Chapter 4: Earth, Moon, Sky

spring tides

( Spring tides are approximately the same, whether the Sun and Moon are on the same or opposite sides of Earth, because tidal bulges occur on both sides. When the Moon is at first quarter or last quarter (at right angles to the Sun's direction), the tides produced by the Sun partially cancel the tides of the Moon, making them lower than usual. These are called neap tides. However, the presence of land masses stopping the flow of water, the friction in the oceans and between oceans and the ocean floors, the rotation of Earth, the wind, the variable depth of the ocean, and other factors all complicate the picture. --> some places have small tides and some have high tides one set of tide predictions doesn't work for the whole planet.

The Challenge of the Calendar

2 purposes: keep track of time over the course of long spans to honor holidays and anniversaries & useful to a large number of people The natural units of our calendar are the day, based on the period of rotation of Earth; the month, based on the cycle of the Moon's phases (see later in this chapter) about Earth; and the year, based on the period of revolution of Earth about the Sun. Difficulties have resulted from the fact that these three periods are not commensurable; that's a fancy way of saying that one does not divide evenly into any of the others he basic period of revolution of Earth, called the tropical year, is 365.2422 days.

Appearance of the Total Eclipse

A solar eclipse starts when the Moon just begins to silhouette itself against the edge of the Sun's disk. A partial phase follows, during which more and more of the Sun is covered by the Moon. About an hour after the eclipse begins, the Sun becomes completely hidden behind the Moon. During totality, the sky is dark enough that planets become visible in the sky, and usually the brighter stars do as well. The corona is the Sun's outer atmosphere, consisting of sparse gases that extend for millions of miles in all directions from the apparent surface of the Sun. Only when the brilliant glare from the Sun's visible disk is blotted out by the Moon during a total eclipse is the pearly white corona visible

Eclipses of the Moon

Although the Sun is about 400 times larger in diameter than the Moon, it is also about 400 times farther away, so both the Sun and the Moon have the same angular size—about 1/2° When the Moon's shadow strikes Earth, people within that shadow see the Sun at least partially covered by the Moon; that is, they witness a solar eclipse. When the Moon passes into the shadow of Earth, people on the night side of Earth see the Moon darken in what is called a lunar eclipse. The shadows of Earth and the Moon consist of two parts: a cone where the shadow is darkest, called the umbra, and a lighter, more diffuse region of darkness called the penumbra. As you can imagine, the most spectacular eclipses occur when an object enters the umbra. If the path of the Moon in the sky were identical to the path of the Sun (the ecliptic), we might expect to see an eclipse of the Sun and the Moon each month—whenever the Moon got in front of the Sun or into the shadow of Earth. However, as we mentioned, the Moon's orbit is tilted relative to the plane of Earth's orbit about the Sun by about 5° (imagine two hula hoops with a common center, but tilted a bit). As a result, during most months, the Moon is sufficiently above or below the ecliptic plane to avoid an eclipse. But when the two paths cross (twice a year), it is then "eclipse season" and eclipses are possible.

locating places in the sky

Astronomers use coordinates called declination & right ascension to measure positions in the sky sky appears to rotate about points above the North and South Poles of Earth—points in the sky called the north celestial pole and the south celestial pole. Halfway between the celestial poles, and thus 90° from each pole, is the celestial equator, a great circle on the celestial sphere that is in the same plane as Earth's equator. We can use these markers in the sky to set up a system of celestial coordinates

The Season At Different Latitudes

At equator all seasons are much the same = every day of the year the sun is half up and half down--> wet & dry seasons rather than based on sunlight Extreme north or extreme south like the artic & Antarctic seasons become more pronounces At the North Pole, all celestial objects that are north of the celestial equator are always above the horizon and, as Earth turns, circle around parallel to it. The Sun is north of the celestial equator from about March 21 to September 21, so at the North Pole, the Sun rises when it reaches the vernal equinox and sets when it reaches the autumnal equinox. Each year there are 6 months of sunshine at each pole, followed by 6 months of darkness. LOOK AT EXAMPLE 4.2

Solar Day

Basic unit of time = day Solar day: rotation period of Earth with respect to the Sun--> set our clocks to sun time

Locating places on earth

Earth's axis of rotation defined the locations of its North & South Poles & its equator halfway between East is the directions toward which the Earth rotates, and west is its opposite NESW is well defined except at the North & South Poles, East and West are ambiguous

The Gregorian Calendar

First, 10 days had to be dropped out of the calendar to bring the vernal equinox back to March 21; by proclamation, the day following October 4, 1582, became October 15 The second feature of the new Gregorian calendar was a change in the rule for leap year, making the average length of the year more closely approximate the tropical year. Gregory decreed that three of every four century years—all leap years under the Julian calendar— (1600,2000) The average length of this Gregorian year, 365.2425 mean solar days, is correct to about 1 day in 3300 years.

