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Confirming Pages PA RT I UNIT 6 The Year UNIT OUTLINE 6.1 Annual Motion of the Sun 6.2 The Ecliptic and the Zodiac 6.3 The Seasons 6.4 The Ecliptic’s Tilt 6.5 Solstices and Equinoxes 6.6 Precession For people long ago, observations of the heavens had more than just curiosity value. Because so many astronomical phenomena are cyclic—that is, they repeat at a regular interval—they can serve as timekeepers. The most basic of these cycles is the rhythm of day and night, as the celestial sphere appears to rotate about the Earth, as described in Unit 5. This cycle is not completely uniform—days and nights alternately lengthen and shorten over the year. This slow rhythm is tied to a gradual shift of the Sun’s apparent position relative to the “fixed stars” on the celestial sphere. The shifting position of the Sun also leads to seasonal changes in the weather and temperature. The motion of the Sun against the celestial sphere provides a means for tracking these changes predictably. We might imagine ancient peoples asking, When is it time to plant crops? Or move to the next location to ensure a ready supply of water? Or prepare for winter? Some societies built monumental structures to mark the changing position of the Sun. An example is the Mayan pyramid at Chichén Itzá. The pyramid is designed so that on the first day of spring or fall, shadow play creates the appearance of a serpent slithering down the staircase (Figure 6.1). We will use much of the terminology for positions in the sky from Unit 5. Review that Unit if terms like celestial sphere, constellation, zenith, or declination are unfamiliar. FIGURE 6.1 On the equinoxes, the Sun casts a shadow that resembles a serpent slithering down the steps of the Mayan pyramid (left side in photograph) at Chichén Itzá. The head of the serpent is depicted in a sculpture at the base of the stairs. Serpent 42 sch12133_ch06.indd 42 11/26/10 2:30 PM Confirming Pages Unit 6 The Year 43 66.1 .1 ANNUAL MOTION OF THE SUN A N I M AT I O N Constellations by season Ancient Egyptians watched for the first signs of the “dog star” Sirius rising just before dawn. They used this to predict when the Nile would flood, and it also marked the start of summer. This is the origin of the phrase “dog days” of summer. FIGURE 6.2 The changing appearance of the evening sky over several months. In early June, the Sun appears to lie in the constellation Taurus and the constellation Gemini is visible in the west just after sunset. A month later the Sun is in Gemini, and a month after that it is in Cancer. As the Earth orbits the Sun, the stars that are visible each night change. The shift is so slow that it is difficult to appreciate from one night to the next, but in the span of a month the changes become obvious. Because these movements repeat after the Earth completes one orbit around the Sun—a year—they are called annual motions. If you watch the sky each evening over several months, you will discover that new constellations appear in the eastern sky and old ones disappear from the western sky. For example, across most of North America, Europe, and Asia on an early July evening, the constellation Scorpius will be visible in the southern half of the sky. However, on December evenings the brilliant constellation Orion, the hunter, is visible instead. The realization that different stars are visible at different times of the year was extremely important to early peoples because it provided a way to predict the changing of the seasons. A farmer might be tricked into planting too early by a short spell of warmer-than-normal weather in the late winter, but experience would teach that each year the stars could reliably predict when spring was arriving. For example, if you live in the Northern Hemisphere and the early evening sky shows Leo in the south instead of Scorpius or Orion, it will soon be time to plant. Even in semitropical climates, where seasonal temperature differences are much smaller, planting with accuracy is also necessary to avoid crop damage due to annual flooding or dry periods. The changing of constellations throughout the year is caused by the Earth’s motion around the Sun. As the Earth revolves (orbits) around the Sun, the Sun’s glare blocks our view of the part of the celestial sphere that lies in the direction of the Sun, making the stars that lie beyond the Sun invisible, as Figure 6.2 shows. For example, in early June, a line from the Earth to the Sun points toward the constellation Taurus, and its stars are completely lost in the Sun’s glare. In the dusk after sunset, however, it is possible to see the neighboring constellation, Gemini, just above the western horizon. A month later, from the Earth’s new position, the Sun lies in the direction of Gemini, causing this constellation to disappear