart of the deceased was weighed against Ma'at, commonly represented as an ostrich feather.

Okavango River

The Okavango flows for about 1,000 miles from central Angola, through Namibia's Caprivi Strip, and into the Kalahari Desert of Botswana. There, rather than flowing into the sea, it terminates in a massive inland swamp known as the Okavango Delta, an area that — especially during the wet season — teems with wildlife in an otherwise inhospitable region.

Mount Everest

The border between Nepal and China straddles the summit of the Himalayan peak Mount Everest, which, at a height of over 29,000 feet, is the tallest mountain in the world. The Khumbu Icefall and the cliff-like Three Steps are hazards faced by potential climbers of Everest. It was first summited by Sir Edmund Hillary and Tenzing Norgay in 1953. Local Sherpas act as guides for mountaineers in the area, though they walked out of the job over dangerous working conditions in 2014 after 16 of them were killed in an avalanche.

Freya

The daughter of Njord and twin sister of Frey, she is also a Vanir hostage living with the Aesir. The goddess of love, passion, and human fertility, her possessions include a cloak that allows her to turn into a falcon, and the necklace Brisingamen. She travels in a chariot drawn by two cats.

Ma'at

The daughter of Ra, she predated the universe and served over the creation of it, ensuring balance between everything. Primarily seen as the keeper of order, Ma'at was responsible for seasons, day and night, rainfall, and star movements. A symbolic offering of Ma'at, in the form a statuette was given to the gods, as Ma'at encompassed all other offerings. Ma'at's aspect as god of justice also showed through her role in death ritual, where her ostrich feather was weighed against the hearts of the dead in the underworld. Judges wore effigies of Ma'at, and the supreme head of courts was said to be the priest of Ma'at.

Balder

The fairest of the Aesir, he is the god of light, joy, and beauty. He dreamed of his own death, so Frigga extracted promises from everything not to harm Balder, but she skipped mistletoe. Loki tricked Balder's blind brother Hoder into killing him with a spear of mistletoe.

Horus

The god of the sky and light and the son of Isis and Osiris. In earlier myth he was the brother of Set, and son of Ra. His mother impregnated herself by the dead Osiris, then hid Horus. When he was grown, he avenged his father's death, driving away Set. In the battle, he lost his eye, but regained it thanks to the god Thoth. Thus Horus came to rule over the earth. He was known to have two faces, that of the falcon, Harsiesis, and that of a child, Harpocrates.

Norns

The goddesses of destiny, represented as the three sisters Urd (or Wyrd), Verdandi (or Verthandi), and Skuld. The counterparts of the Greek Fates, they tend the Well of Fate at the roots of Yggdrasil.

Agamemnon

The king of Mycenae, Agamemnon shares supreme command of the Greek troops with his brother, Menelaus. An epithet of his, "king of heroes," reflects this status. As a commander, however, he often lacks good public relations skills, as shown by his feud with Achilles (book 1) and by his ill-considered strategy of suggesting that all the troops go home (book 2). Upon his return home, Agamemnon is murdered by his wife, Clytemnestra, and her lover, Aegisthus.

Nestor

The king of Pylos, he is too old to participate in the fighting of the Trojan War, but serves as an advisor. He tells tales of "the good old days" to the other heroes.

Menelaus

The king of Sparta, Menelaus is the husband of Helen, the cause celebre of the war. He tries to win Helen back by fighting Paris in single combat, but Aphrodite carried Paris off when it seemed that Menelaus would win. Despite his notionally equal say in commanding the troops with his brother Agamemnon, in practice Agamemnon often dominates.

Priam

The king of Troy and son of Laomedon, he has 50 sons and 12 daughters with his wife Hecuba (presumably she does not bear them all), plus at least 42 more children with various concubines. Neoptolemus, the son of Achilles, kills him in front of his wife and daughters during the siege of Troy.

Lake Volta

The largest man-made lake (by area) in the world, Lake Volta was created by the construction of Ghana's Akosombo Dam across the Volta River in the 1960s. The lake covers the area where the Black Volta and White Volta rivers formerly converged. The Akosombo Dam can provide over a gigawatt of power, enough to supply nearby aluminum smelters utilizing the energy-intensive Hall-Héroult process and the needs of the rest of the country.

Triton

The largest moon of Neptune and the only large moon with a retrograde orbit (that is, an orbit opposite to the rotation of its planet), Triton is the seventh-largest moon in the solar system and is thought to have been captured from the Kuiper Belt. For over 100 years after its 1846 discovery, Triton was thought to be Neptune's only moon; Nereid wasn't discovered until 1949. (There are thirteen known satellites now.) Triton is geologically active and has geysers that are assumed to erupt nitrogen. Because of the activity, impact craters on Triton are relatively scarce; most of the larger craters were formed by volcanic activity. Triton orbits around Neptune in almost a perfect circle. Voyager 2 visited Triton in 1989 and is the only space probe to have done so (and no more are currently planned). Much of Triton's western hemisphere consists of an unexplained series of fissures and depressions sometimes called "cantaloupe terrain." Triton's features are named after various water spirits, monsters, and sacred waters from mythology.

Charon

The largest satellite of the dwarf planet Pluto, Charon wasn't discovered until 1978. (As of 2013, Pluto has five known moons, the last two discovered in 2011 and 2012.) Unlike Pluto, which is covered with nitrogen and methane ices, Charon appears to be covered in water ice and may also have active cryo-geysers. Because the center of mass of the Pluto-Charon system lies outside of either one, Charon doesn't truly orbit Pluto; when Pluto was reclassified as a dwarf planet in 2006, an argument was made (but not accepted) to classify Pluto-Charon as a binary system. The IAU still considers Charon, which is roughly half the size but has only 11% the mass of Pluto, to be a satellite. The New Horizons mission visited Charon and Pluto in 2015. Charon was named by its discoverer, James Christy of the Flagstaff Naval Observatory; the IAU approved the name in 1985. Internationally Charon is pronounced like the Greek mythological figure, "CARE-on", but Christy's choice of name was inspired by his wife Charlene, so NASA and New Horizons personnel use "SHARE-on".

Earths moon

The moon, also called Luna, is the fifth-largest satellite in the solar system, the largest relative to the size of the planet it orbits, and the second densest. The USSR's Luna unmanned spacecraft first reached the moon in 1959, and Apollo 8 became the first manned mission to orbit the moon, in 1968. The 1967 Outer Space Treaty guarantees the rights of all nations to explore the moon for peaceful purposes. The flat dark lunar plains are called maria (singular: mare) and are mainly concentrated on the near side of the moon. The most famous one is Mare Tranquillitatis, the Sea of Tranquility, where Apollo 11 first landed on the moon in 1969. The Apollo program landed on the moon five more times.

Mount Kosciusko

The name of Tadeusz Kosciuszko, the Polish commander who fought in the American Revolutionary War, was appended to the tallest mountain in Australia by European explorers in the 19th century. When it was discovered that Mount Townsend was actually taller, the names were switched so that Kosciuszko would remain the highest peak on the continent. The peak's name in Aboriginal languages, such as Jagungal or Tackingal, translates to "table-top mountain."

Chordata

The phylum that contains humans, Chordata is divided into three subphyla: Urochordata, the sea squirts; Cephalochordata, the lancelets, and Vertebrata, the true vertebrates, which is the most diverse subphylum. Defining traits of chordates include pharyngeal gill slits, a notochord, a post-anal tail, and a dorsal hollow nerve cord. In vertebrates, some of these structures are found only in embryonic stages. The lancelet Amphioxus (Branchiostoma) is often used as a demonstration organism in biology labs.

Stamen

The pollen-producing reproductive organ of the flower consists of a thin filament topped by an anther, which actually contains the pollen.

Mitochondria

The powerhouses of the cell. Mitochondria are double-membrane-bound organelles that are the site of respiration and oxidative phosphorylation, processes that produce energy for the cell in the form of ATP. The inner membrane of a mitochondrion forms folds called cristae, which are suspended in a fluid called the matrix. The mitochondrial matrix contains DNA and ribosomes.

Mount Kenya

The second-tallest mountain in Africa is Mount Kenya, which shares its name with the country in which it is located. British geographer and political theorist Halford Mackinder led the first group to ascend the peak, which bypassed the Darwin Glacier and proceeded up the Diamond Glacier. Like Kilimanjaro, it was formed by a now-dormant volcano, and, like Kilimanjaro, part of its notoriety rests on a book. Facing Mount Kenya, an anthropological study of the Kikuyu people by Jomo Kenyatta, was one of the first such texts by an African ethnographer to gain fame.

Frey

The son of Njord, and twin brother of Freya. He is one of the Vanir, a second group of Norse gods, but lives with the Aesir as a hostage. The god of fertility, horses, sun, and rain, his possessions include the magic ship Skidbladnir. He travels in a chariot drawn by the golden boar Gullinbursti, and had to give away his magic sword to win the hand of the giantess Gerda.

Hector

The son of Priam and Hecuba, he is probably the noblest character on either side. A favorite of Apollo, this captain of the Trojan forces exchanges gifts with Ajax after neither can conquer the other in single combat. He kills Patroclus when Patroclus goes into battle wearing the armor of his friend, Achilles. Killed by Achilles to avenge the death of Patroclus, he is greatly mourned by all of Troy. Funeral games take place in his honor.

Heimdall

The son of nine sisters, he is the god of light and guardians. He guards Bifrost, the rainbow bridge into Asgard. His senses are so sharp, he can see 100 miles by night or day and hear grass growing. He will call the Aesir into battle at Ragnarok with his horn Gjall (or Gjallerhorn).

Golgi apparatus

The stack of flattened, folded membranes that forms the Golgi apparatus acts as the "post office of the cell." Here proteins from the ribosomes are stored, chemically modified, "addressed" with carbohydrate tags, and packaged in vesicles for delivery.

Mount Aconcagua

The subduction of the Nazca Plate formed Mount Aconcagua, the highest point in the Southern Hemisphere. It is near the city Mendoza in Argentina, and straddles the Polish Glacier, which provides a popular route for climbers looking to summit it.

Andromache

The wife of Hector and mother of Astyanax, she futilely warns Hector about the war, then sees both her husband and son killed by the Greeks. After the war she is made concubine to Neoptolemus, and later marries the Trojan prophet Helenus.

Frig

The wife of Odin, and mother by him of Balder, Hoder, Hermod, and Tyr. She is the goddess of the sky, marriage, and motherhood, and often works at her loom, spinning clouds.

Hecuba

The wife of Priam, she suffers the loss of most of her children but survives the fall of Troy. She is later turned into a dog.

Fujita-Pearson and Enhanced Fujita scales

These scales measure tornado strength. The Fujita-Pearson scale was introduced in 1971 by Ted Fujita, a meteorology professor at the University of Chicago, and Allen Pearson, director of what is now the Storm Prediction Center. The original scale went from F0 (wind speeds lower than hurricane force, which should cause little to no damage) to F5 (wind speeds of 260 miles per hour, which should cause "incredible damage"). Fujita also included an "inconceivable damage" category for tornados exceeding 319 miles per hour (theoretically possible, but no such tornado has ever been observed). In 2007 the Enhanced Fujita (EF) scale was introduced, narrowing the speed ranges for each category: an EF5 begins at "just" 200 miles per hour.

Achilles

This "swift-footed" warrior is the greatest on the Greek side. His father is Peleus, a great warrior in his own right, and his mother is Thetis, a sea nymph. The consequences of Achilles' rage at Agamemnon for confiscating his geras (prize of honor) are the subject of the Iliad. Achilles kills Hector, but is killed by a poisoned arrow in the heel, the only vulnerable place on his body.

Cassandra

This daughter of Priam and Hecuba has an affair with Apollo, who grants her the gift of prophecy. Unable to revoke the gift after they quarrel, Apollo curses her by preventing anyone from believing her predictions. Among her warnings is that the Trojan horse contains Greeks. After Troy falls she is given to Agamemnon, who tactlessly brings her home to his wife Clytemnestra. Clytemnestra and her lover Aegisthus then kill Agamemnon and Cassandra, leaving Agamemnon's son Orestes (egged on by his sister Electra) to avenge the deaths and kill Clytemnestra and Aegisthus.

Worship of aton

This is actually a historical episode: during the reign of Amenhotep III (1390-1353 BC), worship of the god Aton (or Aten) — a representation of the disk of the sun — was resurrected. This process was carried to its extreme conclusion by his successor, Amenhotep IV, who eventually declared Aton to be the only god, thereby creating one of the earliest known monotheistic religions. The pharaoh even changed his name to Akhenaton, meaning "Aton is satisfied." The worship of Aton was centered on the capital city Tell-al-Amarna, and was largely confined to upper classes and the pharaonic court; it did not survive Amenhotep. Under his successor, Tutankhamen (King Tut), traditional religious practices were restored.

Sir Percivale

a Knight of the Round Table who accompanies Sir Galahad and Sir Bors on the successful quest for the Holy Grail. Percival is one of the sons of King Pellinore. He was raised in the woods by his (unnamed) mother until he turned 15. Although Percival fails to identify the Holy Grail during an early encounter with the wounded Fisher King that involved a bleeding lance, he later heals the Fisher King's wound at the end of the quest. In some stories, Percival loves a woman named Blanchefleur, and he is named as the father of Lohengrin in many Germanic sources.

Ajax

This prince of Salamis is the son of Telamon. He once fights all afternoon in single combat with Hector; since neither one can decisively wound the other, they part as friends. Ajax's most glorious achievement is fighting the Trojans back from the ships almost singlehandedly. He commits suicide after the armor of Achilles is awarded to Odysseus rather than to himself.

Aeneas

This son of Aphrodite and Anchises often takes a beating but always gets up to rejoin the battle. Knocked unconscious by a large rock thrown by Diomedes, he is evacuated by Aphrodite and Apollo. He succeeds the late Hector as Trojan troop commander and survives the fall of Troy, ultimately settling in Italy. His son Iulus founds Alba Longa, near the site of Rome. That bloodline is the basis of Julius Caesar's claim to have descended from Venus.

Odysseus

This son of Laertes is known for his cleverness and glib tongue. His accomplishments include a successful night raid against King Rhesus, winning the armor of Achilles, and engineering the famous Trojan Horse. His ten-year trip home to Ithaca (where his wife, Penelope, awaits) is the subject of the Odyssey.

The lady of the lake

a character who goes by many other names, among them Nimue and Vivien. In many stories, the Lady of the Lake is responsible for bestowing Excalibur upon King Arthur. She also gave Merlin his powers of sorcery and raised Sir Lancelot after his father's death. The Lady of the Lake is frequently associated with the isle of Avalon and is sometimes conflated with Morgan le Fay.

