Chapters 1-2

What is a planet, a solar system, the Milky Way, a galaxy, the universe?

- A planet is a big object that circles the sun and mostly glows by reflecting light from the sun. - A solar system consists of the Sun, planets and their moons, and other small objects. - The Milky Way is a huge collection of stars, dust and gas. - A galaxy is a large cluster of stars in space that contains trillions of stars orbiting a corresponding center and are close together by the cause of gravity. - The Universe is the full amount of all matter and energy.

Can you define the Astronomical Unit? A Light-Year?

- An Astronomical Unit is the distance from the center of the Sun to the center of the Earth. - A Light-Year is the distance that light travels during one year.

What do we mean by the observable universe? Is it the same thing as the entire universe?

- Given the age of the entire universe and the speed at which light travels, the observable universe is the fraction of the universe that we can see. In other words, the entire universe cannot be seen by the human man. The only part in which people can see is the observable universe (with technology). - The observable universe is not the same thing as the universe. The universe is the total of all matter and energy consisting of planets, stars, moons, and galaxies. The observable universe is a small piece of the entire universe which people can see from Earth.

Define hypothesis and theory?

- When they are first proposed, new models or ideas are sometimes called hypotheses. A hypothesis makes predictions. - experiments check the predictions and if the predictions are validated, it is then a theory. A theory is a work in progress.

How many arcseconds are in a full circle?

1' = 60" arcseconds 360o = 21,600' = 21,600' x 60" = 1,296,000" arcseconds There are 1,296,000 arcseconds in a full circle.

How many arcminutes are in a full circle?

1o = 60' arcminutes 360o = 360o x 60' = 21,600' arcminutes There are 21,600' arcminutes in a full circle

Define the Celestial Equator?

A great circle on the celestial sphere 90° from the celestial poles; where the celestial sphere intersects the plane of Earth's equator

Define Altitude

A star will rise in the east and set in the west and at any given time it will have some height about the horizon which corresponds to the angle between the star and the horizon. When the star is directly overhead at zenith, that angle is 90 degrees. This angle is called altitude.

Define the Celestial Poles?

An imaginary stick through Earth's North and South Poles, representing our planet's axis. It is also points about which the celestial sphere appears to rotate; intersections of the celestial sphere with Earth's polar axis

Define Zenith

An observer's zenith is the point directly overhead.

Describe the developments in astronomy through history.

Ancient astronomers believed that planets today, were "wandering stars" and that the stars were "fixed". This means that they thought they didn't move; however, the planets and stars do move. Also, astronomers had believed that the brightest star is the 1st magnitude, and the faintest star is the 6th magnitude. Today, the 1st magnitude is 100 times brighter than the 6th magnitude stars.

How did ancient astronomers describe the universe?

Ancient astronomers used the geocentric model of the universe. Geocentric model, any theory of the structure of the solar system (or the universe) in which Earth is assumed to be at the centre of it all.

The Moon's angular size is about ½ o. What is this in arcminute? In arcseconds?

Arcminutes: 1o = 60' 1/2o = 30' arcminutes Arcseconds: 1' = 60" 1/2o = 30' = 30' x 60" = 1,800 arcseconds

What is astrology? How is it different from astronomy and does it work?

Astrology is the study of the movements and relative positions of celestial bodies interpreted as having an influence on human affairs and the natural world. Astronomy is the study of the planets and everything within our solar system and galaxy. Yes, Astronomy does work. Astronomers have discovered many different things throughout our galaxy and still today. No, Astrology does not work because there are no known forces or gravity that could cause such effects such as how the positions of the Sun, Moon, or planets at the moment of birth have anything to do with our personality or future. When astrology was tested, it was not proven that it is accurate.

How do the rising and setting times of the stars change from day to day and from month to month? How do the Moon's rising and setting times change on a daily basis? Why do these changes occur?

Because of its motion on the ecliptic, the Sun rises about 4 minutes later each day with respect to the stars. Earth must make just a bit more than one complete rotation (with respect to the stars) to bring the Sun up again. The moon moves eastward each day in its monthly path around us. Since it takes about 30 days to orbit Earth and there are 360° in a circle, the Moon will move about 12° in the sky each day (or about 24 times its own diameter). Therefore, rising and setting at different times each day.

How do the various celestial objects appear to move across the sky as the Earth rotates?

Celestial objects appear to move across the sky as the Earth rotates by apparent motion. Apparent motion means we are seeing an illusion that space is rotating rather than the Earth. The rotation of the Earth on its axis causes all objects to appear to move around the sky once each day.

What are circumpolar stars?

Circumpolar stars are always visible above the horizon, orbiting the north celestial pole (counterclockwise) every day.