in the Sun’s glare. Looking to the west just after sunset, it is possible to see, just barely, the dim stars of the constellation Cancer above the horizon. A month after that, the Sun is in Cancer, and the constellation Leo is visible just above the horizon after sunset. Month by month, the Sun hides one constellation after another. It is like sitting around a campfire and not being able to see the faces of the people on the far side. But if we get up and walk around the fire, we can see faces that were previously hidden. Similarly, the Earth’s motion allows us to see stars previously hidden in the Sun’s glare. Evening twilight on June 1 Cancer Evening twilight on July 1 Evening twilight on August 1 Leo Gemini Leo Cancer Taurus Gemini Cancer Sun sch12133_ch06.indd 43 Sun Sun 11/26/10 2:30 PM Confirming Pages 44 Part One The Cosmic Landscape Gemini Taurus Cancer Aries Leo Virgo Apparent position of Sun in early August Earth Scorpius Pisces August June Libra Apparent position of Sun in early June Aquarius Sagittarius Capricornus FIGURE 6.3 As the Earth orbits the Sun, the Sun appears to move around the celestial sphere through the background stars. The Sun’s path is called the ecliptic. The Sun appears to lie in Taurus in June, in Cancer during August, in Virgo during October, and so forth. Note that the ecliptic is the extension of the Earth’s orbital plane out to the celestial sphere. (Sizes and distances of objects are not to scale.) 6.2 THE ECLIPTIC AND THE ZODIAC 6. The name ecliptic comes from the fact that only when the new or full moon crosses this line can an eclipse occur. (See Unit 8.) Concept Question 1 Can you think of an astronomical reason why the zodiac may have been divided into 12 signs rather than 13— or some other number entirely? sch12133_ch06.indd 44 If we could mark on the celestial sphere the path traced by the Sun as it moves through the constellations, we would see that it moves around the celestial sphere, as illustrated in Figure 6.3. Astronomers call the path that the Sun traces across the celestial sphere the ecliptic. You can see in Figure 6.3 that the ecliptic is the extension of the Earth’s orbit onto the celestial sphere, just as the celestial equator is the extension of the Earth’s equator onto the celestial sphere. The ecliptic passes through a dozen constellations, which are collectively called the zodiac. The word zodiac comes from Greek roots meaning “animals” (as in zoology) and “circle” (as in diameter). That is, zodiac refers to a circle of animals, which is what most of its constellations represent: Aries (ram), Taurus (bull), Gemini (twins), Cancer (crab), Leo (lion), Virgo (virgin), Libra (scale), Scorpius (scorpion), Sagittarius (archer), Capricornus (goat), Aquarius (water bearer), and Pisces (fish). Actually, the ecliptic passes through a thirteenth constellation, Ophiuchus (serpent holder), during the first half of December (between Scorpius and Sagittarius); but this constellation was not included in the zodiac, probably because of some uncertainty in ancient times about the precise path of the Sun and some vagueness about the boundaries of the constellations. The names of some of the constellations of the zodiac may have originated in the seasons when the Sun passed through them. For example, rainy weather in much of Europe during winter was foretold by the Sun’s appearance in the constellation Aquarius (the water bearer). Likewise, the harvest time was indicated by the Sun’s appearance in Virgo (the virgin), a constellation often depicted as the goddess Proserpine, holding a sheaf of grain. 11/26/10 2:30 PM Confirming Pages Unit 6 The Year 45 6.3 THE SEASONS 6. Seasons are not caused by the Earth’s distance from the Sun. The constancy of Earth’s tilt is a consequence of the conservation of angular momentum. (See Unit 20.) Concept Question 2 When it is summer in the Northern Hemisphere, what is the season in the Southern Hemisphere? What does this demonstrate about possible causes of the seasons? Many people believe that we have seasons because the Earth’s distance from the Sun changes. They assume that summer occurs when we are closest to the Sun and winter when we are farthest away. It turns out, however, that the Earth is several million kilometers closer to the Sun in early January, when the Northern Hemisphere is coldest, than it is in July. Clearly then, seasons must have some other cause. To see what causes our seasons, we need to look at how our planet is oriented in space. As the Earth orbits the Sun, our planet also spins or rotates. That spin is around a line—the rotation axis—which we might imagine running through the Earth from its North Pole to its South Pole. The Earth’s rotation axis is not perpendicular to its orbit around the Sun. Rather, it is tipped by 23.5° from the vertical, as shown in Figure 