Seine

Though only the second-longest river in France (behind the Loire), the Seine is of key importance, as it flows through Paris. Starting on the Plateau de Langres near Dijon, the Seine weaves northwest for 485 miles to enter the English Channel near Le Havre. Along the way, it passes through Troyes, Fontainebleau, and Rouen. The Seine is France's chief transport waterway, along with its tributaries the Marne and Oise.

Titania and Oberon

Uranus's largest moons, Titania and Oberon, are named for characters from Shakespeare's A Midsummer Night's Dream. (Other Uranian moons are named for characters from either Shakespeare or Alexander Pope.) They were discovered on the same day in 1787 by William Herschel, who also discovered Uranus itself in 1781. In 1986 Voyager 2 became the only spacecraft to date to visit the Uranian moons. Because Uranus orbits the sun almost on its side and Titania and Oberon orbit Uranus in the same plane as its equator, the moons have extreme seasons: Titania's poles spend over 42 years in nonstop sunlight followed by 42 years of darkness. Most of Titania's features are named for settings or female characters from Shakespeare — its largest crater is Gertrude Crater, after Hamlet's mother — while most of Oberon's are named after settings or male characters from Shakespeare. However, Oberon's largest feature is Mommur Chasma, which is named from a French epic poem.

Nile river

Usually cited as the longest river in the world, the Nile flows about 4,132 miles in a generally south-to-north direction from its headwaters in Burundi to Egypt's Mediterranean Sea coast, where it forms a prototypical delta. Over 80% of the Nile's flow comes from the shorter Blue Nile headstream, which arises from Ethiopia's Lake Tana and meets the longer White Nile — whose headwaters include Lake Victoria — at Khartoum. At the first of the Nile's six cataracts is the Aswan High Dam, which forms Lake Nasser and greatly reduces the annual floods.

Zambezi River

Weaving across southern Africa, the Zambezi rises in eastern Angola, passes through Zambia, flows along the borders of Namibia, Botswana, and Zimbabwe, crosses through Mozambique, and enters the Indian Ocean's Mozambique Channel near Chinde. Namibia's Caprivi Strip was created to allow access to the Zambezi. The Cabora Bassa and Kariba Dams form large lakes of the same name. The most spectacular feature of the Zambezi is Victoria Falls, or Mosi-oa-Tunya ("the smoke that thunders"), which is over a mile wide and is the largest waterfall by flow rate in Africa.

Fuses and circuit breakers

When excess current flows through a fuse, the fuse deforms or disconnects so that no current can flow in the circuit. This protects the equipment in the circuit from being damaged by power surges and also protects the surroundings (as excess current can cause wires to overheat, starting a fire). Fuses typically melt and thus must be completely replaced after tripping; a circuit breaker is a form of fuse that can be reset and restored after the circuit has been verified to be safe.

Mount Fuji

Yamanaka and Kawaguchi are two lakes found along the slopes of Mount Fuji, the tallest mountain in Japan. It is only about an hour's drive from Tokyo, and has significance in the Shinto religion, being sacred to the goddess Sengen-Sama. The mountain was also depicted in the series of prints Thirty-Six Views of Mount Fuji, drawn by Hokusai.

Laocoon

Yet another son of Priam and Hecuba, this priest of Apollo shares Cassandra's doubt about the merits of bringing the Trojan horse into the city. "Timeo danaos et dona ferentes," he says (according to Vergil), "I fear the Greeks, even bearing gifts." Later, while sacrificing a bull, two serpents from the sea crush both him and his two young sons. The death of Laocoon is often blamed on Athena (into whose temple the serpent disappeared) but more likely the act of Poseidon, a fierce Greek partisan.

Interest rate

a "price of money," inasmuch as it is the amount one must pay to use somebody else's money ("borrow") for a given period of time; the term is also used for the payment the lender (e.g., a person with a savings account) receives for loaning out his money. Because inflation lowers the value of currency, economists routinely distinguish between nominal interest rates (the actual amount earned by a lender/saver) and real interest rates (which are corrected for the inflation rate). A savings account earning 5% interest is not acquiring the ability to buy more things if prices are also going up by 5% per year!

Sir Gawain

a Knight of the Round Table and the son of Morgause and King Lot of Orkney, making him the nephew of King Arthur. He is the hero of the Pearl Poet's 14th-century romantic epic Sir Gawain and the Green Knight, in which Gawain's loyalty and resolve are tested by the title Green Knight (secretly Lord Bertilak), who survives his beheading at the hands of Gawain and returns a year later to return the favor. Gawain's brothers Gareth and Gaheris are killed during Lancelot's rescue of Queen Guinevere, sending Gawain into a frenzy.

Sir Galahad

a Knight of the Round Table renowned for his purity and honor. Galahad is the illegitimate son of Sir Lancelot and King Pelles's daughter Lady Elaine of Corbenic. Sir Galahad is the only member of Arthur's corps who can sit in the Siege Perilous, a seat at the Round Table set aside by Merlin for the knight who would complete the quest for the Holy Grail. Galahad's quest for the Holy Grail, which he completed alongside Sir Percival and Sir Bors, ended when he encountered the Fisher King, who asked him to take the chalice to Sarras. Galahad is supposedly descended from the brother-in-law of Joseph of Arimathea, who later visits him and allows him to ascend to Heaven.

capacitator

a charge-storing disconnection in a circuit, usually made of parallel plates separated by a medium that blocks the passage of charge. The inner medium is called a dielectric. The governing equation of a capacitor is Q = C ΔV: the charge stored on the capacitor (Q, typically measured in coulombs) equals the capacitor's capacitance (C, typically measured in farads, or more likely in smaller units like microfarads) times the voltage across the capacitor (ΔV). The total capacitance of a circuit depends on whether capacitors are arranged in parallel or series, but in the opposite fashion as for resistors: If two capacitors having capacitance C1 and C2 are placed in parallel, the total resistance across them is C = C1 + C2; if they are placed in series, then 1/C = 1/C1 +1/ C2. This is because wiring capacitors in parallel effectively expands the area of the parallel plates, so configuring capacitors in parallel increases total capacitance.

Sandstone

a clastic sedimentary rock, which means it is composed of pre-existing fragments of other rocks and minerals (usually quartz and feldspar). Common subgroups of sandstone include arkose (sandstones containing over 25% feldspar), wackes (sandstones with a large amount of clay between mineral grains), and arenites (sandstones with a small amount of clay between mineral grains). Fine-grained sandstones make good aquifers because they are porous enough to allow percolation of water but still filter out pollutants.

Granite

a common igneous rock. Granite is felsic, meaning it is primarily composed of feldspar and quartz, and intrusive, meaning it crystallizes and solidifies underground. Granite can experience large-scale weathering, leading to the formation of structures like Yosemite National Park's Half Dome.

Gross Domestic Product (GDP)

a commonly used measure of the size of a country's or state's economy. GDP is computed by summing consumption, investment, government expenditures, and exports, and then subtracting imports.

Rectifier

a device that converts AC or another variable signal into a positive-voltage signal. A half-wave rectifier simply zeroes out negative voltage, while a full-wave rectifier essentially acts as an absolute value function, converting negative voltage to positive voltage. A rectifier can be combined with a capacitor to approximately convert AC into DC.

Switch

a device that toggles between an open connection and a closed connection. Switches can also be used to toggle between two (or more) different closed connections.

Atlanta

a fierce warrior who drew first blood in the Calydonian Boar hunt led by Meleager, who gifted her the Boar's hide when it was finally killed. Atalanta's father left her to die on a mountaintop because he wanted a son, but she was saved by a she-bear and later became a hunter of Artemis. According to some sources, Atalanta was also the only female Argonaut. She married Hippomenes after he beat her in a footrace by slowing her down with golden apples given to him by Aphrodite; their son, Parthenopaios, was one of the Seven Against Thebes.

Op-amp

a five-port device that changes a voltage difference across two ports, multiplied by a factor (gain), into a voltage on an output port, using power supplied to two power ports. There are various ways to wire an op-amp, often feeding an output back into an input, to accomplish interesting tasks like subtracting and multiplying voltages. An ideal op-amp is assumed to have infinite gain, so when solving a circuit, the two inputs must reach a point with no voltage difference across them. Real op-amps have finite gain, impedance across their inputs, resistance on their output, and a finite bandwidth or spread of operating frequencies when used with alternating current.

Theodore von Karman

a fluid dynamicist by training. As a graduate student at Göttingen, he described the so-called Kármán vortex street (a form of turbulence in flow going past a cylinder, which is responsible for driving the vibration or "singing" of power lines) and went on to provide the theoretical basis for the idea of turbulent flows. He contributed to the construction of the wind tunnel at the Guggenheim Aeronautical Laboratory, which eventually became the Jet Propulsion Laboratory. He also formed the company Aerojet to develop short-takeoff aircraft technologies. The Kármán line is the accepted boundary between the Earth's atmosphere and outer space, and the Kármán ogive is the shape of the cone of rockets curving into the main body, which minimizes drag during rocket burn.

Micas

a group of silicate minerals known for their perfect basal cleavage, meaning they can easily split into thin, parallel sheets. Common varieties of mica include black biotite and transparent muscovite. Mica is often found in igneous rocks called pegmatites and also serves as the main constituent of the metamorphic rock schist. Micas generally have a Mohs hardness between 3 and 4.

Feldspars

a group of silicate minerals that are the most abundant minerals in the Earth's crust. Feldspars are categorized based on whether they contain potassium (orthoclase) or sodium and calcium (plagioclase). The continuous branch of Bowen's reaction series covers the transition from calcium-rich to sodium-rich plagioclases. Orthoclase defines hardness 6 on the Mohs scale.

The Minotaur (Asterion)

a half-man, half-bull monster kept in the Labyrinth on Crete by King Minos. Minos prayed to Poseidon to send a snow-white bull as a sign of support during Minos' quarrel with his brothers for the throne of Crete, but instead of sacrificing the animal to the sea god, Minos kept it for himself. Angered, Poseidon caused Minos' wife Pasiphaë to lust after the bull, so Daedalus built her a wooden cow so she could mate with the bull. The product of this encounter was the Minotaur (lit. "Bull of Minos"). After Minos' son Androgeus was killed by Athenians, Minos demanded seven Athenians male youths and seven Athenian female youths, to be selected by lots every seven or nine years (accounts vary) as retribution; these victims were fed to the Minotaur. On the third drawing of the lots, the Athenian hero Theseus volunteered to vanquish the beast; with the help of Minos' daughter Ariadne, who gave Theseus a ball of string so he could find his way out of the Labyrinth, Theseus slew the Minotaur. On the return voyage from Crete, Theseus forgot to change his sails from black back to white, and his father Aegeus jumped into the sea, believing his son had died.

Quartz

a mineral composed of Si O4 tetrahedra linked in a framework with oxygen atoms shared between neighboring tetrahedra to give an overall chemical formula of Si O2 (silicon dioxide). Inclusions of different elements give different varieties of quartz, including amethyst (purple). Quartz is the second-most abundant mineral in Earth's crust. Quartz is often used in clocks because it exhibits piezoelectricity. Quartz defines hardness 7 on the Mohs scale.

Delaware river

a historically significant river of the eastern United States. From its source in the Catskills, the Delaware flows 301 miles, forming the border between Delaware and New Jersey. The Delaware flows past Trenton, New Jersey and Philadelphia, where it receives the Schuylkill River before emptying into the Atlantic. George Washington crossed the Delaware on Christmas night in 1776 to surprise the Hessians at the Battle of Trenton.

The chimera

a hybrid monster who was also a child of Typhon and Echidna. She is most commonly described as a lioness with a goat's head protruding from her back and a tail that ended in a snake's head. She was a fire-breathing menace to Lycia until Bellerophon slew her on orders from King Iobates. Flying on the back of Pegasus, Bellerophon shot at the Chimera and ultimately killed the beast by affixing a block of lead to his spear and causing the Chimera to melt the block with her fiery breath, suffocating her in the process.

The Sphinx

a hybrid monster whose parentage varies widely from source to source. She was a lion-bodied, winged monster with the face of a human, who terrorized the city of Thebes in the generations before Oedipus. She would give a riddle — "What creature has one voice and yet becomes four-footed and two-footed and three-footed?" — and eat anyone who was unable to answer correctly. It is possible that the Sphinx was sent to Thebes from Ethiopia by either the goddess Hera or the war god Ares. Eventually, Oedipus correctly answered the riddle— "Man" — and the Sphinx threw herself off her mountainside perch to her death.

Existentialism

a loosely defined movement of 19th- and 20th-century thinkers who focused on the importance of leading an "authentic" life. Many thinkers that are now thought of as "existentialists" would either not have recognized the term, as it emerged after their time, or would not have accepted it as a description of their outlook. The movement is often identified with the French philosopher Jean-Paul Sartre, who asserted that "existentialism is a humanism." Others associated with the movement include the 19th-century Danish philosopher Søren Kierkegaard (who wrote Either/Or), the 20th-century German philosopher Martin Heidegger (who wrote Being and Time), and Sartre's contemporary Albert Camus (who wrote The Myth of Sisyphus).

Snake river

a major river of the northwestern United States. Rising in Yellowstone National Park, the Snake bisects southern Idaho along its 1,078-mile route. Flowing through Pocatello and Boise, the Snake River forms much of the border between Idaho and Oregon, including the famous Hells Canyon. The Snake River was a vital route for travelers headed west on the Oregon Trail, who followed the river most of the way to its mouth on the Columbia River.

Red River of the south

a major river of the southern Great Plains. The Red River forms most of the border between Oklahoma and Texas along its 1,360-mile length. That border formed part of the border between Mexico (and later the Republic of Texas) and the United States after the 1819 Adams-Onís Treaty. The Red runs through the Louisiana cities Shreveport and Alexandria before emptying into the Mississippi south of Natchez, Mississippi.

Elasticity

a measure of how much one economic variable changes in response to a change in a different variable, expressed in the form of "Every 1% change in the independent (second) variable leads to an x% change in the value of the first (dependent) variable." A common example is the price elasticity of demand, which measures the degree to which the quantity demanded changes when the price is altered.

Saffir-Simpson Scale

a measure of wind speed and damage from hurricanes. It was developed in 1971 by Herbert Saffir, a civil engineer, and Robert Simpson, then the director of the National Hurricane Center. It rates hurricanes on a 1-to-5 scale: a 1 corresponds to a wind speed of 74 to 95 miles per hour, which causes "some damage." A 5 causes "catastrophic damage," with wind speeds over 157 miles per hour and affected areas uninhabitable for weeks or months.