Declination

From the celestial equator toward the north (positive) or south (negative) --> like latitude Polaris, the star near north celestial pole has a declination of almost + 90 degrees

Prime Meridian

Greenwich England = Prime Meridian ( the longitude = 0 degrees) Greenwich was selected because it was between continental Europe and the USA & b/c it was where more of the development of the measuring of longitude occurred

The Seasons & Sunshine

How does the Sun's favoring one hemisphere translate into making it warmer for us down on the surface of Earth? When we lean into the Sun, sunlight hits us at a more direct angle and is more effective at heating Earth's surface June is more direct and intense in NH and hence more effective at heating In September and March, Earth leans "sideways"—neither into the Sun nor away from it—so the two hemispheres are equally favored with sunshine.

Eclipses of the Sun & Moon

Much of the time, the Moon looks slightly smaller than the Sun and cannot cover it completely, even if the two are perfectly aligned. In this type of "annular eclipse," there is a ring of light around the dark sphere of the Moon. If moon is somewhat nearer and moon completely covers Sun = total solar eclipse total eclipse of the Sun occurs at those times when the umbra of the Moon's shadow reaches the surface of Earth If the Sun and Moon are properly aligned, then the Moon's darkest shadow intersects the ground at a small point on Earth's surface. Anyone on Earth within the small area covered by the tip of the Moon's shadow will, for a few minutes, be unable to see the Sun and will witness a total eclipse. observers on a larger area of Earth's surface who are in the penumbra will see only a part of the Sun eclipsed by the Moon: we call this a partial solar eclipse. Between Earth's rotation and the motion of the Moon in its orbit, the tip of the Moon's shadow sweeps eastward at about 1500 kilometers per hour along a thin band across the surface of Earth. The thin zone across Earth within which a total solar eclipse is visible (weather permitting) is called the eclipse path. Within a region about 3000 kilometers on either side of the eclipse path, a partial solar eclipse is visible. It does not take long for the Moon's shadow to sweep past a given point on Earth. The duration of totality may be only a brief instant; it can never exceed about 7 minutes.

Early Calendar

Stonehedge: tones are aligned with the directions of the Sun and Moon during their risings and settings at critical times of the year (such as the summer and winter solstices), and it is generally believed that at least one function of the monument was connected with the keeping of a calendar. Mayan Calendar: did not attempt to correlate their calendar accurately with the length of the year or lunar month. Rather, their calendar was a system for keeping track of the passage of days and for counting time far into the past or future. Among other purposes, it was useful for predicting astronomical events, such as the position of Venus in the sky Ancient Chinese: In addition to the motions of Earth and the Moon, they were able to fit in the approximately 12-year cycle of Jupiter, which was central to their system of astrology. The Chinese still preserve some aspects of this system in their cycle of 12 "years"—the Year of the Dragon, the Year of the Pig, and so on—that are defined by the position of Jupiter in the zodiac. Western Calendars: --> Sumerians --> Egyptians and Greeks --> Julian Calendar-->which approximated the year at 365.25 days, fairly close to the actual value of 365.2422. The Romans achieved this approximation by declaring years to have 365 days each, with the exception of every fourth year. The leap year was to have one extra day, bringing its length to 366 days, and thus making the average length of the year in the Julian calendar 365.25 days --> dropped trying to base calendars off of moon and sun

Lunar Phases (figure 4.14)