6.4A. As our planet moves along its orbit, its rotation axis maintains nearly exactly the same tilt and direction, as illustrated in Figure 6.4B. That is, the Earth behaves much like a giant gyroscope or spinning top. In fact, every spinning object shows this tendency to maintain its orientation. This is why you need to put spin on a frisbee to keep it from flipping over when you throw it, and it is why a quarterback puts “spin” on a football. The constancy of our planet’s tilt as we move around the Sun causes sunlight to fall more directly on the Northern Hemisphere for half of the year and more directly on the Southern Hemisphere for the other half of the year, as illustrated in Figure 6.4B. This in turn changes the amount of heat each hemisphere receives from the Sun. A surface directly facing the Sun receives the most concentrated sunlight. If the surface receives the sunlight at an angle, the light is spread out over a larger area and therefore is less concentrated, as illustrated in Figure 6.5. An astronomer might express this in terms of the energy received per square meter. A portion of the Earth directly facing the Sun receives about 1300 watts on every square FIGURE 6.4 (A) The Earth’s rotation axis is tilted 23.5° to its orbit around the Sun. (B) The Earth’s rotation axis keeps nearly the same tilt and direction as it revolves (orbits) around the Sun. As a result, sunlight falls more directly on the Northern Hemisphere during half of the year and on the Southern Hemisphere during the other half of the year. (Sizes and distances are not to scale.) North Pole Equator A A N I M AT I O N March 20 North Pole Earth’s rotation axis North Pole INTERACTIVE Seasons June 21 December 21 September 22 B sch12133_ch06.indd 45 11/26/10 2:30 PM Confirming Pages 46 Part One The Cosmic Landscape FIGURE 6.5 The portion of the Earth’s surface directly facing the Sun receives more concentrated light (and thus more heat) than other parts of the Earth’s surface. The same size “beam” of sunlight (carrying the same amount of energy) spreads out over a larger area where the surface is “tilted.” North Pole Summer A Equ ator Winter Full beam falls on A. Sunlight A Only portion of beam falls on A. The “tilted” surface receives less light and heats less. On June 21 at latitude 23.5°N, the Sun is straight overhead at noon—0° from the zenith. Because the relative angle of the Earth’s surface depends on the difference of latitudes, at 40°N the Sun is 16.5° (= 40° – 23.5°) from the zenith. On December 21, when the Sun is overhead at 23.5°S, the difference in latitudes places the Sun 63.5° from the zenith. Seasonal differences between the north and south are not caused by one hemisphere being closer to the Sun than the other hemisphere. sch12133_ch06.indd 46 meter. Where the surface is tilted at an angle to the Sun’s light, the same 1300 watts are spread out over a larger area on the ground, and each square meter of the Earth’s surface receives only a fraction as much energy. You take advantage of this effect instinctively when you warm your hands at a fire by holding your palms flat toward the fire. You also may have experienced the high temperature of pavement or a beach around noon, when the Sun is shining most directly on it, whereas the same surface will be cooler in the late afternoon, even though it is not shaded, when sunlight strikes it more obliquely. Because of the 23.5° tilt of the Earth’s axis, when the Earth is at the point of its orbit where the North Pole is most tipped toward the Sun, the Sun will pass straight overhead for someone at a latitude of 23.5°N (Figure 6.5). This occurs on about June 21 each year. Half a year later, the same is true at a latitude of 23.5°S. At latitudes between these, the Sun shines straight down at noon at other times of year. At regions farther north or south, the Sun can never be straight overhead, and the different angles of the Sun as the Earth orbits it can lead to strong differences in heating during the year. Figure 6.6 illustrates the difference between summer and winter at a latitude of 40°N. In late June the Sun gets most nearly overhead. Six months later the Sun’s light strikes the ground much more obliquely. The same size bundle of sunlight is spread out over a much larger area in winter, as shown in the bottom panel of Figure 6.6. The direct sunlight produces the strongest heating, whereas the large angle in December produces the least. From Figure 6.6 you can also see that this makes the seasons reversed between the Northern and Southern Hemispheres; when it is summer in one, it is winter in the other. An important point here is not to confuse the “directness” of the Sun’s light with one hemisphere being closer to the Sun. It is true that the Northern Hemisphere of the Earth is a few thousand kilometers closer to the Sun than the Southern