Calydonian Boar

a monstrous beast sent by Artemis to wreak havoc in Calydon after King Oeneus neglected to honor her while sacrificing to the gods. Oeneus's son Meleager led a group of heroes — including Theseus, the twins Castor and Polydeuces (or Pollux), and Achilles's father Peleus, as well as the huntress Atalanta — on what became known as the Calydonian Boar Hunt. Atalanta drew first blood, and Meleager finished off the beast. Meleager, who had fallen in love with Atalanta, then insisted on honoring her by giving her the hide. Meleager's uncles protested, Meleager killed them, and Meleager's mother avenged the death of her brothers by burning up the log that represented Meleager's lifespan, killing him.

Pragmatism

a movement that values ideas based on their practical application (what is sometimes known as the "cash value" of an idea). Its prominent thinkers include William James (who wrote Pragmatism) and John Dewey (who wrote Democracy and Education). The founder of pragmatism was James's friend C. S. Peirce, who expounded its ideas in such essays as "The Fixation of Belief" and "How to Make Our Ideas Clear."

enoplasmic reticulum

a network of tube-like membranes continuous with the nuclear envelope. Part of it are "rough" because they are covered in ribosomes, while other parts are "smooth" because they aren't. In the ER, proteins undergo modifications and folding to yield the final, functional protein structures.

Tristan and iseult

a pair of lovers who predate the stories of King Arthur but nonetheless appear in the Vulgate and Post-Vulgate Cycles. Sir Tristan was a knight who brings Iseult the Fair back to Cornwall to marry his uncle King Mark after killing Morholt, an Irish knight extorting the king. During the return journey, the pair ingest a powerful potion and fall deeply in love with each other, but Iseult nevertheless marries Tristan's uncle. The love potion, however, forces the pair to continuously seek one another out, and King Mark eventually discovers their affair. Tristan escapes his execution and later marries a different woman known as Iseult of the White Hands. Their story inspired Richard Wagner's opera Tristan und Isolde.

Transformer

a pair of solenoids connected to a central core, which permits the circulation of magnetic flux. Because the flux in the core is constant, an alternating current in one coil produces a changing magnetic field, inducing an alternating current in the second coil. If the coils have different numbers of turns of wire (as is typical), the current in the second coil will have a different voltage than the current in the first coil. Therefore, transformers are used to "step up" or "step down" voltage, for example in power transmission lines and in adapters for household electronics.

ground

a point on a circuit that is connected to the ground. This is to prevent accidental discharge of electricity from an unexpected connection—a person who touches an ungrounded circuit might cause electricity to pass through their body. This lets ground act as a known reference point of voltage for analyzing circuits: any grounded point in the circuit is typically understood to have zero voltage.

Merlin

a powerful wizard who serves as Arthur's chief advisor. When Merlin was a child, King Vortigern was told that the boy's blood was necessary to keep his tower from constantly collapsing; however, Merlin identified a pool beneath the tower in which two dragons fought as the source of the instability. Some sources credit Merlin with constructing the Round Table as well as Stonehenge. Merlin's primary apprentice is the sorceress Morgan le Fay, Arthur's half-sister on his mother's side. Some sources have Merlin wind up trapped in an enchanted tomb (possibly in a cave, possibly in a tree) by a figure identified as Vivien or Nimue (the Lady of the Lake). In other tales, Merlin dies and is buried in the legendary forest Brocéliande.

Cassandra

a princess of Troy, one of the children of King Priam and Queen Hecuba. She and her twin brother Helenus were both priests of Apollo and thus blessed with the gift of prophecy; however, after Cassandra spurned the advances of Apollo, he cursed her to never have her prophecies believed. During the Trojan War, Cassandra was raped in the Temple of Athena by Ajax the Lesser, which led Athena to wreck his ship on his journey home. After the war, Cassandra was made the concubine of Agamemnon, and the two were killed by Clytemnestra and Aegisthus when they returned to Mycenae.

Utilitarianism

a school of ethics that advocates the maximization of "utility," which is often identified with pleasure or happiness. Its most influential early thinker was Jeremy Bentham, who asserted that it is the "greatest happiness of the greatest number that is the measure of right and wrong." The foremost utilitarian was Bentham's disciple, John Stuart Mill, who expounded his views in such books as Utilitarianism and On Liberty.

Limestone

a sedimentary rock primarily composed of calcium carbonate. Limestone often metamorphoses into marble. Because limestone is made from carbonate minerals, it is soluble in acid, and erosion of limestone can form karst topography. Limestone is commonly used in the production of lime and cement and as a pH buffer in soil conditioners.

Mauna Loa

a shield volcano on the Big Island of Hawaii that is the most massive above-sea volcano on Earth. It should not be confused with Mauna Kea, a dormant volcano that is the tallest mountain in Hawaii. Both Mauna Kea and Mauna Loa are taller than Mount Everest when measuring from base-to-summit rather than from sea level. Along with Kilauea, Mauna Loa forms the Hawaiʻi Volcanoes National Park, which was established in 1916 by Woodrow Wilson. When Mauna Loa erupted in 1942, four months after the attack on Pearl Harbor, the U.S. government gagged the press from reporting on the eruption.

Kilauea

a shield volcano that is the most active of the five volcanoes on the Big Island of Hawaii. The name "Kilauea" means "spewing" or "spreading" in the Hawaiian language. In Hawaiian mythology, the volcano is the home of the fire goddess Pele. The three primary craters of Kilauea are Halemaʻumaʻu Crater, Puʻu Crater, and Puʻu ʻOʻo Crater. In addition, part of the volcano is covered by the Kaʻu Desert, a plain of dried lava and volcanic ash. The Hawaiian Volcano Observatory is on the rim of Kilauea's caldera.

Medea

a sorceress from the island of Colchis; her father was King Aeëtes, and her aunt was the witch Circe. Medea encountered the hero Jason when he and the Argonauts came to Colchis in search of the Golden Fleece, and she helped him yoke fire-breathing oxen and sow dragon's teeth. Medea left Colchis with Jason, and on the voyage home she killed both her brother Absyrtus and the giant bronze automaton Talos. She and Jason had several children together, but Jason ultimately left Medea for the princess Glauce; in vengeance, she killed two of her children and fled on a golden chariot.

Calcite

a stable mineral form of calcium carbonate. Less stable forms of calcium carbonate (called polymorphs), such as aragonite, eventually convert to calcite. Calcite is found in the shells of many marine organisms, including plankton, echinoderms, and bivalves. This makes calcite a major constituent of sedimentary rocks, especially limestone. Many optical devices utilize calcite because it exhibits birefringence (double refraction). Calcite defines hardness 3 on the Mohs scale.

Krakatoa

a stratovolcanic island in the Sunda Strait between the Indonesian islands of Sumatra and Java. Prior to its massive 1883 eruption, there were three separate volcanic peaks: Perboewatan (completely destroyed by the eruption), Rakata (extant), and Danan (almost completely destroyed). The 1883 eruption of Krakatoa killed more than 35,000 people and is believed to have produced the loudest sound ever. Krakatoa is one of the largest volcanoes in the Pacific Ring of Fire. The continued activity of Krakatoa formed a new island, Anak Krakatau (or "Child of Krakatoa") in 1927.

Mount Etna

a stratovolcano in eastern Sicily that plays a significant role in ancient Greek myths. Zeus trapped the monster Typhon under Mount Etna, and the volcano is also the site of Hephaestus forges. Mount Etna overlooks the Sicilian town of Catania. It most recently erupted in 2015. Earlier eruptions occurred in 1928 (destroying the village of Mascali) and in 1669, after which the volcano was thought to have gone dormant. There is a large depression on the side of Mount Etna known as Valle del Bove, or "Valley of the Ox."

Mount Pinatubo

a stratovolcano in the Zambales Mountains on the eastern coast of the island of Luzon in the Philippines. In the aftermath of its 1991 eruption, the global temperature briefly decreased by almost a full degree Fahrenheit due to the ejection of aerosols and sulfuric acid. That eruption was ten times more powerful than the 1980 eruption of Mount Saint Helens; it killed at least 800 people and forced the evacuation of some 20,000 people. In its aftermath, monsoon rains created Lake Pinatubo in the resulting crater. In 2010, the indigenous Aeta people were formally granted domain over the volcano.

Mount Vesuvius

a stratovolcano on the Gulf of Naples in Campania in south-central Italy. Mount Vesuvius, which last erupted in 1944, is the only active volcano on mainland Europe. The most notable eruption of Mount Vesuvius was in AD 79 when it buried the Roman cities of Pompeii, Herculaneum, and Stabiae. The only surviving eyewitness accounts of that eruption are a pair of letters written by Pliny the Younger to Tacitus. The 1908 Summer Olympics were relocated from Rome to London after a surprise eruption of Mount Vesuvius.

Mount Fuji

a stratovolcano on the island of Honshu, is Japan's tallest mountain. Mount Fuji is one of the "Three Holy Mountains" of Japan, along with Mount Haku and Mount Tate. Mount Fuji last erupted in December 1707 in the lava-less Hoei Eruption. Because of its beautiful shape, Mount Fuji is a frequent muse for artists, including the Japanese artist Katsushika Hokusai, whose woodblock series Thirty-Six Views of Mount Fuji includes The Great Wave off Kanagawa. Aokigahara, popularly known as the "Suicide Forest," is on the volcano's northwestern slope.

Mount Erebus

a stratovolcano, is the second-highest volcano in Antarctica (but tallest active one) and the southernmost active volcano in the world. Mount Erebus sits on Ross Island. It took its name from the primordial personification of darkness in Greek mythology. The northeast slope of Mount Erebus is known as "Fang Ridge," and the volcano's underground magma produces unique pieces of feldspar known as "Erebus crystals." In 1979 an Air New Zealand jet crashed into the side of Mount Erebus, killing 257 people.

Arachne

a talented Lydian weaver who constantly boasted that her skills were better than that of any of the gods. Arachne challenged the goddess Athena to a weaving competition, during which Arachne wove a tapestry depicting the gods' transgressions against mankind and Athena depicted four contests between mortals and the gods. The result of the competition depends on the sources: some say that Arachne won but Athena was insulted by her images and destroyed Arachne's work, and others say that Athena's weaving was superior. In all versions, though, Athena turns Arachne into a spider.

Tariff

a tax placed on the import (common) or export (rarer) of a good. Tariffs have various — often controversial — sociopolitical goals, but economists agree that they raise consumer prices relative to equilibrium (resulting in a lower quantity demanded). Tariffs are similar to quotas, in which a government limits the amount of a good that may be imported.

Gene Kranz

a trained aeronautical engineer and pilot, he is best known for his Flight Directorship during the Mercury and Apollo projects. He presided over a departure from NASA's traditional incremental testing structure, in favor of a more aggressive, manned campaign ("all-up testing"). As part of the Apollo 13 "white team," Kranz and his associates engineered the free-return trajectory and human-factors management that brought the Apollo 13 crew back safely after a near-fatal spacecraft anomaly.

Columbia River

a vital waterway of the Pacific Northwest. Rising in the Rockies of British Columbia, the Columbia flows through Lake Revelstoke before entering Washington state. Grand Coulee Dam along the Columbia in Washington forms Lake Roosevelt. When it was completed in 1943, Grand Coulee was the largest hydroelectric plant in the world; it is still America's largest electric power plant. The Columbia receives the Yakima and Snake Rivers before forming much of the Washington-Oregon border, receiving the Willamette River in Portland before emptying into the Pacific where Lewis and Clark sighted the ocean.

Infrared spectroscopy (IR)

acquires information about the chemical groups present in a compound based on which wavelengths of infrared light the bonds in those groups absorb. When IR-active bonds absorb infrared light, they undergo a change in dipole moment and are excited to a higher-energy vibrational mode, which have names like "stretching," "wagging," and "scissoring." The output of an IR experiment (called an "IR spectrum") is a graph of absorbance on the y-axis against wavenumbers, measured in cm−1 (inverse centimeters), on the x-axis. The most distinguishable IR peak is that of a carbonyl group, which displays a very strong absorbance at 1700 cm−1, while peaks below 1500 cm−1 produce a complex pattern unique to the compond being analyzed, called the fingerprint region. Samples are typically prepared by grinding them into a potassium bromide pellet or by creating a mull with the oil Nujol.

Stoicism

an ancient Greek school that idealized freedom from emotions. It was founded by Zeno of Citium, who taught at the "painted porch" in Athens. (The name of the movement derives from the Greek word "stoa," which means "porch.") An important Stoic idea was that of the "pneuma," or the "breath of life," which is the life force that structures matter and the soul. Important thinkers include Epictetus, a slave whose views were recorded by his student Arrian in the Discourses, and the Roman emperor Marcus Aurelius.

Robert Goddard

an early explorer of the theory and practice of launching rockets. His work provided the foundation for modern rocketry: in particular, gimballed engines (engines where the exhaust nozzles can change direction allowing the rocket to be steered), fuel pumps, steering with vanes, and gyroscopic stabilization. His seminal monograph on the subject was 1919's A Method of Reaching Extreme Altitudes. The next year, he put forward a concept for a rocket launch to the moon that was widely ridiculed in the press as being unrealistic. Goddard is the namesake of the NASA Goddard Spaceflight Center in Maryland.

Resistor

an element that impedes the flow of current, creating a voltage drop. Resistance is measured in ohms, named for Georg Ohm and symbolized by a capital omega (Ω). Resistors are governed by Ohm's law, V = IR. In other words, the voltage drop across a resistor (V) equals the current passing through the resistor (I) times the resistor's resistance (R). According to Joule's first law, the power dissipated by a resistor is P = I2R. Combining that fact with Ohm's law yields P = IV. The total resistance of a circuit depends on whether resistors are arranged in parallel (different branches connecting to the same pair of nodes) or series (same branch). If two resistors having resistance R1 and R2 are placed in series, the total resistance across them is R = R1 + R2; if they are placed in parallel, then 1/R = 1/R1 +1/ R2.A rheostat, or variable resistor, can be made by taking a material of uniform resistivity and changing how much of the material is in the circuit. A potentiometer is a rheostat with two terminals and a connection in the middle, so the middle terminal can set a precise, variable dividing point between the ends; therefore, a potentiometer can be used as a voltage divider. Rheostats and potentiometers are often operated by knobs or sliders. A Wheatstone bridge is a diamond-shaped arrangement of two resistors of known resistance, one rheostat, and a resistor of unknown resistance. With a DC voltage source between the two known resistors, and also between the rheostat and the unknown resistor, this setup can be used to find the resistance of the unknown resistor: an ammeter (device to measure current) is connected across the two other nodes and the rheostat is adjusted until no current flows. The formula for solving a Wheatstone bridge uses Kirchhoff's laws: the total current flowing into any point in a circuit equals the total current flowing out of the point, and the total voltage change around any loop of a circuit is 0 V.