Sun moves 1/12 its path around the sky each month, but we can assume the Sun' light is constant through a moon's four week cycle Move moves completely around earth in that time how much of moon face we see illuminated depends on the angle the sun makes with the Moon The moon = new when it is in the same general direction in the sky as the Sun ( A)--> moon invisible to us because new mon is the same part of the sky as the sun, it rises at sunrise & sets at sunset Moves 12 degrees in the sky each day (24 times its own diameter) A day or two later = thin crescent first appears, as we begin to see a small part of the Moon's illuminated hemisphere--> reflects a little sunlight towards us along one sides--> increases in size on successive days as Moon moves farther & farther around the sky away from Sun (B)--> moon is moving eastward away from the Sun, it rises later & later each day the Moon is one-quarter of the way around its orbit (position C) and so we say it is at the first quarter phase. Half of the Moon's illuminated side is visible to Earth observers. Because of its eastward motion, the Moon now lags about one-quarter of the day behind the Sun, rising around noon and setting around midnight During the week after the first quarter phase, we see more and more of the Moon's illuminated hemisphere (position D), a phase that is called waxing (or growing) gibbous. Eventually, the Moon arrives at position E in our figure, where it and the Sun are opposite each other in the sky--> full moon Moon rises at sunset and setting at sunrise Highest and most noticeable in midnight More likely to notice or remember b/c of bright celestial light During the two weeks following the full moon, the Moon goes through the same phases again in reverse order returning to new phase after about 29.5 days. About a week after the full moon, for example, the Moon is at third quarter, meaning that it is three-quarters of the way around (not that it is three-quarters illuminated—in fact, half of the visible side of the Moon is again dark). At this phase, the Moon is now rising around midnight and setting around noon. And, since the Moon's orbit is tilted relative to the path of the Sun in the sky, Earth's shadow misses the Moon most months. That's why we regularly get treated to a full moon. The times when Earth's shadow does fall on the Moon are called lunar eclipses

Right Ascension

The arbitrarily chosen point where we start counting is the vernal equinox (a point in the sky where the ecliptic (the sun's path) crosses the celestial equator)--> like longitude Can be expression as units of angle (degrees) or units of time--> b/c celestial sphere appears to turn around Earth once a day as our planet turns on its axis Thus the 360° of RA that it takes to go once around the celestial sphere can just as well be set equal to 24 hours. Then each 15° of arc is equal to 1 hour of time. For example, the approximate celestial coordinates of the bright star Capella are RA 5h = 75° and declination +50°

The Seasons

The difference between seasons gets more pronounced the farther north or south from the equator we travel, and the seasons in the Southern Hemisphere are the opposite of what we find on the northern half of Earth

Darwin & The Slowing of the Earth

What Darwin calculated for the Earth-Moon system was that the Moon will slowly spiral outward, away from Earth. As it moves farther away, it will orbit less quickly (just as planets farther from the Sun move more slowly in their orbits). Thus, the month will get longer. Also, because the Moon will be more distant, total eclipses of the Sun will no longer be visible from Earth Moving away at 3.8 cm each year = billions of year day and month will be the same length

How does the Sun favoring a hemisphere make it warmer for us down on the surface?

When we lean into the Sun, sunlight hits us at a more direct angle and is more effective at heating Earth's surface June is more direct and intense in NH and hence more effective at heating Second effect has to do with the length of the time the Sun spends above the horizon--> hours of daylight increase in summer and decrease in winter In June the Sun is north of the celestial equator and spends more time with those who live in the Northern Hemisphere--> it rises high and is above the horizon for as long as 15 hours Thus not only direct rays but also a longer period of time opposite in the Southern Hemisphere

eclipse of the moon

a lunar eclipse is visible to everyone who can see the Moon. Because a lunar eclipse can be seen (weather permitting) from the entire night side of Earth, lunar eclipses are observed far more frequently from a given place on Earth than are solar eclipses An eclipse of the Moon is total only if the Moon's path carries it though Earth's umbra. If the Moon does not enter the umbra completely, we have a partial eclipse of the Moon. because Earth is larger than the Moon, its umbra is larger, so that lunar eclipses last longer than solar eclipses The Moon is opposite the Sun, which means the Moon will be in full phase before the eclipse, making the darkening even more dramatic. As the Moon begins to dip into the shadow, the curved shape of Earth's shadow upon it soon becomes apparent. Moon is red == sunlight that has been bent into Earth's shadow For an eclipse where the Moon goes through the center of Earth's shadow, each partial phase consumes at least 1 hour, and totality can last as long as 1 hour and 40 minutes. Not dangerous to look at On arrival, make sure the scene is saved enter, and then perform a rapid scan of the patient, noting whether any blood or bodily fluids are present period select the proper PPE according to the task you are likely to perform. Typically, globally used for all patient Contacts.