Hemisphere during the northern summer. However, the effect of this difference in distance is tiny. Compared to the millions of kilometers of distance to the Sun, this difference in distance between the two hemispheres does not produce a change of even one-tenth of a degree in the temperature. By contrast, the differing angle at which the Sun shines on higher latitudes during the year changes the solar energy absorbed by the ground by a factor of two or more. 11/26/10 2:31 PM Confirming Pages Unit 6 The Year 47 Earth‘s orbit Earth on June 21 Earth on December 21 Sun North Pole Large angle between overhead and Sun Small angle between overhead and Sun Sunlight Eq North Pole Sunlight ua tor Equa tor December 21 June 21 Point exactly overhead —the Zenith Sun in summer —high in sky Summer Sun a sti le or at qu y sk in th ols pa r s n’s me Su um s on le Ce y sk in e th tic pa sols n’s r Su inte w on S Smaller summer area more heat for a given piece of ground warmer Winter Sun Sun in winter —lower in sky tic E e N Larger winter area less heat for a given piece of ground Cooler The summer and winter beams carry the same amount of energy, but spread that energy over very different amounts of ground. FIGURE 6.6 Why the Sun at noon is high in the sky in summer and low in the sky in winter. On June 21, from a latitude of 40°N, the noontime Sun is at an angle of just 16.5° (= 40° – 23.5°) from the zenith. On December 21, the Sun is at an angle of 63.5° (= 40° + 23.5°) from the zenith. 6.4 THE ECLIPTIC’S TILT 6. The tilt of the Earth’s rotation axis not only causes heating differences, it makes the Sun appear to move north and south on the celestial sphere. As discussed in Section 6.3, on June 21 when the Northern Hemisphere is tipped most toward the Sun, it passes straight overhead for someone at latitude 23.5°N. This means that the Sun lies 23.5° north of the celestial equator. In other words its declination is +23.5° (see Unit 5.5). Likewise, on December 21 the Sun’s declination is –23.5°. The Sun lies north of the celestial equator for half of the year and south of the celestial equator for the other half of the year. Another way of describing the Sun’s annual motion is that the Sun’s path—the ecliptic—must cross the celestial equator, and therefore the ecliptic must be tilted sch12133_ch06.indd 47 11/26/10 2:31 PM Confirming Pages 48 Part One The Cosmic Landscape Earth‘s position in its orbit at different times of year Sun‘s position on celestial sphere at start of each season North Pole Earth To Sun North celestial pole June 21 Sun on September 22 —on Cel. Eq. To Sun North Pole September 22 North Pole To Sun December 21 North Pole Sun on December 21 23.5º South of Cel. Eq. Sun on June 21 23.5º North of Cel. Eq. Ecliptic C Sun on March 20 elestia l Equ a t o r —on Cel. Eq. North Pole To Sun March 20 FIGURE 6.7 As the Earth orbits the Sun, the Sun’s position with respect to the celestial equator changes. The Sun reaches 23.5° north of the celestial equator on June 21 but 23.5° south of the celestial equator on December 21. The Sun crosses the celestial equator on about March 20 and September 22 each year. The times when the Sun reaches its extremes are known as the solstices; the times when it crosses the celestial equator are the equinoxes. (The dates can sometimes vary because of the extra day inserted in leap years.) The Sun’s position in the celestial sphere is shown in star charts by the curving line of the ecliptic—see the foldout star chart in the back of the book. A N I M AT I O N The Sun’s seasonal motion sch12133_ch06.indd 48 with respect to that line, as the sequence of sketches in Figure 6.7 shows. Because the Sun’s declination changes very slowly, its rising and setting is similar to that of any other star at the same declination. And just as the length of time a star spends above the horizon depends on its declination, the same is true of the Sun. The seasonal shifts of the Sun also define three kinds of regions on the Earth: the polar regions, the tropics, and, lying in between these, the temperate latitudes. The polar regions mark the latitudes where the Sun does not rise during some portion of the year. This occurs within 23.5° of the poles—north of 66.5°N, the Arctic Circle, and south of 66.5°S, the Antarctic Circle. The tropics lie between latitudes 23.5°S and 23.5°N, where the Sun passes directly overhead at some time during the year. The northern limit of tropical latitudes, 23.5°N, is called the Tropic of Cancer because the Sun reached its point farthest north when it was in the constellation Cancer at the time this term was defined. The southern limit is the called the Tropic of Capricorn after the constellation where the Sun was at its farthest south position. The Sun is no longer in these constellations at its northern and southern extremes because of