Partons

an older name that was used for the "internal parts" of hadrons before the discovery and widespread acceptance of the quark model. Models based on partons are still used but, for the most part, it was determined that partons were quarks and the term is rarely used at the high school level except in historical contexts.

Quarks

another class of fundamental particle. They also come in six flavors: up, down, charm, strange, top (occasionally called "truth"), and bottom (occasionally "beauty"). The up, charm, and top quarks have a charge of +2/3, while the down, strange, and bottom have a charge of -1/3. All quarks are fermions, and they combine in pairs to form mesons and in triples to form baryons. The enormous mass of the top quark (178 GeV) made it difficult to create in particle accelerators, but its discovery in 1995 confirmed an essential element of the Standard Model of particle physics. The name "quark" comes from the line "Three quarks for Muster Mark" in Finnegans Wake that appealed to Murray Gell-Mann. The study of quarks (and of the strong nuclear force) is quantum chromodynamics.

Mount Saint Helens

another stratovolcano in the Cascades of Washington state. It is about 100 miles south of Seattle and about 50 miles north of Portland. It last erupted in May 1980, killing 57 people in the most devastating volcanic eruption in U.S. history. Perhaps the most famous casualty of that eruption was an innkeeper named Harry Truman (no relation to the president of the same name) who refused to evacuate from the mountain. Because of an avalanche of volcanic debris during that eruption, Mount Saint Helens lost about 1300 feet in height; lahars (mudflows of pyroclastic material) reached all the way to the Columbia River. The volcano is surrounded by Gifford Pinchot National Forest.

Source

any device that provides voltage or current. Direct current (DC) sources like batteries provide constant current, while alternating current (AC) sources like household electric outlets provide current that changes at a regular frequency with a constant average voltage. In alternating-current circuits, capacitors and inductors can help tune a circuit's efficiency by contributing to a phenomenon called impedance, which is the complex analogue of resistance. A circuit's reactance is the imaginary part of its impedance and depends on the frequency of the circuit. When reactance is balanced, we find the frequency at which the circuit loses the least energy, or its resonant frequency.

Hadrons

any particles made out of quarks (alternatively, any particle affected by the strong nuclear force). Generally, this means the baryons and the mesons. All hadrons are colorless (in the sense of the combined color of their constituent quarks). The name "hadron" comes from the Greek for "thick."

Yellow river

at 3,400 miles, China's second-longest; it is also the most important to the northern half of the country. It rises in Qinghai province and flows into the Bohai Gulf of the Yellow Sea. The river's name comes from the extraordinary amount of loess silt that it carries, an average of 57 pounds for every cubic yard of water. Among its notable features is the Grand Canal, rebuilt during the Ming Dynasty, that links it to the Yangtze.

Sirens

beautiful women who appeared harmless and sang a beautiful song to passing sailors, only to prove vicious and bloodthirsty when the sailors ventured too close. The Greeks often said that the Sirens were the daughters of the river god Achelous, while the Romans named their father as Phorcys. In the Argonautica, Chiron warns Jason that Orpheus will be instrumental on his journey, and Orpheus later saves all of Jason's crew (save Butes) by playing his lyre when they pass the Sirens to drown out their beautiful and alluring song. Odysseus also encountered the Sirens, tying himself to the mast of his ship so that he could safely hear their song while his crew plugged their ears with beeswax, on the advice of the sorceress Circe.

Rhine river

begins in the Swiss Alps, passes through Lake Constance (in German, the Bodensee), flows west along the German-Swiss border, then turns north to form part of the German-French border. The river then flows north and joins with the Meuse and Scheldt to enter the North Sea at a delta in the Netherlands. Cities along its course include Basel, Strasbourg, Mainz, Bonn, Cologne, and Rotterdam, and tributaries include the Main, Mosel, and Ruhr. The Rhine has played a strategic role in most German conflicts since the time of the Gallic Wars, but was not established as an international waterway until the Rhine Commission of 1815. German myth tells of the Lorelei, a nymph who lured sailors on the Rhine to their deaths.

Oxygen

by mass, the most common element in Earth's crust. It was discovered independently by Carl Scheele and Joseph Priestley; Priestley originally called it "dephlogisticated air." Oxygen normally exists in elemental form as a diatomic gas (O2), but it can also exist in a triatomic form, ozone (O3), which is known for its role in blocking UV rays in Earth's stratosphere. Diatomic oxygen is — despite having an even number of electrons — paramagnetic, meaning it has unpaired electrons. This points out a problem with traditional valence bond theories, which predict that oxygen should be diamagnetic; molecular orbital theory correctly explains this behavior. Because oxygen is easily capable of accepting electrons, reactions in which a species gives up electrons are known as oxidation reactions.

Titrations

calculate the concentration of a solution by adding in small volumes of a reactant of known concentration until a chemical change, like a pH indicator changing color, occurs. Acid-base titrations are usually performed in a thin glass tube called a buret and use pH indicators like phenolphthalein and bromothymol blue. The Henderson-Hasselbalch equation can be used to calculate the pH at any point during a titration. The equivalence point is the point at which equal amounts of acid and base have been mixed and there is a sharp inflection point in the pH curve. Redox titrations use an oxidation-reduction reaction instead of an acid-base reaction. In complexometric titrations, the analyte forms a coordination complex with the titrant. Karl Fischer titrations use electrolysis to determine the amount of water in a substance.

Calorimetry

calculates the heat or enthalpy change of a chemical or physical process by using specialized vessels to measure a change in temperature. A very simple calorimeter can be made by placing a thermometer in an insulating polystyrene coffee cup and sealing it with a lid, causing the reaction inside to occur at constant pressure. Bomb calorimeters are very sturdy containers used to conduct combustion reactions at constant volume. Isothermal titration calorimetry (ITC) is a variant which can be used to determine the binding affinites and stoichiometry of proteins and enzymatic reactions. Differential scanning calorimetry (DSC) is another variant that measures how a compound's heat capacity changes with temperature; it is commonly used to determine properties of polymers such as their melting point or glass transition temperature.

Seeded plants

can be divided into gymnosperms (cycads, ginkgos, conifers, and gnetophytes) and angiosperms (phylum Anthophyta, or flowering plants). Most of these plants produce male gametophytes that grow into the female, allowing fertilization to take place in relatively dry conditions. Many of them also exhibit secondary growth of woody tissues, allowing them to grow even taller than the pterophytes.

Decibel scale

can describe any kind of power, but is most commonly used to describe the intensity of sound waves. The decibel (abbreviated dB) is one tenth of a larger unit, the bel (abbreviated B), named after inventor Alexander Graham Bell. The intensity of a sound in decibels is given by the formula I = 10 log(P1/P0), where P1 is the intensity of the sound being measured (in watts per square meter) and P0 is a reference intensity, which is based on the least powerful sound wave that can be detected by the average human ear (namely 10-12 W/m2). A 10-dB increase in sound intensity corresponds to multiplying the energy of the sound wave by 10. A normal conversation has a volume of about 70 dB.

Pterophytes

can grow taller thanks to vascular tissues (see number 4 below) that provide structural support and transport water and other materials throughout the plant. Many of them do have true leaves and roots. Pterophytes have a prominent sporophyte stage that grows from a small, short-lived gametophyte. Pterophyte gametophytes may be dioicous or monoicous, producing both sperm and egg on the same plant.

Calyx

composed of sepals, specialized green leaves that protect the flower as a bud and provide support for the fully bloomed flower

Baryons

composite (i.e., non-fundamental) particles made from three quarks. The most common examples are the proton (two up quarks and one down quark) and the neutron (two down quarks and one up). All baryons are fermions. Quarks possess a characteristic called "color" (which has nothing to do with visual color) which can be either red, green, or blue (arbitrary names; again, no relation to the colors we see). A baryon must have one quark of each color so that the "total color" (analogous to mixing red, green, and blue light) is colorless (i.e., "white"). The word "baryon" comes from the Greek for "heavy." The total number of baryons is conserved (again, counting anti-baryons as -1).

Mesons

composite particles generally made from a quark and an anti-quark. There are dozens of examples including the pion, kaon, J/Psi, Rho, and D. All mesons are bosons. The quark and anti-quark must have the same color (such as red and anti-red) so that the resulting meson is colorless (or "white"). It is also possible to make mesons out of two (or more) quarks and the same number of anti-quarks, but this kind of particle (a "tetraquark") is rare, both in nature and in quiz bowl.

Helen of troy

considered the most beautiful mortal woman during the Age of Heroes. Helen was the daughter of Zeus and Leda, and her siblings were Castor, Polydeuces (or Pollux), and Clytemnestra. When Helen married Menelaus, the king of Sparta, Helen's father Tyndareus forced the Greek kings to swear an oath to fight for her if she were kidnapped. When she was abducted by (or eloped with) Paris, a prince of Troy, the whole Greek world plunged into the Trojan War. For this reason, Christopher Marlowe had Doctor Faustus refer to Helen as "the face that launched a thousand ships."

Rational functions

consist of one polynomial divided by another polynomial. The denominator polynomial cannot be the zero polynomial, because dividing by zero is undefined. Examples therefore include 1/x, x2/(x - 3), and (x2 + 1)/(x2 - 1). Every polynomial can be considered to be a rational function because 1 is a polynomial and dividing by 1 doesn't change an expression (so to consider the polynomial x3 as a rational function, think of it as x3/1). It is often instructive to study the asymptotes of rational functions, which are places in which their graphs approach a line (or occasionally other shape), usually getting infinitely close to but not crossing it. That analysis may require performing long division on the numerator and denominator polynomials to find their greatest common factor.

Hubble Space Telescope Servicing Missions SM1

corrected Hubble's flawed optics by installing COSTAR (a corrective optics system), while removing the High-Speed Photometer to make room. It also replaced the original Wide Field and Planetary Camera, or WFPC "wiff-pick", with WFPC2, which a camera that had optical correction built in.

Euphrates River

defines the western border of Mesopotamia; it also rises in the Zagros Mountains of Turkey and its shores are home to Fallujah and Babylon. It is the longer of the two rivers, with a course of 1,740 miles (compared to the Tigris' 1,180). Both the Tigris and the Euphrates have changed courses several times, leaving ruins in the desert where cities have been abandoned.

Hellmuth Walter

designed the Starthilfe (takeoff assist) rocket propulsion units for the Messerschmidt Me 163 and the vertical-takeoff Bachem Ba 349 aircrafts, used by the German Luftwaffe in World War II. He also was awarded a patent in 1925 for suggesting that a catalyzed decomposition of hydrogen peroxide could provide oxygen for combustion.

Flame tests

detect the presence of elements by dipping a wooden splint or nichrome wire in a sample of the element or its salt, then placing the sample over a Bunsen burner. The unique emission spectrum of the element present then causes the flame to briefly change color. The D lines of sodium produce one of the strongest flame test colors: a bright yellow. Because of this, sodium can contaminate samples, so flames are usually viewed through a cobalt blue glass to filter out the yellow. Other notable flame test colors include red, produced by lithium, calcium and strontium; lilac, produced by potassium; green, produced by barium; and blue, produced by copper, selenium, arsenic, and indium.

pH scale

developed by S. P. L. Sørensen in 1909, is used to quantify acidity. The pH (power of hydrogen) of a solution is defined as the opposite of the (base-10) logarithm of the concentration of protons in a solution: pH = -log10[H+]. (The brackets are standard notation in chemistry for "the concentration of.") Thus, greater concentrations of protons correspond to smaller pH values. At 25°C, the neutral (neither acidic nor basic) pH is 7; solutions with a pH less than 7 are considered acidic, and solutions with a pH greater than 7 are considered basic.

Pauling scale

devised by Linus Pauling in 1932, is one of several scales that measure electronegativity, the extent to which atoms attract electrons in chemical bonds. Higher values correspond to stronger attractions. Francium has the lowest value, about 0.7, while fluorine has the highest value, about 4.0. (Noble gases are not assigned values on the Pauling scale, since they were not known to form any bonds when Pauling devised it.) Differences in electronegativity characterize bonds: the greater the difference, the more ionic the bond.

Phobos

discovered by Asaph Hall III in 1877. At just 3,700 miles above the Martian surface, Phobos orbits more closely to its planet than any other moon in the Solar System. Because it orbits Mars faster than Mars rotates, each day it appears (from the Martian surface) to set twice in the east. Geological features on Phobos, including the Stickney Crater, are primarily named for either astronomers (Stickney was the maiden name of Asaph Hall's wife) or characters from Jonathan Swift's Gulliver's Travels. In 1971 the U.S.'s Mariner IX became the first spacecraft to provide close-up photos of Phobos.

Seedless plants

divided into bryophytes (mosses, liverworts, and hornworts) and pterophytes (ferns, club mosses, quillworts, and horsetails). Both of these groups, like all other plants, reproduce by producing sperm and eggs on a structure called the gametophyte. The gametes fuse to form another structure called the sporophyte, which produces spores that disperse and grow into new gametophytes. Both groups produce flagellated sperm that require water for fertilization. Note that these uses of "bryophyte" and "pterophyte" here are informal, and should not be confused with the actual phyla Bryophyta (true mosses) and Pteridophyta (true ferns).

St. Lawrence River

drains the Great Lakes and serves as a major waterway of eastern Canada. First explored and named by Jacques Cartier in the early 16th century, the St. Lawrence emerges from the northeastern corner of Lake Ontario in the Thousand Islands archipelago, forming the border between Ontario and New York. The St. Lawrence receives the Ottawa and Saguenay Rivers and flows through Montréal and Quebec City. At its mouth, the Gulf of St. Lawrence is one of the world's largest estuaries.

supply and demand

embody the general idea that, at any given price, producers are willing to produce a given amount of a good (the quantity supplied) and consumers are willing to buy a different amount (the quantity demanded). As the price rises, firms will be willing to supply more goods, but fewer will be demanded; conversely, as price falls, consumers will demand more goods, but fewer will be supplied. On a graph showing price and quantity on its two axes, this is represented by a downward-sloping demand curve and an upward-sloping supply curve. Where the two curves intersect, there is an equilibrium; economists predict this equilibrium quantity will be produced (and consumed) in a free market.

