1. Discovering the Universe Ninth Edition Discovering the Universe Ninth Edition Neil F. Comins William J. Kaufmann III CHAPTER 1 Discovering the Night Sky CHAPTER 1 Discovering the Night Sky

4. WHAT DO YOU THINK? 1. Is the North Star—Polaris—the brightest star in the night sky? 2. What do astronomers define as constellations? 3. What causes the seasons? 4. When is Earth closest to the Sun? 5. How many zodiac constellations are there? 6. Does the Moon have a dark side that we never see from Earth? 7. Is the Moon ever visible during the daytime? 8. What causes lunar and solar eclipses?

5. In this chapter you will discover… how astronomers organize the night sky to help them locate objects in it. how astronomers organize the night sky to help them locate objects in it. that Earth’s spin on its axis causes day and night. that Earth’s spin on its axis causes day and night. how the tilt of Earth’s axis of rotation and Earth’s motion around the Sun combine to create the seasons. how the tilt of Earth’s axis of rotation and Earth’s motion around the Sun combine to create the seasons. that the Moon’s orbit around Earth creates the phases of the Moon and lunar and solar eclipses. that the Moon’s orbit around Earth creates the phases of the Moon and lunar and solar eclipses. how the year is defined and how the calendar was developed. how the year is defined and how the calendar was developed.

6. The Night Sky With and Without Light Pollution

7. The universe is huge, and the sizes and distances of objects in the universe vary greatly. Therefore we use scientific notation, which involves powers of ten notation to describe numbers much smaller or much greater than 1. Some common examples of powers of ten: POWERDECIMALNAMEMETRIC PREFIX 10 3 1000one thousandKilo 10 9 1,000,000,000one billionGiga 10 -6 0.000001one millionthmicro

8. THE SCALES OF THE UNIVERSE The range of objects we study are from the extremely small subatomic particles, to objects which are gigantic, such as a galaxy or the size of the known universe itself. Each division up the line indicates an increase in size by 100,000.

9. What Have Astronomers Discovered in Our Universe?

10. In order to more easily locate objects in the sky, we divide the sky into regions named after familiar patterns of stars called constellations. Ancient constellations were imaginary pictures outlined by familiar patterns of stars. Modern astronomers divide the sky into 88 official constellations or regions of space, many of which contain the ancient star patterns.

11. Some Common Guides to Finding Constellations Using the “Big Dipper” as a guide

12. The “Winter Triangle”

13. The “Summer Triangle”

14. Astronomers describe the universe as an imaginary sphere surrounding the Earth on which all objects in the sky can be located, called the CELESTIAL SPHERE.  As viewed from Earth, the celestial sphere appears to rotate around two axis points, the north and south celestial poles, which are located directly above the Earth’s poles.  Between these is the celestial equator, which divides the celestial sphere into northern and southern hemispheres.  We define the position of an object on the celestial sphere using two coordinates, right ascension and declination.

15. Cyclic motions of the Sun and stars in our sky are due to motions of Earth. 1.ROTATION = the spin of Earth on its axis. It takes one day for Earth to complete one rotation. 2.REVOLUTION = the movement of Earth in orbit around the sun. It takes one year for Earth to complete one revolution. 3.PRECESSION = the slow conical (top-like) motion of Earth’s axis of rotation. It takes 26,000 years for Earth to complete one cycle of precession.

16. The Big Dipper The angular distance between the two “pointer stars” at the front of the Big Dipper is about 5°.

17. Estimating Angles with the Human Hand Various parts of the adult human hand extended to arm’s length can be used to estimate angular distances and angular sizes in the sky.

18. Circumpolar Star Trails The stars near the poles of the celestial sphere (shown here) move in trails that circle the pole and never set. They are called circumpolar.

19. The apparent westward motion of the Sun, Moon, and stars across our sky each day is caused by Earth’s rotation. At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising at some point along the eastern horizon. Then, they appear to arc across the sky. Finally, they disappear somewhere along the western horizon. We generalize this motion to make statements such as, “The Sun rises in the east and sets in the west.”