Are more stars circumpolar at the North Pole or in the United States? Explain.

Circumpolar stars never rise or set, instead circling the north celestial pole in counterclockwise loops every day. To put it another way, they are always above the horizon. As a result, more circumpolar stars would be visible near the North Pole.

Method of Eratosthenes I. You are an astronomer on planet Nearth, which orbits a distant star. It has recently been accepted that Nearth is spherical in shape, though no one knows its size. One day, while studying in the library of Alectown, you learn that on the equinox your sun is directly overhead in the city of Nyene, located 1000 kilometers due north of you. On the equinox, you go outside and observe that the rays of the Sun make an angle with the vertical equal to about 10°. What is the circumference of Nearth? (Hint: Apply the technique used by Eratosthenes (page 45) to measure Earth's circumference.)

Considering the Nearth planet is spherical in shape, the star's light rays will not be directly overhead in two different cities. That is why 1,000km south of Nyene, the rays of sun are making an angle with a vertical degree of 10 compared to Nyene which the sun is directly overhead. Since the Nearth planet is a spherical shape, that means that the planet is 360 degrees. Since the difference in shadow length is 10 degrees between the two cities, that means they are 10 degrees apart on a 360-degree surface. All in all, the two cities are 10 degrees apart on a 360 degrees surface with the cities 1,000km apart. The circumference of Nearth is 36,000km. 10/360 = 1/36 = 1,000km/Xkm 36 x 1,000km = 36,000km

A certain comet has a parallax 1/40 as large as the Moon's. How far away is it (compared to the Moon's distance)? SHOW WORK

D = 1/p D = 1/(1/40) D = 40 The distance is 40 times as far away as the moon.

An artificial satellite has a parallax 5 times as big as the Moon's. How far away is it?

D = 1/p D = 1/5 The satellite is 1/5 as far as the Moon.

How does the length of time a star spends above the horizon depend on its declination and on the observer's latitude?

Depending on where you are, stars can either be circumpolar (do not rise or set) or stars could rise and set with no stars being circumpolar (or only have some stars be circumpolar). The closer a star is to the observer's celestial pole, the longer that star will spend above the horizon each day. The closer the observer is to the star's celestial pole, the longer that star will spend above the horizon each day. Therefore, whoever is farther away from the celestial pole, will see that star shorter than the other observer.

Find the Sun's Diameter. The Sun has an angular diameter (α) of about 0o.5 and an average distance (Dist) of about 150 million kilometers. What is the Sun's approximate physical diameter (Dia)? Compare your answer to the actual value of 1,390,000 km. Show work - formula is given. [Dia = Dist x α/57.3o]

Dia = 150,000,000km x 00.5 / 57.3 = 1,308,900km approximately 1,308,900km is the Sun's approximate diameter. This is smaller than the Sun's actual diameter of 1,390,000km by 6%.

Define Summer and Winter Solistices

Either of the two times of the year when the sun is at its greatest distance from the celestial equator. The day that the Earth's North Pole is tilted closest to the sun is called the summer solstice. The winter solstice, or the shortest day of the year, happens when the Earth's North Pole is tilted farthest from the Sun.

Define Vernal and Autumnal Equinoxes?

Either of two times of the year when the sun crosses the plane of the earth's equator and day, and night are of equal length. Vernal (Spring) / Autumnal (Fall)

How does the process of science work?

First you would make an initial observation or assumption, then you would create a hypotheses or model and create predictions. After that, you would perform a test and make observations based on the evidence collected. You would then ask, "does the test support the hypothesis"? If yes, you would use additional predictions to come to a conclusion. If not, you would revise your hypothesis or make a new one.

Define the astronomical unit. How big is it? How was it measured?

For general reference, we can say that one astronomical unit (AU) represents the mean distance between the Earth and our sun. An AU is approximately 93 million miles (150 million km). It's approximately 8 light-minutes. More exactly, one astronomical unit (AU) = 92,955,807 miles (149,597,871 km)

How much of the sky can you see from a given latitude?

For those in the continental United States and Europe, the north celestial pole is neither overhead nor on the horizon, but in between. It appears above the northern horizon at an angular height, or altitude, equal to the observer's latitude. In San Francisco, for example, where the latitude is 38° N, the north celestial pole is 38° above the northern horizon. For an observer at 38° N latitude, the south celestial pole is 38° below the southern horizon and, thus, never visible. As Earth turns, the whole sky seems to pivot about the north celestial pole. For this observer, stars within 38° of the North Pole can never set. They are always above the horizon, day and night. This part of the sky is called the north circumpolar zone. For observers in the continental United States, the Big Dipper, Little Dipper, and Cassiopeia are examples of star groups in the north circumpolar zone. On the other hand, stars within 38° of the south celestial pole never rise. That part of the sky is the south circumpolar zone.