Skepticism

encourages the rejection of truths unless they are supported by sufficient evidence. Academic Skepticism, which states that no truths can be certain, was led by such men as Arcesilaus and Carneades. Another form of skepticism is known as "Pyrrhonian," after Pyrrho of Elis, who is considered the founder of skepticism. The thinker Sextus Empiricus, who lived in the second century AD, provided one of the most complete accounts of skepticism in his Outlines of Pyrrhonism

Positivism

encourages the use of the scientific method to discover the laws that govern society. It was founded by the 19th-century Frenchman Auguste Comte, who believed that society develops through three stages, which he termed the theocratic, metaphysical, and positive. It should not be confused with logical positivism, a 20th-century movement whose leaders included the young Ludwig Wittgenstein (who wrote Tractatus Logico-Philosophicus) and which holds that the only meaningful statements are those that are logically verifiable.

Pistil

female reproductive organ of the flower, is composed of leaf-like carpels. The ovary-containing ovules are at the base of the pistil, while a tube called a style topped by a sticky, pollen-receptive stigma rises from the ovary. There may be one or many pistils in each flower.

Ohio rivers

flows 981 miles through a significant industrial region of the central United States. Historically seen as the border between the northern and southern United States, the Ohio is formed in downtown Pittsburgh by the confluence of the Allegheny and Monongahela Rivers, flowing past Wheeling, Cincinnati, Louisville, and Evansville, forming borders of five states before emptying into the Mississippi at Cairo, Illinois. Other major tributaries of the Ohio include the Kanawha, Kentucky, Tennessee, Wabash, and Cumberland Rivers.

Rio Grande

formed the border between Texas and four Mexican states since 1848. It flows south out of Colorado through New Mexico before reaching the international boundary near El Paso. Texas's Big Bend National Park is named for the sweeping curve the Rio Grande cuts through the Sierra Madre Oriental. After leaving the mountains, the river flows past Laredo and Brownsville before it empties into the Gulf of Mexico.

Carbon

found, by definition, in all organic compounds. It is the fourth most abundant element in the Universe. It has three major isotopes: isotope 12, which is stable; isotope 13, which is used in NMR spectroscopy; and isotope 14, which is radioactive and is the basis of carbon dating. Carbon's ability to form four chemical bonds means that it has many different allotropes. The best-characterized natural allotropes are diamond, which consists of a tetrahedral network of carbon atoms, and graphite, which consists of planes of carbon atoms arranged in hexagons. Fullerenes such as buckyballs and carbon nanotubes, on the other hand, are generally produced synthetically; buckyballs are roughly spherical. More recently, graphene, which is a single layer of atoms shaped like graphite, has proven to have remarkable properties; for example, it is nearly transparent while being about 200 times stronger than an equivalent mass of steel.

differentiable function

functions for which the derivative can be found (because a particular limit, called a difference quotient, exists). Like continuity, differentiability is really a property of a specific point, and a differentiable function is one that is differentiable at every point. Every differentiable function is continuous, but some continuous functions are not differentiable; mathematicians thus say that differentiability is a stronger property than continuity. In terms of graphs, a differentiable function has a "smooth" graph with no corners or cusps (and also, because continuity is required, no holes, jumps, or asymptotes). All polynomials are differentiable, as are the sine and cosine functions, and exponential and logarithmic functions. However, the absolute value function f(x) = |x| is not differentiable because it has a "corner" at x = 0 (but recall that it is still continuous).

Polynomials

functions made of terms added together, in which each term is a number times a product of variables raised to nonnegative-integer powers. For instance, 3x2y and -πx7y2z3 are each terms, so 3x2y - πx7y2z3 is a polynomial. (Individual terms are also considered polynomials.) Much of math is concerned with polynomials involving only one variable, such as -x3 + 2x2. The number at the beginning of each term is called a coefficient. Since simple numbers ("constants") can also be written as the same number times any variable to the zeroth power (that is, 6 is the same as 6x0), numbers are also considered terms and polynomials. So x + 6 is a polynomial, as is just -4. Polynomials can be classified according to their number of terms: a polynomial with one term, like 2x or -12x2, is called a monomial; a polynomial with two terms is called a binomial; and a polynomial with three terms is called a trinomial. Each polynomial has a degree. For polynomials of one variable, the degree is the largest exponent on the variable, so for the polynomial 4x3 - x2, the degree is 3. For polynomials of multiple variables, to find the degree you calculate the sum of the variables' exponents on each term, then choose the largest such sum, so the degree of 3x6y5 - x2y3 is calculated by adding 6+5 = 11 for the first term and 2+3 = 5 for the second and noting that 11 is larger, so the degree is 11. A polynomial that is just a constant has degree 0. A polynomial with degree 1, like 3x, is called linear; a polynomial with degree 2, like 3x2 - 8x + 4, is called quadratic; a polynomial with degree 3 is called cubic; continuing with increasing degrees, the terms are quartic, quintic, sextic, and so on, though terms corresponding to degrees larger than 5 are seldom used. For technical reasons that are beyond the scope of this article, the zero polynomial (i.e., f(x) = 0) is said to have a degree of -∞ or an undefined degree. The fundamental theorem of algebra is the statement that every single-variable polynomial, other than constants, has a root in the complex numbers, which means that if f(x) is a polynomial, then the equation f(x) = 0 has at least one solution where x is some complex number. There are formulas to find those roots for linear, quadratic, cubic, and quartic polynomials, though the latter two formulas are extremely complicated. The Abel-Ruffini theorem, also called Abel's impossibility theorem, is the statement that there is no way to find a formula for the solutions of all quintic or higher-degree polynomials, if the formula must be based on the traditional operations (addition/subtraction, multiplication/division, and exponentiation/taking roots). That impossibility is the topic that began an area of study called Galois gal-wah theory, which is part of abstract algebra.

logarithmic function

functions of the form f(x) = logbx, where b is again a positive number other than 1 (and again called the base). They are the inverses of the exponential functions with the same bases. Logarithmic functions are used to model sensory perception and some phenomena in probability and statistics. The phrase "the logarithm" can refer to a logarithmic function using the base 2 (especially in computer science; this is also called the binary logarithm), e (especially in higher math), or 10 (especially in lower levels of math and physical sciences). The phrase natural logarithm refers to the logarithm base e, and the phrase common logarithm usually refers to the logarithm base 10.

Surjective Function

functions that achieve every possible output. For instance, if you are thinking of functions whose domain and codomain are both the set of all real numbers, then f(x) = tan(x) is surjective, because every real number is an output for some input. But f(x) = x2 is not surjective, because (for instance) -3 is not an output for any real-number input. The term image is sometimes used for the set of all output values that a function actually achieves; a surjective function, then, is one whose image equals its codomain. A function that is both injective and surjective is called bijective, or a bijection. If a function is bijective, then it has an inverse. Furthermore, a function can only have an inverse if it is bijective.

Injective Function

functions that do not repeat any outputs. For instance, f(x) = 2x is injective, because for every possible output value, there is only one input that will result in that output. On the other hand, f(x) = sin(x) is not injective, because (for instance) the output 0 can be obtained from several different inputs (0, π, 2π, and so on). If you have the graph of a function, you can determine whether the function is injective by applying the horizontal line test: if no horizontal line would ever intersect the graph twice, the function is injective.

Continuous functions

functions where the limit approaching each point equals the function's value at that point. In particular, there are no holes, jumps, or asymptotes "in the middle of the graph." Continuity is really a property of a specific point; a continuous function is a function that is continuous at every point. Continuity is often explained as a function's graph being drawable in one motion without lifting the writing utensil from the paper. All polynomials are continuous, as are the sine and cosine functions, exponential and logarithmic functions, and the absolute value function. Some examples of non-continuous functions are many rational functions, as they often have holes or asymptotes; the tangent, cosecant, secant, and cotangent functions, which have asymptotes; and the floor and ceiling functions, which have jumps.

Gauge bosons

fundamental bosons that carry the forces of nature. That is, forces result from particles emitting and absorbing gauge bosons. The strong nuclear force is carried by gluons, the weak nuclear force is carried by the W, Z-, and Z+ particles, the electromagnetic force is carried by the photon, and gravity is carried by the (as yet unobserved) graviton. The name comes from the role of "gauge theories" in describing the forces (which are beyond the scope of this article).

Monocots

produce pollen grains that have a single furrow (monosulcate); flower parts in multiples of three; numerous, fibrous roots; parallel leaf veins; and stems with scattered vascular bundles. They also lack secondary growth, remaining herbaceous throughout their lives.

Mass Spectrometry (mass spec or MS)

identifies an unknown compound by ionizing it, fragmenting it into pieces, then passing it through electromagnetic fields to separate the pieces based on their mass-to-charge ratio (m/z). A mass spectrum plots the abundance of each fragment against the m/z—the spectrum features a base peak (the peak of highest intensity) and a series of peaks whose spacing tells you what elements are present—for instance, a peak spacing of 14 typically indicates a CH2 or methylene unit. The different isotopes of elements can also produce characteristic peak patterns—for instance, two equal-intensity peaks spaced 2 units apart and 79 units away from the next peak indicates bromine. When analyzing proteins, fragmentation is often undesirable, so "soft" ionization methods such as matrix-assisted laser desorption/ionization (MALDI) or electrospray ionization (ESI) are used. In forensics, mass spectrometry is often "coupled" to gas chromatography in GC-MS; gas chromatography first separates the unknown mixture, then mass spec identifies the individual components of the mixture.

LED

ight-emitting diode, is a diode that lights up when current passes through in the forward direction.

Herman Oberth

imagined at the age of 14 a recoil rocket, which could propel itself by exhausting gas from its base. He also lends his name to the Oberth effect, a phenomenon by which rockets operate more efficiently when moving at higher speeds, and formulated the idea behind multiple-stage rocketry. Oberth also mentored many German rocket engineers through the Verein für Raumschiffahrt (Spaceflight Society), whose formation was inspired partially by Oberth's writing.

Cilia and flagella

important organelles of motility, that is, they allow the cell to move. Flagella are long, whip-like structures, while cilia are short, hair-like projections. Both contain a 9 + 2 arrangement of microtubules in cross-section (two microtubules in the middle, nine pairs in a circle around the outside) and are powered by molecular motors of kinesin and dynein molecules.

Homer Hickam

inspired by seeing Sputnik fly overhead to study rocketry in his early teens. From his coal-company town in West Virginia, he and his rocket club, The Big Creek Missile Agency, designed modest-sized rockets and won the propulsion category of the 1960 National Science Fair. After serving in Vietnam, Hickam worked at the US Army Missile Command in Germany and Huntsville. Later, he moved to NASA, where he specialized in astronaut training. He trained crews for the Hubble Space Telescope's deployment extra-vehicular activity (EVA) and the first two servicing missions.

Apollo 1

intended to be a test of the Command/Service Module in low-Earth orbit (LEO). However, a fire on the launchpad during a test killed the three astronauts aboard (Virgil "Gus" Grissom, Roger Chaffee, and Ed White). The fire was exacerbated by the pure-oxygen, positive-pressure environment inside the capsule, and the fact that the capsule door opened inward. Both of these design elements were scrapped in subsequent missions, and the second was replaced with an outward-opening hatch nominally to facilitate spacewalks. Lessons learned from Apollo 1 were also taken into account during the design of the Space Shuttle.

Konstantin Stanislavski

interest in science manifested first through his science fiction writings, but he gained more notoriety for his eponymous rocket equation. The Tsiolkovsky equation, as published in 1903's Exploration of Outer Space by Means of Rocket Device, relates a rocket's speed with its mass, the speed of its exhaust, and the exhaust's mass (an application of Newton's laws of motion.) The equation is also connected with the idea of specific impulse, a measure of a rocket's efficiency. Tsiolkovsky also published theoretical studies on the capabilities of multi-stage, liquid-fueled engines, gyroscopes, and escape velocities.

Typhon and Echidna

known as the Father and Mother of All Monsters due to their numerous monstrous offspring, including the two-headed dog Orthrus, the Nemean Lion, the Hydra, the Chimera, and Cerberus. Typhon was the last son of Gaea and Tartarus, while Echidna's parentage is obscured by ancient sources; most often, she is listed as a daughter of Phorcys and Ceto. In the Theogony, Hesiod describes a climactic battle between Zeus and Typhon following Zeus' defeat of the Titans: Typhon rips out Zeus' sinews and is nearly victorious, but Hermes restores Zeus's sinews and Zeus finally overpowers the giant monster. Typhon was then trapped under Mount Etna, where he is believed to cause earthquakes and volcanic eruptions.

Gold

known to the ancients as a relatively inert metal. Its atomic symbol comes from its Latin name, aurum. It is resistant to attack by most acids, but it (along with platinum) will dissolve in aqua regia, a mixture of concentrated nitric acid and hydrochloric acid. Among all metals, it has the highest electronegativity and electron affinity; it occasionally is found in a -1 oxidation state as Au-. Widely used in jewelry, it also has a number of scientific uses. Ernest Rutherford's gold foil experiment demonstrated the existence of a positively charged nucleus. Scanning electron microscopy (SEM) often requires that specimens be "sputtered," or thinly coated, with gold atoms to allow imaging. Suspensions of gold compounds have been used to treat rheumatoid arthritis.

Sergei Korolev

known until his death by the pseudonym "Chief Designer", headed design and construction for the Soviet long-range ballistic missile program, as well as the R-7 ICBM program. This work urged similar innovations in the U.S. Earlier in his career, Korolev designed a rocket-powered glider, though he was imprisoned and forced into slave labor during the Great Purge. In the 1950s, he personally oversaw Sputnik and Sputnik 2, and he returned to the R-7, modifying it for lunar insertion of robotic probes. His final years were spent devising soft-landing methods for manned lunar missions.

factors of production

land, labor, and capital (i.e., machinery and tools). Modern economists sometimes add a fourth, entrepreneurship.

Apollo-Soyuz Mission

marked a definite end to the Space Race, and was a symbol of the de-escalation of tensions between the US and the USSR. Deke Slayton, an original Mercury 7 astronaut grounded for medical reasons until 1973, was accompanied by Tom Stafford and Vance Brand on the last launch of Apollo before the advent of the Space Shuttle. The mission demonstrated that two dissimilar spacecraft could rendezvous and dock while in space (an Apollo Command Module and Soyuz 19 docked), and also enabled the crew of Soyuz to photograph the Sun's corona through an artificial eclipse created by the Apollo spacecraft. Each spacecraft also carried out independent experiments.

