Astronomy: The Solar System and Beyond 5th edition

1. Astronomy:The Solar System and Beyond 5th edition Michael Seeds

2. Even a man who is pure in heart and says his prayers by night may become a wolf when the wolfbane blooms and the moon shines full and bright. - Proverb from old Wolfman movies Chapter 3

3. Your life is shaped by the cycles of the sky. The rotation of Earth on its axis defines the day and its revolution around the sun defines the year. The orbital motion of the moon across our sky defines the month and divides it into four weeks.

4. Rotation is the turning of a body on its axis. Revolution is the circular motion of a body around a point outside the body. You should be careful to say that Earth rotates once a day on its axis and revolves once a year around the sun. Furthermore, you can say the moon revolves once a month as it orbits Earth.

5. The sun seems to rise and set because Earth rotates on its axis. That is only the most obvious of the solar cycles. The Cycle of the Sun

6. Even in the daytime, the sky is filled with stars, but the glare of sunlight fills our atmosphere with scattered light, and you can only see the brilliant sun. If the sun were fainter, you would be able to see it rise in the morning surrounded by stars. Earth’s rotation causes both the sun and the stars to appear to move westward across the sky. So, at the end of the day, you would see the sun set among the same group of stars. The Annual Motion of the Sun

7. If you were to watch the sun carefully, you would also see it move a short distance eastward relative to the other stars as the day progressed. This is due to Earth’s orbital motion around the sun. As Earth moves counterclockwise along its orbit, the sun seems to shift slowly eastward in the sky. The Annual Motion of the Sun

8. The Annual Motion of the Sun • In January, you would see the sun in front of the constellation Sagittarius. • As Earth moves along its circular orbit, the sun appears to move eastward among the stars. By March, it is in front of Aquarius.

9. The apparent path of the sun against the background of stars is called the ecliptic. If the sky were a great screen, the ecliptic would be the shadow cast by Earth’s orbit. That is why the ecliptic is often called the projection of Earth’s orbit on the sky. The Annual Motion of the Sun

10. Earth circles the sun in 365.25 days and, consequently, the sun appears to circle the sky in the same period. That means the sun, traveling 360°around the ecliptic in 365.25 days, travels about 1°eastward each day, about twice its angular diameter. You don’t notice this motion because you cannot see the stars in the daytime, but the motion of the sun has an important consequence that you do notice: the seasons. The Annual Motion of the Sun

11. The Seasons • The seasons result from a simple fact: Earth’s axis of rotation is tipped 23.5°from the perpendicular to its orbit.

12. As you study the cycle of the seasons, you will notice two important principles. One, the seasons are not caused by any variation in the distance from Earth to the sun. Earth’s orbit is nearly circular, so it is always about the same distance from the sun. The Seasons

13. Two, the seasons are caused by the changes in solar energy that Earth’s northern and southern hemispheres receive at different times of the year. Because of circulation patterns in Earth’s atmosphere, the northern and southern hemispheres are mostly isolated from each other and exchange little heat. When one hemisphere receives more solar energy than the other, it grows rapidly warmer. The Seasons

14. The ancient superstition of astrology is based on the cycle of the sun. You have probably heard of the zodiac, an astronomical term used in astrology. The zodiac is a band 18 degrees wide centered on the ecliptic. The Seasons

15. The Seasons • The signs of the zodiac take their names from the 12 principal constellations along the ecliptic.

16. Astrology was once an important part of astronomy, but the two are now almost exact opposites. Astronomy is a science that depends on evidence, whereas astrology is a superstition that survives in spite of evidence. Thus, the signs of the zodiac are no longer important in astronomy. The zodiac itself is of interest only because it is on the path followed by the planets as they move around the sky. The Seasons

17. The planets of our solar system produce no visible light of their own. You see them by reflected sunlight. Mercury, Venus, Mars, Jupiter, and Saturn are all easily visible to the naked eye. Uranus is usually too faint to be seen, and Neptune is never bright enough. Pluto is even fainter, and you need a large telescope to find it. The Motion of the Planets