20. We can see how different stars appear at different times of day by looking at the position of the Sun against the backdrop of stars. The side of Earth facing the Sun is experiencing “day,” while the side of Earth turned away from the Sun is experiencing “night.” SEPTEMBER MARCH

21. Motion of Stars at the Poles Because Earth rotates around its poles, stars seen from these locations appear to move in huge, horizontal circles. This is the same effect you would get by standing up in a room and spinning around; everything would appear to move in circles around you. At the North Pole, stars move left to right, while at the South Pole, they move right to left.

22. Rising and Setting of Stars at the Equator Standing on the equator, you are perpendicular to the axis around which Earth rotates. As seen from there, the stars rise straight up on the eastern horizon and set straight down on the western horizon.

23. Rising and Setting of Stars at Middle Northern Latitudes Unlike the motion of the stars at the poles, the stars at all other latitudes do change angle above the ground throughout the night. This time-lapse photograph shows stars setting. The latitude determines the angle at which the stars rise and set.

24. Earth also revolves around the Sun, which changes our view of the stars. From our perspective, the Sun appears to move through the stars along a special path called the ecliptic. From an outside view, we see Earth revolve around the Sun. We define the plane of Earth’s orbit as the ecliptic plane.

25. Seasons are caused because Earth’s axis is tilted, and as Earth revolves around the Sun, different parts of Earth receive more direct sunlight (summer), whereas other parts of Earth receive sunlight that is more spread out (winter).

26. The seasons we experience are linked to the motion of the Sun along the celestial sphere. The point of the Sun’s path farthest north on the celestial sphere is called the summer solstice (JUN 21), whereas the point of the ecliptic farthest south is called the winter solstice (DEC 21). The two points on the ecliptic where the Sun crosses the celestial equator are called equinoxes. During the vernal equinox (MAR 21), the Sun is moving north, while during the autumnal equinox (SEPT 21), the Sun is moving south. Remember that the seasonal names of the equinoxes and the solstices refer to seasons in the NORTHERN hemisphere. The seasons occurring in the SOUTHERN hemisphere are exactly opposite.

27. The Sun’s Daily Path and the Energy It Deposits Here On the winter solstice―first day of winter,―the Sun rises farthest south of east, it is lowest in the noontime sky, stays up the shortest time, and its light and heat are least intense (most spread out) of any day of the year in the northern hemisphere. (b) On the vernal equinox―first day of spring―the Sun rises precisely in the east and sets precisely in the west. Its light and heat have been growing more intense, as shown by the brighter oval of light than in (a). (c) On the summer solstice―first day of summer― the Sun rises farthest north of east of any day in the year, is highest in the sky at noontime, stays up the longest time, and its light and heat are most intense of any day in the northern hemisphere. (d) On the autumnal equinox, the same astronomical conditions exist as on the vernal equinox.

28. The Midnight Sun This time-lapse photograph was taken on July 19, 1985, at 69° north latitude in northeastern Alaska. At that latitude, the Sun is above the horizon continuously from mid-May until the end of July.

29. Different parts of the world experience different times of day as Earth rotates. TIME ZONES can be used to calculate the time of day in any given part of the world.

30. Gravitational forces of the Sun and the Moon pulling on Earth as it rotates cause Earth to undergo a top-like motion called precession. Over a period of 26,000 years, Earth’s rotation axis slowly moves in a circular motion.

31. This precession causes the position of the North Celestial Pole to slowly change over time. Today, the North Celestial Pole is near the star Polaris, which we call the “North Star.” However, in 3000 BC, Thuban was close to the North Celestial Pole and in 14,000 AD, Vega will be in this location.

32. Another familiar cycle is the lunar cycle. When the Moon orbits Earth, the amount of the side facing Earth that is lit changes, creating the Moon’s phases. This phase cycle is called the synodic period and is 29½ days long.

33. One common misconception is that the Moon is only visible at night. However, the time of day in which the Moon is in our sky varies depending on its phase. This picture clearly displays the Moon, visible during the day.

34. A synodic month is the time it takes for the Moon to orbit Earth with respect to the Sun and is 29½ days long. A sidereal month is the time it takes for the Moon to orbit Earth with respect to the stars and is 27.3 days long. The two times are different because Earth moves in its orbit around the Sun as the Moon moves in its orbit around Earth.