Gymnosperms

means "naked seed," referring to the fact that their gametophytes develop on the surface of leaves or on the scales of cones.

Angiosperms

means "receptacle seed." Their gametophytes develop enclosed within flowers. Most (but not all) angiosperms fall into one of two classes based on the number of cotyledons, or embryonic seed-leaves, in the plant embryo. Monocots, or Monocotyledonae, have one cotyledon, while dicots, or Dicotyledonae, have two. While there are no other hard-and-fast distinguishing characteristics between the two groups, plants in each category tend to share other characteristics:

Mach number

measure speed. Based on a suggestion by Swiss aeronautical engineer Jakob Ackeret, it was named after Ernst Mach, an Austrian physicist who studied — among other things — the Doppler effect, sensory perception, and the origin of inertia. The Mach number is defined as the ratio of the speed of an object to the speed of sound in the same medium. So an object moving at the speed of sound (which in dry air at 20°C is about 343 m/s) has a speed of Mach 1, and an object moving at twice the speed of sound has a speed of Mach 2.

Richter scale

measures earthquake intensity. Developed by Caltech professor Charles Francis Richter, it measures the shaking intensity associated with earthquakes, as quantified by the amplitude of vibrations on a seismograph. A magnitude-5.0 earthquake will have an amplitude 10 times larger than that of a magnitude 4.0 quake. The energy associated with an earthquake is actually proportional to the 3/2 power of the magnitude: a 1-point difference on the Richter scale corresponds to a 103/2-fold (about 31.6) difference in energy. Because of difficulties in measuring the magnitudes of large earthquakes, the Richter scale has been superseded by the moment magnitude scale, which uses a different formula but retains the logarithmic nature (and correlates to measurements on the Richter scale).

Lysosomes

membrane-bound organelles that contain digestive enzymes that break down proteins, lipids, carbohydrates, and nucleic acids. They are important in processing the contents of vesicles taken in from outside the cell. It is crucial to maintain the integrity of the lysosomal membranes because the enzymes they contain can digest cellular components as well.

Monopoly

occurs when there is only one firm producing goods for a given market. This allows that firm to set the price higher—and thus the quantity sold lower—than would otherwise occur. An oligopoly is a similar market environment where production is dominated by a few firms, whereas a monopsony is the opposite case: a market with only a single consumer.

Rationalism

often contrasted with empiricism, asserts that we gain knowledge through intuition or our rational nature rather than through experience. A version of this doctrine was espoused by the Greek philosopher Plato in his Theory of Forms, which states that abstract ideas ("forms") are more real than the material world of the senses. Later rationalists include René Descartes (who wrote Meditations on First Philosophy) and Baruch Spinoza (who wrote Ethics).

Beaufort Wind Scale

on the voyage of the HMS Beagle in 1831, which was led by Robert FitzRoy, who had been trained by the scale's namesake, Sir Francis Beaufort, a rear admiral in the British Navy. The Beaufort scale is primarily based on wind speed, but also incorporates descriptions of wave height, sea conditions, and land conditions. It starts at 0, corresponding to "calm" winds with a speed less than 1 knot. In most parts of the world, it stops at 12, which is designated "hurricane-force winds." Scores of 2 to 6 are called "breezes," and scores of 7 to 10 are called "gales." Since 1946 the median wind speed for each step has been defined as 1.87 B3/2, where B is the number on the Beaufort scale.

Potomac River

one of America's most historic waterways. Rising at Fairfax Stone in West Virginia, the Potomac runs 405 miles, forming the border between Virginia and Maryland. Washington, D.C. was sited on the Potomac at its confluence with the Anacostia River. George Washington's plantation Mount Vernon was on the Potomac, while Robert E. Lee's two invasions north of the Potomac were major events of the Civil War.

Mercury

one of just two elements that is a liquid at standard temperature and pressure (the only other one is bromine). It has been known since antiquity, and is found in ores such as cinnabar. Older names for it, reflecting its liquid nature, include hydrargyrum (the source of its symbol) and quicksilver. Because it is a very dense liquid, it is commonly used in barometers to measure atmospheric pressure; the pressure exerted by the atmosphere equals the pressure exerted by a column containing 760 millimeters of mercury. Alloys of mercury with other metals are called amalgams, some of which have been used as dental fillings. Chronic exposure to mercury can cause psychological problems; its use in hatmaking led to the expression "mad as a hatter." More recently, concerns about mercury exposure have led to the banning of mercury in thermometers.

Leptons

one of the classes of "fundamental particles" (meaning that they cannot be broken down into smaller particles). There are six "flavors" of leptons: the electron, the muon, the tauon, the electron neutrino (usually just called "the neutrino"), the muon neutrino, and the tauon neutrino. The three neutrinos are neutral and nearly massless (they were once thought to be entirely massless), while the other three have a charge of -1. All neutrinos are fermions, and the total number of leptons is always conserved (counting regular leptons as +1 particle and anti-leptons as -1 particle). The word "lepton" comes from the Greek for "light" (the opposite of "heavy," not illumination), even though the muon and tauon are relatively massive.

Lernaean Hydra

one of the offspring of Typhon and Echidna. The Hydra was a multi-headed water serpent that breathed poisonous gas and had toxic blood, and every time one head was cut off, two more grew back in its place. The Hydra dwelled in the Spring of Amymone in the swamp or lake of Lerna near the Peloponnese, beneath which was said to be an entrance to the Underworld. The Hydra was killed by Heracles as his second labor for Eurystheus during a battle in which Heracles' nephew Iolaus provided aid by cauterizing the neck stumps after Heracles cut each head off, preventing additional heads from growing back. After killing the monster, Heracles dipped his arrows in the Hydra's blood; the poisoned arrows were later used against the Stymphalian Birds, Geryon, and the centaur Nessus.

Fermions

particles with half-integral spin. Spin is a form of "intrinsic angular momentum," possessed by particles as if they were spinning around their axis (but they aren't). The values cited for spin are not (usually) the real magnitude of that angular momentum, but the component of the angular momentum along one axis. Quantum mechanics restricts that component to being n/2 times Planck's constant divided by 2π for some integer n. If n is even, this results in "integral" spin, if it is odd, it results in "half-integral" spin. Note that the exact value of the spin itself is a real number; it's the multiplier of h/2π that determines whether it is "integral" or not. The most significant thing about fermions is that they are subject to the Pauli Exclusion Principle: No two fermions can have the same quantum numbers (i.e., same state). The name "fermion" comes from the name of the Italian-American physicist Enrico Fermi.

Bosons

particles with integral spin. All particles are either bosons or fermions. The spin of a composite particle is determined by the total spin (i.e., the component of its intrinsic angular momentum along one axis) of its particles. For instance, an alpha particle (two protons and two neutrons) has four half-integral spin values. No matter how they are added up, the result will be an integral spin value (try it!), so an alpha particle is a (composite) boson. The Pauli Exclusion Principle does not apply to bosons (in fact, bosons prefer to be in the same quantum state). The name "boson" comes from the name of the Indian physicist Satyendra Nath Bose.

Diode

permits the flow of current in only one direction. Ideally, a diode permits any amount of current in one direction but has a very high resistance to current in the opposite direction. In practice, there is a minimum voltage that must be provided before current will flow, called the bias voltage, and there is a voltage past which the resistance of the diode reduces and current flows backward, called the breakdown voltage. In a Zener diode, the breakdown voltage is well-defined with as sharp a transition as possible, so engineers can design a circuit around the breakdown voltage characteristic.

Quadratics

polynomials of degree 2. The graph of a quadratic equation will be in the shape of a parabola that opens straight up (if the coefficient on the x2 term is positive) or straight down (if that coefficient is negative). It is possible to find the roots of a quadratic by graphing it, factoring it, completing the square on it, or using the quadratic formula (itself derived by completing the square) on it. If the quadratic is in the form ax2 + bx + c, then the expression b2 - 4ac, which appears in the quadratic formula, is called the discriminant. If the discriminant is positive, the quadratic will have two real roots; if the discriminant is zero, the quadratic will have one real root (said to have a multiplicity of 2); and if the discriminant is negative, the quadratic will have two non-real complex roots (and if the coefficients of the quadratic are real numbers, the complex roots will be conjugates of each other).

Ultraviolet-Visible Spectrophotometry (UV-Vis)

quantifies the presence of compounds by shining light in the ultraviolet-to-visible range on a molecule, then measuring which wavelengths are absorbed. Parts of a molecule which absorb visible light are referred to as chromophores; since different molecules have different chromophores, the wavelength of maximum absorbance, or lambda-max, can be used to identify a compound. Beer's law is then used to quantify the concentration of the present compound. Samples are held in small square plastic or quartz tubes called cuvettes. The Woodward-Fieser rules empirically estimate the lambda-max from the types of bonds and functional groups in a molecule. UV-Vis has many uses in biology—for example: calculating the OD600 to measure bacterial growth rate, determining nucleic acid quality with the 260/280 ratio, measuring protein concentration in the Lowry and Bradford protein assays, and monitoring protein folding by measuring tryptophan or tyrosine absorbance.

Inflation

rate is the pace at which prices are rising, usually expressed in the form "2.5% per year." This is equivalent to the rate at which currency loses its value. Inflation is often measured by the consumer price index, which establishes a standard basket of goods that a family might buy and seeing how its total price changes over a year.

unemployment

rate refers to the percentage of the population that is actively seeking work but cannot find a job. Unemployment can be cyclical (common in seasonal industries like fieldwork), frictional (the natural time between jobs that exists in most labor markets), or structural (when workers' skills do not match those required by open jobs).

Hubble Space Telescope Servicing Missions SM2

replaced the Goddard High-Resolution Spectrograph (GHRS) and Faint Object Spectrograph (FOS) with improved successors.

Trigonometric Functions

represent relations between angles and sides of triangles. They are often illustrated using points and segments related to a circle of radius 1 centered at the origin, called the unit circle. By far the most commonly discussed trigonometric functions are the sine, cosine, tangent, cosecant, secant, and cotangent functions. There are many interesting relationships between these: the graphs of sine and cosine are translations of each other; the tangent function equals the sine function divided by the cosine function; the cosecant, secant, and cotangent functions are the reciprocals of the sine, cosine, and tangent functions, respectively; and there are many other relationships called trigonometric identities.

The jordan river

rises in Syria from springs near Mount Hermon. It flows south to Lake Merom, through the Sea of Galilee, and into the Dead Sea, which is 1,300 feet below sea level. The river forms the nation of Jordan's boundary with the West Bank and northern Israel. In the New Testament, the river was the site of the baptism of Jesus by John the Baptist. In modern times, about 80% of its water is diverted for human use, a figure that has led to the shrinking of the Dead Sea and serious contention among bordering nations.

Brahmaputra River

runs 1,800 miles from its source in the Tibetan Himalayas; it starts eastward across the plateau, then turns south into the Indian state of Assam, and then enters Bangladesh, where it merges with the Ganges to form the world's largest delta. While serving as a historical route to Tibet, the river is also prone to disastrous flooding.

Odd function

satisfy the rule f(-x) = -f(x) for every x in the domain of the function. The graph of an odd function remains the same when it is rotated 180° around the origin. Odd functions are so named because if a polynomial's exponents (on the variable) are all odd, then the polynomial is an odd function; for instance, x3, 4x7, and -x5 + 2x3 are all odd. There are other odd functions, such as the sine and cube root functions.

Even functions

satisfy the rule f(-x) = f(x) for every x in the domain of the function. The graph of an even function has reflection symmetry over the y-axis. Even functions are so named because if a polynomial's exponents (on the variable) are all even, then the polynomial is an even function; for instance, x2, 3x6, and -x8 + 7x4 are all even. There are other even functions, though, such as the cosine and absolute value functions.

Apollo 11

saw the first Moon landing and moonwalk by astronauts Neil Armstrong and Buzz Aldrin (astronaut Michael Collins piloted the Command Module in lunar orbit and never walked on the moon). After the Lunar Module landed in the Sea of Tranquility, Armstrong said, "Houston, Tranquility Base here. The Eagle has landed." Later stepping onto the lunar surface, he said, "That's one small step for [a] man, one giant leap for mankind." According to Chris Kraft, NASA officials chose Armstrong as the first to walk on the Moon because he was more humble than Aldrin, and because he was the Commander. However, the stated reason was that Armstrong's seat was closer to the door. Apollo 11, like other Apollo missions, launched atop a Saturn V booster.

Apollo 13

supposed to land in the Fra Mauro region of the Moon. An improperly-refurbished No. 2 oxygen tank and subsequent improper repairs caused the tank to rupture during a routine "cryo-stir" before entering lunar orbit. The explosion also damaged the No. 1 oxygen tank and caused further leakage. To bring back the astronauts, the orbiter was put on a free-return trajectory around the Moon. NASA engineers also solved power-management, water-conservation, and trajectory planning problems with the help of Ken Mattingly, the primary Command Module pilot who had been grounded due to exposure to German measles. All three astronauts returned to Earth safely, and the landing site was re-assigned to the subsequent Apollo 14 mission.

Chromatography

separates a complex mixture into its individual components, commonly illustrated by the separation of pen ink into many colors. Chromatography involves two components: a mobile phase, which moves, and a stationary phase, which interacts differently with different components of the mobile phase to produce a separation. For example, in thin-layer chromatography, a mixture is spotted on one end of a plate of silica gel (the stationary phase), then a solvent (the mobile phase) carries the components across the plate and separates them based on their polarity—polar substances will strongly interact with the polar silica gel and not move very far, while nonpolar substances will move very far. In gas chromatography (GC), substances are vaporized and run through a packed column, where the time it takes each component to travel through—the retention time—is determined. High-performance liquid chromatography (HPLC) is like gas chromatography, but the sample remains in the liquid phase and is pushed through using pressures of up to 400 atmospheres. Ion-exchange chromatography uses stationary phases with acidic or basic functional groups to remove charged compounds; it can be used for water softening.

Distillation

separates a mixture of liquids based on their boiling point by heating, causing the more volatile component to vaporize and condense in a different container while the other components remain in the original vessel. In the laboratory, the vapors are usually cooled with a water-based Liebig condenser or Vigreux condenser, and the product is collected in a round-bottomed receiving flask. In industry, multiple rounds of distillation are performed in a single "column" packed with trays, each of which can be modeled as a "theoretical plate." Oil refining relies on fractional distillation, in which different products are pulled out of the mixture at various trays along the column. Azeotropes are mixtures that cannot be separated because at the specified pressure and composition, both components boil at the same temperature.

liquid-liquid extraction

separates mixtures based on their relative solubilities in two immiscible solvents, such as oil and water. It is commonly performed by placing the mixture and solvents into a separatory funnel, shaking, then using the stopcock to remove one of the two phases. The partition coefficient quantifies the desired compound's relative solubility in the two phases. The material left over after the desired material has been extracted is called the raffinate. A variant of liquid-liquid extraction that uses phenol and chloroform as the two solvents is used to isolate DNA from cells.