18. All the planets of the solar system move in nearly circular orbits around the sun. If you were looking down on the solar system from the north celestial pole, you would see all the planets moving counterclockwise through their orbits, with the planets farthest from the sun moving the slowest. The Motion of the Planets

19. When you look for planets in the sky, you will always find them near the ecliptic, because the planes of their orbits lie in nearly the same plane as Earth’s orbit. As Mars, Jupiter, and Saturn orbit the sun, they appear to move eastward along the ecliptic. Mars moves completely around the ecliptic in slightly less than two years, but Saturn, being farther from the sun, takes nearly 30 years. The Motion of the Planets

20. Venus and Mercury follow slightly different paths in the sky. They are never seen far from the sun because their orbits are inside Earth’s orbit. So, instead of making a circuit of the ecliptic like Mars, Jupiter, and Saturn, Venus and Mercury move alternately eastward and westward along the ecliptic as they orbit the sun. The Motion of the Planets

21. The Motion of the Planets • They appear at times above the western horizon just after sunset or above the eastern horizon just before sunrise.

22. The Motion of the Planets • Venus is easier to locate because its larger orbit carries it higher above the horizon than does Mercury’s orbit. • Mercury’s orbit is so small that it can never get farther than 27°50’ from the sun. Consequently, it is hard to see against the sun’s glare, and is often hidden in the clouds and haze near the horizon.

23. By tradition, any planet visible in the evening sky is called an evening star, even though planets are not stars. Similarly, any planet visible in the sky shortly before sunrise is called a morning star. Perhaps the most beautiful is Venus, which can become as bright as magnitude –4.7. The Motion of the Planets

24. As Venus moves around its orbit, it can dominate the western sky each evening for many weeks. However, eventually it’s orbit carries it back toward the sun, and it is lost in the haze near the horizon. In a few weeks, it reappears in the dawn sky, a brilliant morning star. The Motion of the Planets

25. Building Scientific Arguments • If Earth had a significantly elliptical orbit, how would its seasons be different? • If Earth had an elliptical orbit, perihelion would occur in July and aphelion in January.

26. At perihelion, Earth would be closer to the sun, and the entire surface of Earth would be a bit warmer. If that happened in July, it would be summer in the northern hemisphere and winter in the southern hemisphere, and both would be warmer than they are now. It could be a dreadfully hot summer in Canada and southern Argentina could have a mild winter. Building Scientific Arguments

27. Six months later, at aphelion, Earth would be a bit farther from the sun and if that occurred in January, winter in northern latitudes could be frigid. Argentina, in the southern hemisphere, could be experiencing an unusually cool summer. Building Scientific Arguments

28. Of course, this doesn’t happen. Earth’s orbit is nearly circular, and the seasons are caused not by a variation in the distance of Earth from the sun but by the inclination of Earth in its orbit. Nevertheless, Earth’s orbit is slightly elliptical. Earth passes perihelion about January 4 and aphelion about July 4. Although Earth’s oceans tend to store heat and reduce the importance of this effect, this very slight variation in distance does affect the seasons. Building Scientific Arguments

29. Now, use your scientific argument to analyze the seasons. Does the elliptical shape of Earth’s orbit make your winters warmer or cooler? Building Scientific Arguments

30. The Cycles of the Moon • The moon orbits eastward around Earth once a month. • When you begin looking for the moon, you may not seeit if it is a cloudy night or if the moon is in the wrong part of its orbit. • However, keep trying on successive evenings and, within a week or two, you will see the moon.

31. The Cycles of the Moon • Watch for the moon on following evenings: you will see it following its orbit around Earth and cycling through its phases as it has done for billions of years.

32. The Motion of the Moon • If you watch the moon night after night, you will notice two things about its motion. • First, you will see it movingeastward against thebackground of stars. • Second, you will notice thatthe markings on itsface don’t change. • These two observations willhelp you understand themotion of the moon and theorigin of the moon’s phases.