35. During a new or full moon phase, when the Moon, Sun, and Earth are aligned, the Moon may enter the shadow of Earth, or the shadow of the Moon may reach Earth, creating eclipses. However, these eclipses do not occur during every full or new moon because the Moon’s orbit is tilted by 5  with respect to the Earth- Sun (ecliptic) plane.

36. PENUMBRAL = the Moon appears dimmed. PARTIAL = part of the Moon enters the umbra of Earth’s shadow and is darkened. TOTAL = all of the Moon enters Earth’s shadow and becomes a reddish color, only lit from light bending around Earth’s atmosphere.

37. During a total lunar eclipse, the Moon moves in and out of the umbra of Earth’s shadow.

38. If you are located where the umbra of the Moon’s shadow reaches, you will see a total solar eclipse, during which the entire disk of the Sun is covered by the Moon, revealing the faint solar corona surrounding the Sun.

39. Unlike lunar eclipses, solar eclipses occur at specific places on Earth, indicated by the arrow.

40. Eclipse Paths for Total and Annular Eclipses 2001–2020 This map shows the eclipse paths for the 14 total solar and 13 annular eclipses that occur between 2001 and 2020. In each eclipse, the Moon’s shadow travels along the eclipse path in a generally eastward direction across Earth’s surface.

41. Sometimes eclipses occur when the Moon is too far away from Earth to completely cover the Sun in our sky. When this occurs, the Moon appears in the center and a thin ring, or “annulus,” of light surrounds it. These are called annular eclipses.

42. Summary of Key Ideas

43. Sizes in Astronomy Astronomy examines objects that range in size from the parts of an atom (  10  15 m) to the size of the observable universe (  10 26 m). Astronomy examines objects that range in size from the parts of an atom (  10  15 m) to the size of the observable universe (  10 26 m). Scientific notation is a convenient shorthand for writing very large and very small numbers. Scientific notation is a convenient shorthand for writing very large and very small numbers.

44. Patterns of Stars The surface of the celestial sphere is divided into 88 unequal areas called constellations. The surface of the celestial sphere is divided into 88 unequal areas called constellations. The boundaries of the constellations run along lines of constant right ascension or declination. The boundaries of the constellations run along lines of constant right ascension or declination.

45. Earthly Cycles The celestial sphere appears to revolve around Earth once in each day-night cycle. In fact, it is the Earth’s rotation that causes this apparent motion. The poles and equator of the celestial sphere are determined by extending the axis of rotation and the equatorial plane of Earth out onto the celestial sphere.

46. Earthly Cycles Earth’s axis of rotation is tilted at an angle of 23½° from a line perpendicular to the plane of Earth’s orbit (the plane of the ecliptic). This tilt causes the seasons. Equinoxes and solstices are significant points along Earth’s orbit that are determined by the relationship between the Sun’s path on the celestial sphere (the ecliptic) and the celestial equator.

47. Earth’s axis of rotation slowly changes direction relative to the stars over thousands of years, a phenomenon called precession. Precession is caused by the gravitational pull of the Sun and Moon on Earth’s equatorial bulge. Earth’s axis of rotation slowly changes direction relative to the stars over thousands of years, a phenomenon called precession. Precession is caused by the gravitational pull of the Sun and Moon on Earth’s equatorial bulge. The length of the day is based upon Earth’s rotation rate and the average motion of Earth around the Sun. These effects combine to produce the 24-hour day upon which our clocks are based. The length of the day is based upon Earth’s rotation rate and the average motion of Earth around the Sun. These effects combine to produce the 24-hour day upon which our clocks are based. Earthly Cycles

48. The phases of the Moon are caused by the relative positions of Earth, the Moon, and the Sun. The Moon completes one cycle of phases in a synodic month, which averages 29½ days. The phases of the Moon are caused by the relative positions of Earth, the Moon, and the Sun. The Moon completes one cycle of phases in a synodic month, which averages 29½ days. The Moon completes one orbit around Earth with respect to the stars in a sidereal month, which averages 27.3 days. The Moon completes one orbit around Earth with respect to the stars in a sidereal month, which averages 27.3 days. Earthly Cycles