precession (Section 6.5). The length of time the Sun is above the horizon is another critical heating factor during different seasons. When the Northern Hemisphere is tilted toward the Sun (between March 20 and September 22), the North Pole remains in sunlight continuously for six months. When the Sun is 23.5° north of the celestial equator on June 21, it is 66.5° from the north celestial pole. Therefore it becomes circumpolar for anyone north of latitude 66.5°N. For these arctic latitudes, you can therefore see 11/26/10 2:31 PM Confirming Pages Unit 6 The Year Concept Question 3 If the shape of the Earth’s orbit were unaltered but its rotation axis were shifted so that it had no tilt with respect to the orbit, how would seasons be affected? 49 the “midnight sun.” Half a year later, on December 21, the Sun is 23.5° south of the celestial equator, so the Sun does not rise in those northern arctic regions. At northern temperate latitudes, when the Sun is north of the celestial equator, there is sunlight for more than 12 hours each day. The days are longer the higher your latitude and the farther north of the celestial equator the Sun is. The situation reverses during the other half of the year and in the Southern Hemisphere. (Differences in the length of the day are examined in more detail in Unit 7.2.) Thus, because of the Earth’s tilted axis, in summer we receive more hours of sunlight and the Sun’s light strikes the surface more directly. 6.5 SOLSTICES AND EQUINOXES 6. The tilt of the ecliptic with respect to the celestial equator means that during the year, the points on the horizon where we see the Sun rise and set are not due east and west, except when the Sun is crossing the celestial equator. This occurs on two days of the year called the equinoxes, from the Latin for “equal night,” so named because the length of the night is approximately equal to the length of the day on those dates. The vernal equinox occurs near March 20, when the Sun is moving from the Southern Hemisphere of the celestial sphere into the Northern Hemisphere. Six months later the autumnal equinox occurs near September 22 as the Sun crosses the celestial equator on its way south. In the Northern Hemisphere, these dates mark the first day of spring and fall, respectively, but in the Southern Hemisphere this is reversed. On every other day of the year the Sun rises either north or south of due east in a regular, predictable fashion as illustrated in Figure 6.8A. From the vernal equinox to the autumnal equinox (during the Northern Hemisphere spring and summer, and the Southern Hemisphere fall and winter), the Sun rises in the northeast and sets in the northwest. During the rest of the year, the Sun rises in the southeast and sets in the southwest. Ancient peoples all over the world used the northward and southward journeys of the Sun to track the seasons. They built a variety of structures to detect the Sunrise direction June 21 (Summer solstice) March 20, September 22 (Equinoxes) December 21 (Winter solstice) December solstice Equinox June solstice East South North Solar observatory West A B FIGURE 6.8 (A) The direction of the rising and setting Sun changes throughout the year. At the equinoxes, the rising and setting points are due east and due west. The sunrise direction shifts slowly north from March until the summer solstice, after which it shifts back, reaching due east at the autumnal equinox. The sunrise direction continues moving south until the winter solstice, then reverses direction again back to the north. (B) The oldest known astronomical observatory in the Americas is found in Chankillo, Peru. This ancient observatory marked the shifting position of sunrise with a series of 13 towers along a ridge built about 2300 years ago. sch12133_ch06.indd 49 11/26/10 2:31 PM Confirming Pages 50 Part One The Cosmic Landscape North To summer sunrise B A FIGURE 6.9 Stonehenge. (A) Massive stones were erected by ancient Britons more than 4000 years ago to mark the changing position of the Sun. (B) Diagram showing how an observer in the stone circle would see the rising Sun framed by a pair of standing stones on the summer solstice. The exact dates of the equinoxes and solstices vary slightly from year to year, mainly because of differences in the calendar due to leap years, but also because of slight variations in the Earth’s orbit. Concept Question 4 During the course of the year, the sunset (and sunrise) position shifts. How should the amount of the shift depend on latitude? limits of the Sun’s motion. For example, Figure 6.8B shows an ancient observatory in Peru designed to track the Sun’s progress throughout the year. An even older structure built to mark these journeys is Stonehenge, the ancient stone circle in England (Figure 6.9A). Although its exact use in ancient times is lost to us, it appears