How did Galileo contribute to our understanding of planetary motion?

Galileo showed that objects will stay in motion unless a force acts to slow them down. Galileo spotted sunspots on the sun ("imperfections"). Mountains and valleys were seen on the moon by Galileo's telescope. Galileo realized that the stars are farther than Tycho thought (Tycho thought he had measured stellar distances, so lack of parallax seemed to rule out an orbiting Earth). Galileo's observations of phases of Venus showed that it orbits the Sun and not the Earth. When Galileo pointed his telescope into the night sky in 1610, he saw for the first time in human history that moons orbited Jupiter. If Aristotle were right about all things orbiting Earth, then these moons could not exist. Galileo also observed the phases of Venus, which proved that the planet orbits the Sun. While Galileo did not share Bruno's fate, he was tried for heresy under the Roman Inquisition and placed under house arrest for life.

What were four of Galileo's discoveries that were important to astronomy?

Galileo's discoveries concerning the Moon, Jupiter's moons, Venus, and sunspots backed the astronomers' belief that the Sun (rather than the Earth) was the center of the Universe at the time. Galileo's work was instrumental in the development of modern space missions and telescopes. Galileo's 3-power magnification telescope, Jupiter's four moons, Venus's phases, the surface of the moon, its craters, mountains, rages, and dark patches that he assumed were water, and the Milky Way mass of stars were all major astronomical discoveries.

Explain, according to both geocentric and heliocentric cosmologies, why we see retrograde motion of the planets.

Geocentric: The planets travel on epicycles on their own deferents that are nearest to Earth, and they move in the opposite direction of their greater motion on the deferent, making them appear to move backwards. Heliocentric: In its orbit around the Sun, the Earth travels faster than the outer planets. As a result, the Earth overtakes and passes the outer planets, causing them to seem to reverse their orbits.

Explain the statement: The farther away we look in distance, the further back we look in time.

Light years takes time to travel through space. Light years is a measurement of distance. Considering light takes time to travel, the further an item is from the planet (Earth), the longer it will take for light to reach back to Earth. The time it takes for light to reach Earth might be thought of as a time delay, and we are thus looking back in time. The longer the delay, the further back in time we are looking. Which also means since light takes so long to reach earth that people are seeing incredibly old light generated from trillions of years ago.

How do we measure the distance to the Moon? How do we use this result to find the distances of other objects?

Moon's distance by means of its parallax. From Earth's poles, Moon appears to jump about 2o against background of stars. *D=1/p* We can measure the distance of any object close enough to have a measurable parallax. Any thin triangle with an angle of 2o is about 30 times as long as it is wide.

Briefly describe our solar system's location and motion within the Milky Way galaxy.

Our solar system is located in the Milky Way Galaxy's furthest reaches. This solar system is located more than halfway between the center of the galaxy and the edge of the galaxy. Our solar system revolves around the Milky Way Galaxy's core. The Local Group is orbited by the Milky Way Galaxy. Our Local Supercluster is orbiting our Local Group.

Do you know the differences between the Ptolemaic, Copernican, and Tychonic theories of planetary motion? What observational tests helped us decide among these three theories?

Ptolemaic: Earth is in the center of the universe. Moon goes around the earth. Mercury and Venus go around the Earth. Mercury and Venus are close to the Sun. They moved on the epicycles. The centers were on the line joining the Sun to the Earth. Was not proven. Ptolemy placed the Earth at the center of his geocentric model. Using the data he had, Ptolemy thought that the universe was a set of nested spheres surrounding the Earth. He believed that the Moon was orbiting on a sphere closest to the Earth, followed by Mercury, then Venus and then the Sun. Copernican: Not more accurate than the Ptolemaic model. Copernicus used this model to determine the layout of the solar system. Still used the idea of perfect circles. This model positioned the Sun at the center of the Universe, motionless, with Earth and the other planets orbiting around it in circular paths, modified by epicycles, and at uniform speeds. Tychonic: Most accurate (one arcminute) naked eye measurements (stars and planets) Earth must be at the center of the Solar System. Other planets go around the sun. The Tychonic model is a theoretical model of the universe that hypothesizes that the earth is the center of the universe. The sun, moon, and stars revolve around the earth. The world beyond Moon's orbit was unchangeable; the phenomenon was below the moon. Tycho observed no daily parallax against the fixed stars. This implied it was at least further away than the Moon and those planets that do show such parallax. All models predicted the retrograde motion. Galileo applied observations to the question of the model of the Universe and adopted the Copernican model.

What is retrograde motion?