49. Eclipses The shadow of an object has two parts: the umbra, where direct light from the source is completely blocked; and the penumbra, where the light source is only partially obscured. The shadow of an object has two parts: the umbra, where direct light from the source is completely blocked; and the penumbra, where the light source is only partially obscured. A lunar eclipse occurs when the Moon moves through Earth’s shadow. During a lunar eclipse, the Sun, Earth, and the Moon are in alignment with Earth between the Sun and the Moon, and the Moon is in the plane of the ecliptic. A lunar eclipse occurs when the Moon moves through Earth’s shadow. During a lunar eclipse, the Sun, Earth, and the Moon are in alignment with Earth between the Sun and the Moon, and the Moon is in the plane of the ecliptic.

50. Eclipses A solar eclipse occurs when a strip of Earth passes through the Moon’s shadow. During a solar eclipse, the Sun, Earth, and the Moon are in alignment with the Moon between Earth and the Sun, and the Moon is in the plane of the ecliptic. A solar eclipse occurs when a strip of Earth passes through the Moon’s shadow. During a solar eclipse, the Sun, Earth, and the Moon are in alignment with the Moon between Earth and the Sun, and the Moon is in the plane of the ecliptic. Depending on the relative positions of the Sun, Moon, and Earth, lunar eclipses may be penumbral, partial, or total, and solar eclipses may be annular, partial, or total. Depending on the relative positions of the Sun, Moon, and Earth, lunar eclipses may be penumbral, partial, or total, and solar eclipses may be annular, partial, or total.

51. angle angular diameter (angular size) annular eclipse arc angle autumnal equinox celestial equator celestial sphere circumpolar star constellation declination degree diurnal motion eclipse path ecliptic equinox gravitation line of nodes lunar eclipse lunar phase north celestial pole partial eclipse penumbra penumbral eclipse precession precession of the equinoxes revolution right ascension rotation scientific notation sidereal month sidereal period solar corona solar day solar eclipse south celestial pole summer solstice synodic month terminator time zone total eclipse umbra vernal equinox winter solstice zenith zodiac Key Terms

52. WHAT DID YOU THINK? Is the North Star—Polaris—the brightest star in the night sky? Is the North Star—Polaris—the brightest star in the night sky? No. Polaris is a star of medium brightness compared with other stars visible to the naked eye.

53. WHAT DID YOU THINK? What do astronomers define as constellations? What do astronomers define as constellations? Astronomers sometimes use the common definition of a constellation as a pattern of stars. Formally, however, a constellation is an entire area of the celestial sphere and all the stars and other objects in it. Viewed from Earth, the entire sky is covered by 88 different-sized constellations. If there is any room for confusion, astronomers refer to the patterns as asterisms.

54. WHAT DID YOU THINK? What causes the seasons? What causes the seasons? The tilt of Earth’s rotation axis with respect to the ecliptic causes the seasons. They are not caused by the changing distance from Earth to the Sun that results from the shape of Earth’s orbit.

55. WHAT DID YOU THINK? When is Earth closest to the Sun? On or around January 3 of each year.

56. WHAT DID YOU THINK? How many zodiac constellations are there? There are 13 zodiac constellations, the least-known one being Ophiuchus.

57. WHAT DID YOU THINK? Does the Moon have a dark side that we never see from Earth? Half of the Moon is always dark. Whenever we see less than a full Moon, we are seeing part of the Moon’s dark side. So, the dark side of the Moon is not the same as the far side of the Moon, which we never see from Earth.

58. WHAT DID YOU THINK? Is the Moon ever visible during the daytime? The Moon is visible at some time during daylight hours almost every day of the year. Different phases are visible during different times of the day.

59. WHAT DID YOU THINK? What causes lunar and solar eclipses? When the Moon is crossing the ecliptic in the full or new phase, the shadows of Earth or the Moon, respectively, then fall on the Moon or Earth. These shadows on the respective surfaces are eclipses.