Discovering the Universe Ninth Edition

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

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

6. The Night Sky Without Light Pollution … and With

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: POWER DECIMAL NAME METRIC PREFIX 103 1000 one thousand Kilo 109 1,000,000,000 one billion Giga 10-6 0.000001 one millionth micro

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. • ROTATION = the spin of Earth on its axis. It takes one day for Earth to complete one rotation. • REVOLUTION = the movement of Earth in orbit around the sun. It takes one year for Earth to complete one revolution. • 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. Angular Distance (example from 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.” MARCH SEPTEMBER

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. Where is the Sun? • At noon, the Sun is in the constellation Leo. • In which constellation is it at 6 PM? • Leo • Taurus (3 constellations right of Leo) • Scorpius (3 constellations left of Leo) • Ophiuchus

26. 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).

27. 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.

28. 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 • 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.

29. 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.

30. Where on Earth is this? • Is this location North or South of the Arctic Circle?? • North • South • Precisely on the Arctic Circle • Cannot be determined

31. 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.

32. 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.

33. Which is NOT a reason for the seasons • Angle of sunlight at high noon • Distance of Sun from Earth • Number of hours of daylight • Tilt of the Earth’s Axis

34. 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.

35. Find your Birthday. Find your Sign. Is it what you thought?

36. 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.

37. Test your understanding • What fraction of the Moon is illuminated • by the Sun at any point in time? • A. All of it • 50% • None • Depends on the phase of the Moon

38. 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.

39. 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.

40. Test your Understanding • Which is longer, a sidereal month or • a synodic month? • Sidereal • Synodic • Both are equal • Depends on the month

41. 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. “Ecliptic” has to do with “eclipses.”

42. Lunar Eclipse: Moon passes through Earth’s Shadow PENUMBRAL =the Moon appears dimmed. PARTIAL =part of the Moon enters Earth’s umbra and is darkened. TOTAL =all of the Moon enters Earth’s umbra and becomes a reddish color, only lit from light bent by Earth’s atmosphere.

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

44. 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.

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

46. Test your understanding • At what phase of the Moon can • a solar eclipse occur? • Full • New • First or Last Quarter • Any; only the Sun is involved in • a solar eclipse

47. Solar Eclipse – Actual Scale

48. 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.

49. 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.

50. Test your understanding • About how often does a lunar eclipse occur? • Every month • Every 6 months • Once per year • Once in a blue moon