Bryophytes

small enough that water and nutrients can diffuse to all parts of the plant without any specialized vascular tissue. They lack true leaves and roots, instead fastening themselves to the ground with rhizoids. Unlike other land plants, bryophytes have a prominent gametophyte stage that is usually dioicous, meaning that an individual plant produces only one type of gamete (either sperm or egg). The short-lived sporophyte grows from the female gametophyte.

Petals

specialized leaves, often brightly colored to attract pollinating species. Collectively, they are called a corolla.

Mount Rainier

stratovolcano that is the highest peak in the Cascades and in the state of Washington. It is located about 50 miles southeast of Seattle. The three principal peaks of Mount Rainier are Columbia Crest (the tallest), Point Success, and Liberty Cap; the Little Tahoma Peak is a remnant of an older, much larger Mount Rainier that has since eroded away. Mount Rainier has not erupted since the mid-to-late nineteenth century. The Nisqually Glacier, on Mount Rainier's southwestern slope, is used to track climate change. The Emmons Glacier on Mount Rainier is the largest glacier by area in the contiguous U.S., while the Carbon Glacier is the largest by volume. The entirety of Mount Rainier is in Mount Rainier National Park, the fifth national park established in the U.S.

Scholasticism

taught at medieval Christian universities and sought to reconcile Christian thought with classical thinkers such as Aristotle. This school included Thomas Aquinas, who provided five arguments for the existence of God (called the "quinque viae") in his Summa Theologica. Other thinkers include Pierre Abelard (who wrote Sic et Non) and Peter Lombard (who wrote The Four Books of Sentences).

Dicots

tend to have pollen with three furrows (tricolpate); flower parts in multiples of four or greater; taproot systems; stems with rings of vascular tissue; and branching leaf veins. Many of them exhibit secondary growth that produces wood.

Nucleus

the "command central" of the cell because it contains almost all of the cell's DNA, which encodes the information needed to make all the proteins that the cell uses. The DNA appears as chromatin through most of the cell cycle, but condenses to form chromosomes when the cell is undergoing mitosis. Within the nucleus there are dense bodies called nucleoli, which contain ribosomal RNA. In eukaryotes, the nucleus is surrounded by a selectively-permeable nuclear envelope.

Irrawaddy River

the chief river of Myanmar (also known as Burma). It flows 1,350 miles past Yangon (formerly Rangoon) and Mandalay to the Gulf of Martaban, an arm of the Bay of Bengal. Its delta is one of the world's most important rice-growing regions, and its name is thought to come from the Sanskrit word for "elephant."

The indus River

the chief river of Pakistan and the source of the name of India. It rises in Tibet and flows 1,800 miles to a delta on the Arabian Sea southeast of Karachi. The five major tributaries of the Indus — the Jhelum, Chenab, Ravi, Beas, and Sutlej Rivers — are the source of the name of the Punjab region, which is Persian for "land of the five rivers." The Indus is the cradle of the Indus Valley Civilization, one of the world's earliest urban areas, whose main cities were Mohenjo-Daro and Harappa.

Mekong

the chief river of Southeast Asia. It originates in eastern Tibet, forms much of the Laos-Thailand border, flows south through Cambodia, and enters the South China Sea in southern Vietnam just south of Ho Chi Minh City. The capital cities Vientiane and Phnom Penh are on the Mekong. The building of dams and clearing of rapids are a source of diplomatic conflict among China, Laos, and Cambodia.

Danae

the daughter of King Acrisius of Argos. The king consulted an oracle to ask if he would ever have a son, and the oracle replied that he wouldn't, but his daughter would, and that grandson would overthrow him. In response, he shut Danaë up in a chamber in his palace, but Zeus appeared in the form of a shower of gold and impregnated her; shortly thereafter, her son Perseus was born. Acrisius then cast Danaë and Perseus out to sea in a chest, but with the assistance of Poseidon they were rescued by the fisherman Dictys. Danaë had no interest in marrying Dictys's brother King Seriphos, who agreed not to pursue her if Perseus could kill the Gorgon Medusa.

Ariadne

the daughter of King Minos and Queen Pasiphaë, making her a princess of Crete. Minos's palace in Knossos is sometimes referred to as the "dancing ground of Ariadne." When Theseus came to Crete to kill the Minotaur, Ariadne fell in love with him and gave him a ball of twine to help him navigate the Labyrinth. Ariadne and Theseus left Crete together, but he later abandoned her on the island of Naxos where she was saved by Dionysus, whom she married. Her wedding diadem was placed in the heavens as the constellation Corona Borealis.

Queen Guinevere

the daughter of Leodegrance and the wife of King Arthur. In one story, Guinevere is abducted by Meleagant (or Melwas), a king of Somerset, and rescued by Lancelot, beginning an illicit affair between the two. After the affair is revealed to Arthur (in some sources by Mordred, in others by Agravain), Arthur orders her to be burned at the stake; she is rescued from that fate by Lancelot in a battle that results in the deaths of Sir Gareth and Sir Gaheris and the permanent exile of Lancelot. Some sources say that Guinevere spent her final days hiding in the Tower of London or in a nearby convent.

Tigris

the eastern of the two rivers that define the historic region Mesopotamia (meaning "the Land Between Two Rivers"), which was home to the ancient civilizations Sumer and Akkad. It rises in Turkey, then flows southeast by Mosul, Tikrit, and Baghdad before joining the Euphrates to make the Shatt-al-Arab, which empties into the Persian Gulf.

Penelope

the ever-faithful wife of Odysseus, the king of Ithaca whose long journey home is the subject of The Odyssey. While Odysseus was away, Penelope was courted by dozens of suitors led by Antinous, but she held them off with cunning: she told them she'd wed one when she finished weaving a burial shroud for Odysseus's father Laertes, but she unwove her progress each night. When Odysseus returned, she tested his identity by asking the maid to move their bed, which cannot be done since the bed—crafted by Odysseus—had one post that was a living olive tree. Her only child is a son, Telemachus.

SpaceX CRS-1

the first commercial mission to resupply the International Space Station (ISS). A structural failure in one of the nine Merlin engines that make up the Falcon 9 rocket first stage necessitated a longer burn with the remaining eight engines. That correction resulted in a proper orbital insertion for the primary payload (a Dragon resupply vehicle), but an unstable, decaying orbit for its secondary payload (an ORBCOMM satellite). This was taken as a proof of concept by SpaceX for the redundant, multiple-engine design. The craft successfully berthed with the ISS and successfully carried out its primary mission, re-supplying the space station and returning cargo to Earth.

Hydrogen

the first element on the periodic table and, by far, the most common element in the Universe. In addition to the main isotope (also called protium), there are two other significant isotopes of hydrogen: deuterium (2H or D), which has one neutron, and tritium (3H or T), which has two neutrons. It naturally exists as a diatomic gas (H2), which was discovered by the British chemist Henry Cavendish. Hydrogen is highly flammable when exposed to high temperatures or electric current, a fact demonstrated by the Hindenburg disaster. It can react with nonmetals by losing an electron to form the H+ ion, or react with metals to form the hydride ion, H-.

comparative advantage

the idea that every pair of potential trading partners (two firms, two neighbors, two countries, etc.) can benefit by trade if they are producing at least two goods. The counterintuitive aspect is that this result is true for every pair, no matter how unproductive one of the parties might be (in absolute terms). Broadly speaking, each party should specialize in what it does best (relative to the other party) and then trade for everything else. David Ricardo formulated this theory with a famous example involving Britain's cloth industry and Portugal's wine industry.

Pandora

the first human woman in Greek mythology. Hephaestus sculpted her from clay as a punishment for humanity after Prometheus stole fire from the gods. The primary myth of Pandora relates how she released all the evils of the world by opening a jar ("Pandora's Box"), and when she closed the lid only hope remained within. Pandora married the Titan Epimetheus, and their daughter Pyrrha survived the Greek flood with her husband Deucalion.

Sir Lancelot

the foremost among the Knights of the Round Table, an expert swordsman and jouster who is the primary figure of the Vulgate Cycle. The son of King Ban of Benwick, Lancelot was raised by the Lady of the Lake, which earned him the epithet "du Lac" or "of the Lake." Another of his epithets is "Knight of the Cart," which he earned for riding in a dwarf's cart while searching for Guinevere after she was kidnapped. Aside from his adulterous affair with Queen Guinevere, Lancelot is known for fathering Sir Galahad with Elaine of Corbenic, who had tricked Lancelot into sleeping with her by disguising herself as Guinevere. After his betrayal of Arthur was revealed, Lancelot fled to France and was therefore not present during the Battle of Camlann.

Gluons

the gauge bosons that carry the strong nuclear force and bind hadrons together. Gluons have no charge and no mass, but do have color (in the sense of quarks). This color cannot be observed directly because the gluons are part of the larger hadron. The name comes from their role in "gluing" quarks together.

The Ganges river

the holiest river of Hinduism. It rises in the Himalayas and flows a comparatively short 1,560 miles to the world's largest delta, on the Bay of Bengal. Among that delta's distributaries are the Hooghly (on whose banks Kolkata — formerly Calcutta — may be found) and the Padma (which enters Bangladesh). Approximately one in every twelve human beings lives in the Ganges Basin, a population density that is rapidly polluting the river; a significant source of that pollution is cremated remains.

Io

the innermost of the four Galilean moons of Jupiter (the moons discovered by Galileo), the fourth-largest moon in the solar system, the densest moon, and the most geologically active body in the solar system, with more than 400 volcanoes. Io's features are named for characters from the Io story in Greek mythology; fire, volcano, and thunder deities from other mythologies; and characters from Dante's Inferno. Io plays a significant role in shaping Jupiter's magnetosphere. Pioneer 10 first passed by Io in December 1973.

Inverse Trigonometric Functions

the inverse functions of the trigonometric functions. (Note that "inverse" in this case refers to a function that "undoes" another, not to "multiplicative inverse," also called "reciprocal.") They are sometimes just called "inverse sine," etc, and are also given with the prefix "arc": arcsine, arccosine, arctangent, arccosecant, arcsecant, and arccotangent. They are sometimes notated in the form sin-1 for arcsine, but that notation can be confusing, so NAQT tends to prefer "arcsine," etc. Because the trigonometric functions are not bijective (see below), to have inverses it is necessary to restrict the domains of the inverse trigonometric functions; for instance, arcsin(x) is only defined for x between -1 and 1, inclusive.

Ganymede

the largest moon in the solar system and the only one known to have its own magnetosphere. The third of the Galilean satellites, Ganymede was also first photographed close-up by Pioneer 10 in 1973. Galileo made six flybys of Ganymede between 1996 and 2000. Based on a suggestion from Simon Marius, Ganymede (along with many of the Jovian satellites) is named for one of Jupiter's lovers in Roman mythology; Ganymede is the only such moon named for a male figure. Many of Ganymede's features, including the Enki Catena, are given names from Egyptian and Babylonian mythology, although its largest dark plain is Galileo Regio. Ganymede is scheduled to be orbited by the European Space Agency's Jupiter Icy Moon Explorer (JUICE), currently slated for a 2022 launch.

Titan

the largest moon of Saturn and the second largest in the solar system. Until Voyager 1 visited in 1980, it was thought to be larger than Ganymede. It is the only known satellite with a dense atmosphere — so dense that it makes observation of surface features nearly impossible except from close up — and also the only known satellite for which there is evidence of stable bodies of surface liquid. Discovered in 1655 by Christiaan Huygens, it was visited by the Cassini-Huygens mission in 2004. Titan's albedo features, such as the highly reflective area Xanadu, are named for sacred or enchanted places from world literature and mythology. Because of its nitrogen-rich atmosphere and the presence of surface liquid, Titan is often thought to be the most likely place in the solar system for microbial life to exist outside of Earth.

Helium

the lightest noble gas and the second most abundant element in the Universe (after hydrogen). Discovered by Sir William Ramsey, Pierre Janssen, and Norman Lockyer, it has two stable isotopes, helium-3 and helium-4, with helium-4 by far the more common. Because of their different quantum properties (the helium-3 nucleus is a fermion, while the helium-4 nucleus is a boson), the isotopes of helium actually have significantly different physical properties. Helium-4 can exist in a zero-viscosity state known as superfluidity when its temperature drops below the lambda point. Helium has the lowest boiling point of any element; liquid helium is used for devices that need intense cooling, such as MRI machines. Most helium on Earth results from radioactive decay, since the helium nucleus is equivalent to an alpha particle.

Mississippi River

the longest or second-longest in North America, depending on how you count. (The Missouri River could be considered longer.) Referred to by Abraham Lincoln as "the father of waters," the Mississippi begins at Lake Itasca, Minnesota, and flows 2,340 miles to a vast delta on the Gulf of Mexico, forming portions of ten state borders and the world's third-largest drainage basin. The Mississippi picks up numerous major tributaries including the Illinois, Missouri, Ohio, Arkansas, and Red Rivers, and flows past numerous major cities, including Minneapolis, St. Louis, Memphis, and New Orleans.

The yangtze

the longest river in China and Asia, and the third longest in the world. It rises in the Kunlun Mountains, flows across the Tibetan Plateau, passes the cities of Chongqing, Wuhan, Nanjing, and Shanghai, and empties into the South China Sea. Its basin is China's granary and is home to nearly a third of Chinese citizens. The river is dammed by the Three Gorges Dam, the world's largest, which reduced flooding but displaced 1.5 million people and buried over 1,300 known archaeological sites.

Mackenzie River

the longest river of Canada. Flowing 1,080 miles out of the Great Slave Lake, the river flows past Fort Providence and Fort Simpson in Canada's Northwest Territories, emptying into a vast delta on the Beaufort Sea. The Mackenzie is the largest river flowing into the Arctic Ocean from North Am

Ribosomes

the machines that coordinate protein synthesis, or translation. They consist of several RNA and protein molecules arranged into two subunits. Ribosomes read the messenger RNA copy of the DNA and assemble the appropriate amino acids into protein chains.

invisible hand

the metaphorical notion that producers and consumers acting in their own narrow interest (e.g., by trying to maximize income from their business) will create an overall benefit to society. This might happen by their rewarding technologies that make production more efficient, rewarding firms who can sell at lower cost, rewarding inventors who discover new processes, and so on. The phrase was introduced by Adam Smith in his 1776 book The Wealth of Nations, but the modern understanding (and broad-based application of the metaphor) is due to later thinkers.