that it was laid out so that seasonal changes in the Sun’s position could be observed by noting through which stone arches the Sun rose or set. For example, when the Sun reaches its farthest point north, an observer standing at the center of this immense circle of vertical stones would see the rising Sun framed by standing stones outside the main circle (Figure 6.9B). The Sun reaches its farthest point north or south on the celestial sphere, 23.5° from the celestial equator, midway between the equinoxes. At these times of year, the Sun pauses in its north–south motion and changes direction. Accordingly, these times are called the solstices, meaning the Sun (sol ) stops its northward or southward motion and begins to reverse direction. The winter solstice is particularly celebrated by many northern cultures with holidays and festivals, often symbolizing rebirth as this date marks the beginning of longer days. The solstices occur close to June 21 and December 21. In terms of celestial coordinates (Unit 5.5), the Sun is at a declination of 0° on the equinoxes, while it is at plus or minus 23.5° on June 21 and December 21, respectively. The Sun’s position at the moment it crosses the celestial equator on the vernal (March) equinox is used to define 0 hours (or 0h) for the right ascension system. On June 21 the Sun moves to a right ascension of about 6 hours, then 12 hours on September 22, then 18 hours on December 21, before returning to 0 hours of right ascension a year later (see Figure 6.7). The seasons “officially” begin on the solstices and equinoxes, with northern spring running from the vernal equinox to the solstice in June. Even though the longest day is on the first day of summer, the hottest period of the year occurs roughly six weeks later, as shown for four cities in Table 6.1. The delay, known as the lag of the seasons, results from the oceans and land being slow to warm up in summer. Similarly, there is about a six-week lag after the shortest day of the year until the coldest period of the year. TABLE 6.1 Monthly Average Temperatures in Four Cities (in Degrees Celsius) City Latitude Buenos Aires Boston Rome Sydney 34°S 42°N 42°N 34°S sch12133_ch06.indd 50 Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. 23.5 −2.2 7.1 22.1 22.7 −1.6 8.2 22.1 20.6 2.5 10.5 21.0 16.7 8.2 13.7 18.4 13.3 14.1 17.8 15.3 10.4 19.4 21.7 12.9 10.0 22.5 24.4 12.0 11.1 21.5 24.1 13.2 13.2 17.3 20.9 15.3 16.0 11.5 16.5 17.7 19.3 5.5 11.7 19.5 22.0 0.0 8.3 21.2 11/26/10 2:31 PM Confirming Pages Unit 6 The Year 51 6.6 PRECESSION 6. Precession is caused primarily by the Moon’s gravitational pull trying to “straighten out” the direction of the Earth’s spin. You may be amused to learn that horoscopes listed in newspapers are also incorrect by about 1 month. If you thought your “sign” was Cancer, then you are probably “a Gemini”; if Gemini, then Taurus; etc. FIGURE 6.10 Precession makes the Earth’s rotation axis swing around, slowly tracing out a circle in the sky, somewhat like a spinning top. Spinning top If you watch a spinning top, you will see that it “wobbles,” often more extremely as it slows down. That it wobbles is another way of saying that its rotation axis slowly shifts direction (Figure 6.8). The spinning Earth wobbles too, in a motion called precession. Precession occurs very slowly for the Earth. A single “wobble” takes about 26,000 years, but it has both interesting and important consequences. Currently the Earth’s North Pole points very close to the star Polaris. But this is only temporary. When the Egyptian pyramids were built 4000 years ago, the “North Star” was Thuban (meaning “the star”) in the constellation Draco (Figure 6.10; also see Looking Up #1 at the front of the book). In the future the axis will continue shifting direction past Polaris, and it will not point close to any bright stars for thousands of years. In about 7000 years the south celestial pole will be very close to a star slightly brighter than Polaris in the constellation Vela. In that future time there will be a “South Star.” In 12,000 years the rotation axis will have shifted so that the north celestial pole points fairly close to the bright star Vega. Then we will have a new, much brighter “North Star.” The changing direction of the Earth’s pole does not alter the Earth’s orbit, so the ecliptic and the constellations of the zodiac remain the same. However, it does change which constellation the Sun is in on the equinoxes and solstices. Several thousand years ago the Sun was in Cancer on the first day of summer, giving us the name “Tropic of Cancer” for the northernmost latitude where the Sun is ever directly overhead. Today it is in Gemini on the first day of summer. Because astronomers base the zero point of right ascension on the Sun’s position at the vernal equinox (Section 6.5), celestial