Retrograde Motion is an actual or apparent motion of a body in a direction opposite to that of the (direct) motions of most members of the solar system or of other astronomical systems with a preferred direction of motion.

Is the ecliptic the same thing as the celestial equator? Explain.

The Celestial Equator is not the same as the ecliptic. The celestial equator is the projection of Earth's equator into the sky, while the Ecliptic is the Sun's apparent yearly journey through the sky.

How did the Greeks explain planetary motion?

The Greeks explained the planetary motions in terms of geometry/mathematics

Define Right Ascension

The Right Ascension of a celestial object is the amount of time that elapses between the transit of the Vernal Equinox across any given meridian and the transit of that celestial body across the same meridian.

Can you find your latitude from the altitude of the celestial pole?

The altitude (distance above the horizon) of the NCP is equal to your latitude north of the equator; Altitude of zenith = 90° (straight overhead) always. Altitude of celestial pole = observer's latitude.

Define Declination

The angular distance of a point north or south of the celestial equator.

Define Ecliptic

The apparent annual path of the Sun on the celestial sphere

What is the Celestial Sphere?

The apparent sphere of the sky; a sphere of large radius centered on the observer; directions of objects in the sky can be denoted by their position on the celestial sphere

How did we come to be?

The big bang created hydrogen, helium, and a little lithium. Stars created other elements in the process of nuclear fusion. The elements in our body were created in the nuclear furnace of a star. Because of this Carl Sagan said that we are made of star stuff!

In what sense are we "star stuff"?

The elements in our body were created in the nuclear furnace of a star. At the conclusion of the star's lifetimes, these stars burst, reusing their material back into the Galaxy's reservoir. In this way, we are all made of "star dust" that has been recycled.

Define horizon, zenith, and meridian.

The horizon is where the sky and the ground meet. The zenith is the point directly overhead an observer. The meridian is an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north.

Define the Horizon

The horizon is where the sky meets the ground

Briefly describe the major levels of structure (such as planet, star, galaxy, and larger scale structure) in the universe.

The major levels of structure in the universe consist of stars, planets, moons, and galaxies. A star is a ball of gas that creates heat and light by blending hydrogen and helium. A planet is a big object that circles the sun and mostly glows by reflecting light from the sun. A moon is a celestial body that circles another planet. The sun and the planets that circle it make up a solar system. A galaxy is a large cluster of stars in space that contains trillions of stars orbiting a corresponding center and are close together by the cause of gravity. A cluster, also known as a group, is a grouping of galaxies that are connected by gravity. A super cluster is a massive area in which galaxies and clusters are densely packed together. The full amount of all matter and energy is referred to as the universe.

Define Meridian

The meridian is an imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north.

How did astronomy evolve into the view of the universe that we have today?

The scope of astronomy has been expanding since its beginnings. The stars and planets were once reckoned as lights in a flat Earth's sky.Later, The Earth was seen as a sphere, hanging in space at the center of the universe, which revolved around it. Eventually, It was realized that the planets are real worlds, and the stars, faraway Suns. Then it was realized that Earth is not the center of all things, but a planet, circling the Sun, along with all the others. For a while our solar system was thought unique, or at least very rare, but today we know that most stars have planets, and that some are probably the abodes of life.

Why do the stars rise and set?

The stars rise in the East and sets in the West because of Earth's rotation.

What is the nature of science?

The ultimate judge in science is always what nature itself reveals based on observations, experiments, models, and testing. Science is not merely a body of knowledge, but a method by which we attempt to understand nature and how it behaves. This method begins with many observations over a period of time.

What is our place in the universe?

We are located on Earth in our solar system. Our solar system is in the Milky Way Galaxy's furthest reaches. This solar system is located more than halfway between the center of the galaxy and the edge of the galaxy. Our solar system revolves around the Milky Way Galaxy's core.

Suppose you were making a model of the celestial sphere with a ball. Briefly describe five things you would need to mark on your celestial sphere.

When creating a celestial sphere, the celestial north pole, which is immediately over Earth's north pole, must be marked. You'll also need to indicate the celestial south pole, which lies immediately over Earth's south pole, as well as the celestial equator, which is Earth's equator projected into space. Finally, there's the ecliptic, which is the route taken by the sun as it seems to round the celestial sphere once a year.

In what ways did the work of Copernicus and Galileo differ from the views of the ancient Greeks and of their contemporaries?

While everyone before them believed that the planets and the Sun orbited the earth, Copernicus and Galileo created heliocentric models of the universe (a geocentric model).

What does the universe look like from Earth?

With the naked eye, we can see more than 2,000 stars as well as the Milky Way from Earth.

Why do the constellations depend on latitude and time of year?

Your location determines which constellations are hidden by Earth. The time of year determines the location of Sun in the sky.