Tides are caused by

an actual flow of water over Earth's surface toward the two regions below and opposite the Moon, causing the water to pile up to greater depths at those places The rotation of Earth would carry an observer at any given place alternately into regions of deeper and shallower water. An observer being carried toward the regions under or opposite the Moon, where the water was deepest, would say, "The tide is coming in"; when carried away from those regions, the observer would say, "The tide is going out." During a day, the observer would be carried through two tidal bulges (one on each side of Earth) and so would experience two high tides and two low tides. The actual tides we experiences are a combo of the large effect of the Moon & the smaller effect of the Sun When sun & moon are lines up = new or full moon = tides produces reinforce each other and are greater than normal

Great Circle

any circle on the surface of a sphere whose center is at the center of the sphere Earth's equator is a great circle on Earth's surface, halfway between the North and South Poles. We can also imagine a series of great circles that pass through both the North and South Poles. Each of these circle is called the meridian= they are each perpendicular to the equator cross it at right angles Any point of the surface of Earth will have a meridian passing through it The meridian specifies the east-west location = longitude ( the number of degrees of arc along the equator between your meridian and the one passing through Greenwich, England

Mean Standard Time

based on the average value of the solar day over the course of the year--> contains exactly 24 hours--> still inconvenient determined by position of the Sun For example, noon occurs when the Sun is highest in the sky on the meridian (but not necessarily at the zenith). But because we live on a round Earth, the exact time of noon is different as you change your longitude by moving east or west. Within each zone, all places keep the same standard time, with the local mean solar time of a standard line of longitude running more or less through the middle of each zone--> six zones Daylight saving time is simply the local standard time of the place plus 1 hour. It has been adopted for spring and summer use in most states in the United States, as well as in many countries, to prolong the sunlight into evening hours

Refraction

bending of light passing through air or water that allow us to see little over the horizon the Sun appears to rise earlier and to set later than it would if no atmosphere were present the atmosphere scatters light and provides some twilight illumination even when the Sun is below the horizon. Astronomers define morning twilight as beginning when the Sun is 18° below the horizon, and evening twilight extends until the Sun sinks more than 18° below the horizon Leadesr to small correction in many of our statements about the seasons. At equinoxes sun above the horizon for a little longer than 12 hours and & below for a little less than 12 Most dramatic at the poles--> more than a week before it reaches the equator Warmer and colder are a little after the times when we get the most/least sunlight because it takes time for Earth to warm up

Phases

different appearances with the Moon starting dark and getting more and more illuminated by sunlight over the course of two weeks --> moon disks fade becoming dark again two weeks later

Latitude

north-south location is the number of degrees of arc you are away from the equator along the meridian Measured north or south of equator (0 to 90 degrees)--> latitude of equator = 0 degrees and north pole = 90 degrees

Sidereal month

period of its revolution about Earth measure with respect to the star is little over 27 days The time interval in which the phases repeat (full to full) is the solar month(little over 29.5 days) The difference results from Earth's motion around the Sun. The Moon must make more than a complete turnaround the moving Earth to get back to the same phase with respect to the Sun. the Moon changes its position on the celestial sphere rather rapidly: even during a single evening, the Moon creeps visibly eastward among the stars, traveling its own width in a little less than 1 hour. The delay in moonrise from one day to the next caused by this eastward motion averages about 50 minutes The Moon rotates on its axis in exactly the same time that it takes to revolve about Earth. As a consequence, the Moon always keeps the same face turned toward Earth --> synchronous rotation

Tides

results from the gravitations forces exerted by the Moon at several points on Earth Forces differ slightly from one another= all parts are not equally distant from the moon or in the same direction the differences among the forces of the Moon's attraction on different parts of Earth (called differential forces) cause Earth to distort slightly. Side closest is most attracted that side opposite moon = differential forces tend to stretch Earth into prolate spheroid (a football shape) , long diameter pointed towards the moon Measurements of the actual deformation of Earth show that the solid Earth does distort, but only about one-third as much as water would, because of the greater rigidity of Earth's interior. Because the tidal distortion of the solid Earth amounts—at its greatest—to only about 20 centimeters, Earth does not distort enough to balance the Moon's differential forces with its own gravity. Hence, objects at Earth's surface experience tiny horizontal tugs, tending to make them slide about. These tide-raising forces are too insignificant to affect solid objects like astronomy students or rocks in Earth's crust, but they do affect the waters in the ocean