coordinates change a little bit every year. Precession also slowly alters Earth’s climate. At this time we are closest to the Sun during the northern winter. In about 13,000 years we will be farthest from the Sun during the northern winter. This will make seasons in the Northern Hemisphere more severe at that time. Precession is suspected to be one of the components that affect long-term changes in climate, which may have triggered past ice ages. Deneb Spinning and precessing top CYGNUS AD 8000 CEPHEUS Alderamin Toward Vega Earth’s rotation axis slowly precesses to new direction. Toward Polaris LYRA Vega AD 14000 Eltanin North Pole in A.D. 14,000 0 North Pole now URSA MINOR Polaris Today Rastaban DRACO Kocab HERCULES Thuban 4000 BC sch12133_ch06.indd 51 11/26/10 2:31 PM Confirming Pages 52 Part One The Cosmic Landscape KEY POINTS • The Sun appears to shift position among the stars during the course of the year as the Earth orbits it. • The Sun’s path on the celestial sphere is called the ecliptic, and the 12 constellations it moves through are known as the zodiac. • The Earth’s spin axis is tilted 23.5° relative to its orbit around the Sun, so the ecliptic is tilted relative to the celestial equator. • Because of this tilt, the Northern Hemisphere receives sunlight at a more direct angle and for a longer period of time each day during half of the year. In the other six months the opposite is true. • The changing amount of solar heating causes the seasons, and they are opposite in the Southern Hemisphere. • The changing direction of sunrise and sunset throughout the year has been observed by peoples back to ancient times. • The direction of the spin of the Earth’s axis has been found to change very slowly over a 26,000-year period, an effect called precession. KEY TERMS Antarctic Circle, 48 rotation axis, 45 Arctic Circle, 48 solstice, 50 autumnal equinox, 49 Tropic of Cancer, 48 ecliptic, 44 Tropic of Capricorn, 48 equinox, 49 vernal equinox, 49 precession, 51 year, 43 revolve, 43 zodiac, 44 rotate, 45 CONCEPT QUESTIONS Concept Questions on the following topics are located in the margins. They invite thinking and discussion beyond the text. 1. Why the zodiac is divided into 12 parts. (p. 44) 2. Differences in northern and southern seasons. (p. 45) 3. Seasons if the Earth’s axis were not tilted. (p. 49) 4. The positions of sunset at different latitudes. (p. 50) REVIEW QUESTIONS 5. What is the ecliptic? What is the zodiac? 6. What causes the seasons? 7. When it is winter in Australia, what season is it in the United States? 8. Where is the Sun located on the celestial sphere during the equinoxes and solstices? 9. Why is the summer solstice not the hottest day of the year? 10. How does the Sun’s position on the horizon at sunset change through the course of the year? sch12133_ch06.indd 52 11. What effect does precession of the Earth’s rotation axis have on the Sun’s location in the zodiac? QUANTITATIVE PROBLEMS 12. Suppose the Earth’s axis were tilted by 10° instead of 23.5°. Where would the tropics and arctic regions be? How would seasons be different? 13. Suppose the Earth’s axis were tilted by 50° instead of 23.5°. Where would the tropics and arctic regions be? How would seasons be different? 14. Suppose the Earth’s axis were tilted by 90° instead of 23.5°. Where would the tropics and arctic regions be? How would the seasons be different? 15. Describe the motion you would see on the solstices and the equinoxes if you were observing the Sun from the Arctic Circle, at a latitude of 66.5°N. 16. If you wished to observe a star with a right ascension of 12h, what would be the best time of year to observe it? What would be the best time to observe a star with a right ascension of 6h? (Also see Unit 5.5.) TEST YOURSELF 17. From what location on Earth will the Sun always rise due east and set due west? a. The North Pole d. A latitude of 23.5°N b. The South Pole e. Nowhere c. The equator 18. On what day(s) of the year are nights longest at the equator? a. They are the same length throughout the year there. b. The solstices c. The equinoxes d. Around June 21 e. Around December 21 19. During winter in either hemisphere the temperature is lower because the Sun a. stops moving. b. is farthest south. c. doesn’t rise as high in the sky. d. has a lower temperature. e. is farther away due to the Earth’s eccentric orbit. 20. For someone in the Southern Hemisphere, which of the following is correct? a. The Sun rises in the west. b. The Sun rises in the southeast on December 21. c. Summer occurs when the Sun is rising lowest in the sky. d. The Sun is in the opposite sign of the zodiac than for an observer in the Northern Hemisphere. e. All of the above. 21. What is the slow shift of the position of the celestial poles? a. solstice d. equinox b. ecliptic e. year c. precession 11/26/10 2:32 PM