33. The moon moves rapidly among the constellations. If you watch the moon for just an hour, you can see it move eastward against the background of stars by slightly more than its angular diameter. The moon is about 0.5°in angular diameter, so it moves eastward a bit more than 0.5°per hour. The Motion of the Moon

34. In 24 hours, it moves 13°. Each night when you look at the moon, you see it about 13°eastward of its location the night before. This eastward movement is the result of the motion of the moon along its orbit around Earth. The Motion of the Moon

35. The Cycle of Phases • The changing shape of the moon as it orbits Earth is one of the most easily observed phenomena in astronomy. • Everyone has noticed the full moon rising dramatically and seen a thin crescent moon hanging in the evening sky.

36. When you study the phases of the moon, you will notice three important points. First, the moon always keeps the same side facing Earth. The ‘man in the moon’ is produced by familiar features on the moon’s near side, and the far side is never visible from Earth. The Cycle of Phases

37. The Cycle of Phases • Second, the changing shape of the moon, as it passes through its cycle of phases, is produced by sunlight illuminating different parts of the side of the moon youcan see.

38. Third, there is a difference between the orbital period of the moon around Earth, and the length of the lunar phase cycle. That difference is a good illustration of how your view from Earth is produced by the combined motions of Earth and other heavenly bodies, such as the sun and moon. The Cycle of Phases

39. The Cycle of Phases • We always see the same side of the moon looking down on us, but the changing shadows make the ‘man in the moon’ shift his moods as the moon cycles through its phases. • Occasionally, something peculiar happens, and the moon darkens and turns copper-red in a lunar eclipse.

40. Lunar Eclipses • A lunar eclipse can occur at full moon if the moon moves through the shadow of Earth. • Because the moon shines only by reflected sunlight, you see the moon gradually darken as it enters the shadow.

41. Lunar Eclipses • Earth’s shadow consists of two parts. • The umbra is the region of total shadow. In the umbra of Earth’s shadow, you would see no part of the sun. • If you moved into the penumbra, however, you would be in partial shadow and would see part of the sun peeking around the edge of Earth. Thus, in the penumbra, the sunlight is dimmed but not extinguished.

42. Lunar Eclipses • In about an hour, the moon reaches the umbra, and you see the umbral shadow darken part of the moon. • It takes about an hour for the moon to enter the umbra completely and become totally eclipsed.

43. Lunar Eclipses • The period of total eclipse, totality, may last as long as 1 hour 45 minutes, though the timing of the eclipse depends on where the moon crosses the shadow.

44. Lunar Eclipses • When the moon is totally eclipsed, it does not disappear completely. • Although it receives no direct sunlight, the moon in the umbra does receive some sunlight that is refracted (bent) through Earth’s atmosphere.

45. Lunar Eclipses • If you were on the moon during totality, you would not see any part of the sun because it would be entirely hidden behind Earth. • However, you would be able to see Earth’s atmosphere illuminated from behind by the sun. The red glow from this ‘sunset’ illuminates the moon during totality and makes it glow coppery red.

46. Lunar Eclipses • If the moon passes a bit too far north or south, it may only partially enter the umbra, and you see a partial lunar eclipse. • The part of the moon that remains outside the umbra in the penumbra receives some direct sunlight. The glare is usually great enough to prevent you from seeing the faint coppery glow of the part of the moon in the umbra.

47. Lunar Eclipses • A penumbral lunar eclipse occurs when the moon passes through the penumbra, but misses the umbra entirely. • Because the penumbra is a region of partial shadow, the moon is only partially dimmed. A penumbral eclipse is not very impressive.

48. Although there are usually no more than one or two lunar eclipses each year, it is not difficult to see one. You need only be on the dark side of Earth when the moon passes through Earth’s shadow. That is, the eclipse must occur between sunset and sunrise at your location. Lunar Eclipses

49. As you live on planet Earth, you can see a phenomenon that is not visible from most planets. It happens that the sun is 400 times larger than our moon and, on the average, 390 times farther away. So, the sun and moon have nearly equal angular diameters, of 0.5°. Thus, the moon is just the right size to cover the bright disk of the sun and cause a solar eclipse. Solar Eclipses

50. Solar Eclipses • If the moon covers the entire disk of the sun, you see a total eclipse. • If it covers only part of the sun, you see a partial eclipse.