Sidereal

rotation period of Earth with respect to other stars (astronomers also use this) A solar day is slightly longer than a sidereal day because Earth not only turns but also moves along its path around the Sun in a day When Earth has completed one rotation with respect to the distant star and is at day 2, the long arrow again points to the same distant star. However, notice that because of the movement of Earth along its orbit from day 1 to 2, the Sun has not yet reached a position above the observer. To complete a solar day, Earth must rotate an additional amount, equal to 1/365 of a full turn. The time required for this extra rotation is 1/365 of a day, or about 4 minutes. So the solar day is about 4 minutes longer than the sidereal day. Because our ordinary clocks are set to solar time, stars rise 4 minutes earlier each day. Astronomers prefer sidereal time for planning their observations because in that system, a star rises at the same time every day. LOOK AT EXAMPLE 4.3

The International Date Line

set by international agreement to run approximately along the 180° meridian of longitude. By convention, at the date line, the date of the calendar is changed by one day. Crossing the date line from west to east, thus advancing your time, you compensate by decreasing the date; crossing from east to west, you increase the date by one day. To maintain our planet on a rational system of timekeeping, we simply must accept that the date will differ in different cities at the same time

Sun's Illumination at Specific Dates of the Year

summer solstice: the Sun shines down most directly upon the Northern Hemisphere of Earth. It appears about 23° north of the equator, and thus, on that date, it passes through the zenith of places on Earth that are at 23° N latitude To a person at 23° N (near Hawaii, for example), the Sun is directly overhead at noon. This latitude, where the Sun can appear at the zenith at noon on the first day of summer, is called the Tropic of Cancer. As Earth turns on its axis, the North Pole is continuously illuminated by the Sun; all places within 23° of the pole have sunshine for 24 hours. The Sun is as far north on this date as it can get; thus, 90° - 23° (or 67° N) is the southernmost latitude where the Sun can be seen for a full 24-hour period (sometimes called the "land of the midnight Sun"). That circle of latitude is called the Arctic Circle. On June 21st all places within 23 degree of the South Pole- south of Antarctic Circle- no sun for 24h Reversed 6 months later, about December 21 (winter solstice)--> Arctic Circle that has 24 hour night and Antarctic Circle that has the midnight Sun. At latitude 23° S, called the Tropic of Capricorn, the Sun passes through the zenith at noon. Days are longer in the Southern Hemisphere and shorter in the north. In the United States and Southern Europe, there may be only 9 or 10 hours of sunshine during the day. It is winter in the Northern Hemisphere and summer in the Southern Hemisphere (44 degrees) Less tilt MIDLER SEASONAL CHANGES--> LOOK AT EXAMPLE 4.1 Halfway between the solstices, on about March 21 and September 21, the Sun is on the celestial equator. From Earth, it appears above our planet's equator and favors neither hemisphere. Every place on Earth then receives roughly 12 hours of sunshine and 12 hours of night. The points where the Sun crosses the celestial equator are called the vernal (spring) and autumnal (fall) equinoxes.

The Turning Earth

the apparent rotation of the celestial sphere could be accounted for either by a daily rotation of the sky around a stationary Earth or by the rotation of Earth itself Jean Foucault: demonstration of rotation--> started swinging pendulum evenly, if earth had not been turning there would have been no alteration to the path, but the pendulum's place of motion was turning --> Earth was turning beneath it

apparent solar time

time reckoned by the actual position of the Sun in the sky or below the horizon--> represented by sundials During the first half of the day, the Sun has not yet reached the meridian (the great circle in the sky that passes through our zenith). We designate those hours as before midday (ante meridiem, or a.m.), before the Sun reaches the local meridian. We customarily start numbering the hours after noon over again and designate them by p.m. (post meridiem), after the Sun reaches the local meridian Although apparent solar time seems simple, it is not really very convenient to use. The exact length of an apparent solar day varies slightly during the year. The eastward progress of the Sun in its annual journey around the sky is not uniform because the speed of Earth varies slightly in its elliptical orbit. Another complication is that Earth's axis of rotation is not perpendicular to the plane of its revolution. --> does not advance at a uniform rate.