Nitrogen

the most abundant element in Earth's atmosphere. Nitrogen, which was first isolated as "noxious air" by Daniel Rutherford, exists primarily as a diatomic molecule containing two triple-bonded nitrogen atoms (N2). Because nitrogen gas is extremely stable, N2 is unusable for many biological and chemical purposes. To make it useful, it often undergoes fixation to convert it into usable nitrogen species such as the ammonium ion (NH4+) — as it is by bacteria in the root nodules of legume plants—or ammonia gas (NH3), as is done industrially in the Haber-Bosch process. Conversely, its stability makes it useful in preventing unwanted combustion reactions. It also has a relatively low boiling point (-196°C), which makes liquid nitrogen useful as a refrigerant.

Basalt

the most common igneous rock—over 90% of volcanic rocks on Earth are basalt! Basalt is mafic, meaning it is rich in magnesium and iron, and extrusive, meaning it cools and solidifies on the Earth's surface. Lava containing gas bubbles may cool into vesicular basalt. Basalt primarily composed of vesicles is known as scoria.

Aluminum

the most common metal in Earth's crust, and the first metal in the p block of elements. First isolated by Hans Christian Ørsted, its primary ore is bauxite, from which it is refined using large amounts of electric current, via electrolysis, through the Bayer and Hall-Héroult processes. (Because aluminum exists only in a +3 oxidation state, it takes three moles of electrons to produce one mole of aluminum; as a result, it has been estimated that 5% of all electricity in the U.S. goes to purifying aluminum.) It is found in the mineral corundum, which is found in many gems, including sapphires and rubies; the specific impurities found in a gem determine its color. It is also found in aluminosilicates, such as feldspar.

Iron

the most common metal in the Earth, and one of the major components of the Earth's core. Iron was known to the ancients; its atomic symbol comes from the Latin name ferrum. Iron is the namesake of ferromagnetism; one of its ores is magnetite, Fe3O4, which contains iron in both of its most common oxidation states, 2+ and 3+. Iron(II) sulfide, FeS2, is formally known as pyrite, but because of its appearance has long been known as fool's gold. Iron can react with oxygen in the air to form iron(III) oxide, Fe2O3, in a relatively slow but exothermic process; this process is used in "all-day" heat patches. Hydrated iron(III) oxide is better known as rust; rust only forms when iron is exposed to both oxygen and water. Its isotope 56 is one of the most strongly bound of all nuclei, with the lowest mass per nucleon. Iron is one of the heaviest elements that is normally produced by stellar nucleosynthesis. The largest use of iron is in steel.

Polyphemus

the most famous Cyclops in Greek mythology, is the son of Poseidon and the sea nymph Thoosa. The most notable myth involving Polyphemus is his appearance in Book IX of Homer's Odyssey: after Odysseus and his crew land on Polyphemus' island after escaping the Lotus-Eaters, Polyphemus eats two of Odysseus' crew, imprisons the rest in his cave, and eats four more before the survivors can escape. To escape, Odysseus gets Polyphemus drunk on wine and blinds the one-eyed giant with a stick; the next morning, Odysseus and his crew ride out of Polyphemus' cave, hiding underneath the Cyclops' sheep. When Polyphemus asks Odysseus' name, Odysseus responds "No one" or "No man" (translations vary), and Polyphemus prays to his father Poseidon to make Odysseus' journey home treacherous. In another myth, Polyphemus falls in love with the nymph Galatea, who in turn loves the human Acis. Polyphemus then kills Acis with a boulder out of jealousy.

Colorado river

the most significant river of the southwestern United States. Beginning in Rocky Mountain National Park in Colorado, the Colorado River runs southwest for 1,450 miles to the Gulf of California in northwestern Mexico. The Colorado formed numerous canyons along much of its length, most notably the Grand Canyon in Arizona. The Colorado also has significant dams, including Hoover Dam near Las Vegas (forming Lake Mead) and Glen Canyon Dam in Arizona (forming Lake Powell).

Medusa

the only mortal member of the Gorgons, a trio of monstrous daughters of Phorcys and Ceto who had brass hands, fangs, and venomous snakes for hair; the other two were Stheno and Euryale. Many early sources state that Medusa was born a monster, though Ovid's Metamorphoses state that Medusa was a beautiful woman until she was raped by Poseidon in Athena's temple and cursed by the goddess. Gazing directly into Medusa's eyes resulted in the onlooker being turned into stone. She was beheaded by the hero Perseus, who was sent to retrieve her head by the tyrant Polydectes, whom Perseus then killed with the head. Perseus gave the head of Medusa to Athena, who placed it on her shield, the aegis. When Medusa was beheaded, the winged horse Pegasus and the giant warrior Chrysaor emerged, her sons by Poseidon. According to Ovid, Medusa's head was also used to petrify the Titan Atlas.

Voskhod 1 and Voskhod 2

the only two missions of the Voskhod program, and were superseded quickly by the Soyuz program. The spacecrafts made use of Sergei Korolev's designs. Korolev himself was preoccupied with the Moon race, again in a position of favor after the fall of Khrushchev (though Korolev died before the designs were used). Voskhod 1 was the first flight to contain multiple astronauts, and Voskhod 2 was the platform for the first EVA (extra-vehicular activity or "spacewalk"). Alexey Leonov, who conducted the spacewalk on Voskhod 2, also participated in the later Apollo-Soyuz Test Project.

Tagus

the principal river of the Iberian Peninsula. Rising in east-central Spain, it flows west for roughly 645 miles to the Atlantic, passing through Lisbon, Portugal on the way. The cities of Toledo and Santarém are on the Tagus, and hydroelectric dams on the river produce huge artificial lakes, including the Sea of Castile.

Paris(Alexander)

the son of Priam and Hecuba, he is destined to be the ruin of his country. He fulfills this destiny by accepting a bribe when asked to judge which of three goddesses is the fairest. When he awards Aphrodite the golden apple, Aphrodite repays him by granting him the most beautiful woman in the world; unfortunately, Helen is already married to Menelaus. Known less for hand-to-hand fighting than for mastery of his bow, he kills Achilles with an arrow but dies by the poisoned arrows of Philoctetes.

Cerburus

the three-headed (or, according to Hesiod's Theogony, 50-headed) dog who guarded the gates to the Underworld. A child of Typhon and Echidna, Cerberus is described as a hellhound with a mane of snakes, the claws of a lion, and the tail of a deadly snake. As Heracles' twelfth and final labor, he had to bring Cerberus back from the Underworld, which he did following an intense wrestling match. Prior to the task, Heracles was instructed in the Eleusinian Mysteries, and freed Theseus from being stuck on a chair in Hades. In Virgil's Aeneid, the Cumaean Sibyl gives Cerberus three drugged honeycakes so that she and Aeneas can enter the Underworld.

Empiricism

the view that all knowledge derives from sensory experience. John Locke, whose works include An Essay Concerning Human Understanding, thought that the mind starts out as a tabula rasa — (blank slate) and that we gain knowledge through experiences. Other empiricist philosophers include George Berkeley (who wrote A Treatise Concerning the Principles of Human Knowledge) and David Hume (who wrote An Enquiry Concerning Human Understanding).

Clytemnestra

the wife of King Agamemnon and the sister of Castor, Polydeuces, and Helen. When Agamemnon's troops were stuck at Aulis en route to Troy, he sacrificed their daughter Iphigenia, which made Clytemnestra very angry. She later began an affair with Aegisthus, Agamemnon's cousin; when Agamemnon returned to Mycenae after the Trojan War, Clytemnestra and Aegisthus murdered him and his concubine Cassandra. In response, Clytemnestra and Agamemnon's son Orestes killed both Clytemnestra and Aegisthus, continuing the curse of the House of Atreus.

Hecuba

the wife of King Priam, and therefore the Queen of Troy, during the Trojan War. Her children included Paris—who kidnapped Helen—and Hector, the great Trojan warrior who was slain by Achilles. In Book VI of the Iliad, she leads the Trojan woman in prayer to Zeus on behalf of the Trojan warriors. After the war, Hecuba was grief-stricken upon learning of the death of her youngest daughter Polyxena, and she was given to Odysseus as a slave.

Exponential functions

those of the form f(x) = bx, where b (called the base) is a positive number other than 1. Exponential functions are used to model unrestricted growth (such as compound interest, and animal populations with unlimited food and no predators) and decay (such as radioactive decay). The phrase "the exponential function" refers to the function f(x) = ex, where e is a specific irrational number called Euler's number, about equal to 2.718. Exponential functions have the interesting property that their derivatives are proportional to themselves.

Periodic functions

those whose graph repeats a pattern (specifically, the graph has translational symmetry). Technically speaking, a function of one variable f is periodic if f(x+p) = f(x) for every x in the domain of the function and some positive number p, which is called the period, because the graph repeats itself every p units. While the trigonometric functions are the periodic functions most commonly encountered by high school math students, some other functions like triangle waves are also periodic; in general, functions representing waves tend to be periodic. A Fourier fur-ee-ay series is a way to rewrite (almost) any periodic function in terms of only sine and cosine functions.

Vostok 1 and Vostok 6

transported the first human and the first woman into space (respectively, Yuri Gagarin and Valentina Tereshkova). Gagarin's April 12, 1961 flight is still celebrated as Yuri's Night. During Vostok 1, Gagarin completed a single orbit around Earth before re-entering and parachuting out of his capsule. Vostok 6 was largely uneventful, though Tereshkova did note minor physical pains and also could not reach the scientific experiments aboard. Both Vostok 1 and 6 used Vostok-K boosters.

Xylem

transports water and soluble nutrients from the roots to the leaves.contains distinct elongated cells called tracheids that have lignified cell walls and help provide structural support. Vessel elements are also reinforced by lignin, but they are open at each end at perforation plates and connect to form long tubes for water transport. Xylem functions via transpirational pull and osmosis.

STS-51-L and STS-107 (the Challenger and Columbia accidents respectively)

two Space Shuttle (or Space Transportation System) launches known mostly for their accidents and subsequent losses of life. The Challenger disaster was attributed by the Rogers Commission (the investigative body set up after the accident) to poor performance of the solid rocket booster (SRB) O-rings. The O-rings lost integrity and became brittle at low temperatures, such as those present on the morning of the launch. The failure of the O-rings caused "blow-by," where hot gasses escaped the booster joint, ultimately resulting in the destruction of Challenger. The Rogers Commission also cited both NASA and SRB contractor Morton Thiokol for a failure to redesign the SRB joint, known to be dangerous—manifestations of "go fever." According to the Columbia Accident Investigation Board, the disaster on STS-107 was due to a piece of foam from the external fuel tank hitting and breaching the left wing of the Orbiter during launch. The breach damaged the heat shielding, allowing hot gas to enter the Orbiter during re-entry. Ultimately, that damage caused the vehicle to disintegrate over Texas. Both missions resulted in the complete loss of crew (seven crew were on each mission) and orbiters. Both modes of failure had been previously observed in earlier missions, but went un-fixed because those missions were successful.

Nuclear Magnetic Resonance (NMR)

uses magnetic fields to determine the arrangement of nuclei in a molecule. Typical NMR methods only work on nuclei that have nonzero spin, such as 1H and 13C—the nonzero spin means that the nuclei oscillate between two spin states in a phenomenon called Larmor precession. NMR measures the frequency at which each nucleus oscillates, whose deviation from a reference nucleus (such as tetramethylsilane) depends on the local electron density and is called the chemical shift. Nuclei that have more electron density (due to proximity to electron-donating groups like alkyl groups) are said to be "shielded" and have lower chemical shifts, while those that have less electron density (due to proximity to electron-withdrawing groups like halogens) are said to be "deshielded" and have higher chemical shifts. Peaks in NMR can be "split" into several peaks due to J-coupling if they are adjacent to identical nuclei, like the three hydrogens of a methyl group. NMR is the theoretical basis for magnetic resonance imaging (MRI) in medicine.

Wernher von Braun

was at one time a student of Oberth in the Spaceflight Society. Von Braun is best-known for leading Nazi Germany's development and construction of the V-2 rocket (literally, "vengeance weapon"), which wreaked destruction on southern England during World War II. After World War II, he was recruited to the US as part of Operation Paperclip, and proposed to launch a space station carrying a nuclear arsenal (though he hedged the concept as "particularly dreadful"). He presided over Mercury-Redstone development as Director of NASA's newly-opened Marshall Spaceflight Center, which produced the rockets used in the Mercury spaceflights, and the Saturn V rocket used to launch the Apollo spacecraft.

Hubble Space Telescope Servicing Missions SM4

was originally cancelled after the Columbia Disaster by then-NASA Administrator Sean O'Keefe, but was revived by Michael Griffin, who took over the post in 2005. SM4 installed WFC3 (made from some parts of the original WFPC) and the Cosmic Origins Spectrograph (COS), and conducted other repairs. No further repairs are planned, as much of HST's functionality will be replicated with the James Webb Space Telescope, scheduled for launch in 2021.

Sulfur

widely known in the ancient world, and is referred to in the Bible as brimstone. Its nature as an element was first recognized by Antoine Lavoisier. Its most stable allotrope is an eight-membered ring that exists as a yellow solid. It is most often isolated by injecting superheated steam into the ground in the Frasch process. As an element, it is used in the vulcanization process to cross-link the polymer strands of rubber to increase strength; similarly, sulfur-sulfur bonds hold many proteins together. Industrially, though, the majority of sulfur is used to make sulfuric acid, H2SO4 (in fact, sulfuric acid is the most widely produced chemical in the chemical industry). Sulfur compounds are noted for their strong and unpleasant odors; small quantities of hydrogen sulfide, H2S, are frequently added to natural gas — which is normally odorless — to help people notice gas leaks.

James Watson and Francis Crick Nobel

won in 1962 for discovering the molecular structure of DNA. They published the well-known double-helix structure based on "Photo 51," an X-ray diffraction image of DNA produced by Rosalind Franklin. Because Franklin died in 1958 (the Nobel committee does not award prizes posthumously), Francis and Crick instead shared the prize with Maurice Wilkins, a colleague of Franklin who provided them with the diffraction data. The pair also determined, with the help of Chargaff's rules, that the base pairs of adenine-thymine and cytosine-guanine were structurally similar within the DNA molecule.