Chapter 2 Discovering the Universe for Yourself 2.3 Short Answer Questions

2.3 Short Answer Questions Choose the letter for the real motion of space from the list below that is responsible for the apparent motion of space as seen from Earth. A. Earth rotates once each day. B. Earth revolves around the Sun once each year. C. The direction of Earth's axis in space precesses with a period of 26,000 years. D. Stars appear to move randomly in the local solar neighborhood. E. The universe is expanding. 1) Polaris will no longer be the North Star 1,000 years from now. Answer: C 2) In the year A.D. 15,000, Vega will be a better north star than Polaris. Answer: C 3) The Big Dipper will look different 100,000 years from now than it does today. Answer: D 4) The Moon rises in the east and sets in the west. Answer: A 5) The stars of Orion's belt rise in the east and set in the west. Answer: A 6) A million years from now, Alpha Centauri will no longer be the nearest star system to our own. Answer: D 7) If Earth's axis had no tilt, would we still have seasons? Why or why not? Answer: We would no longer have seasons, because the Sun's light would hit at the same angle all throughout the year, depending only on where you lived. The slight change in distance between Earth and the Sun during the year would not produce much of an effect. 12 8) Consider the following statement, and explain whether or not it is sensible: If you had a very fast spaceship, you could travel to the celestial sphere in about 100 years. Answer: This statement does not make sense because the celestial sphere is a concept and not a physical object. 9) Consider the following statement, and explain whether or not it is sensible: When I looked into the dark fissure of the Milky Way with my binoculars, I saw what must have been a cluster of distant galaxies. Answer: This statement does not make sense because we cannot see through the band of light we call the Milky Way to external galaxies; the dark fissure is gas and dust blocking our view. 10) Why does the Milky Way appear as a band of light in the sky? Answer: The solar system lies in the outer parts of the thin disk of a spiral galaxy. Thus when we look along the plane of the disk, we see large numbers of stars that, to the naked eye, merge into a band of light. When we look out of the plane of the disk, there are very few stars and the night sky is much darker. 11) Consider the following statement, and explain whether or not it is sensible: Although all the known stars appear to rise in the east and set in the west, we might someday discover a star that will appear to rise in the west and set in the east. Answer: This statement does not make sense. The stars aren't really rising and setting, they only appear to rise in the east and set in the west because the Earth rotates. 12) At what altitude and in what direction in your sky does the north or south celestial pole appear? Answer: Answers will vary with your latitude; latitude = altitude of NCP (or SCP in Southern Hemisphere). 13) Consider the following statement, and explain whether or not it is sensible: My sign is Ursa Major because the Sun was in Ursa Major when I was born. Answer: Not sensible: The Sun appears only in the constellations of the zodiac–and Ursa Major is not one of these. 14) Consider the following statement, and explain whether or not it is sensible: Last night I saw Jupiter in the constellation Ursa Major. Answer: This statement does not make sense because Jupiter, like all the planets, is always found very close to the ecliptic in the sky. The ecliptic passes through the constellations of the zodiac, so Jupiter can appear to be only in one of the 12 zodiac constellations–and Ursa Major is not one of these. 13 15) Answer each of the following questions for our local sky. A. Where is the north celestial pole in our sky? B. Is Polaris a circumpolar star in our sky? Explain. C. Describe the meridian in our sky. D. Describe the celestial equator in our sky. Answer: A. Answers will vary with latitude; here is a sample for 40°N: The north celestial pole appears at an altitude of 40°, in the direction due north. B. Yes, for any location in the Northern Hemisphere; no, for any location in the Southern Hemisphere. Polaris is circumpolar because it never rises or sets in our sky. It makes a daily circle, less than 1° in radius, around the north celestial pole. C. The meridian is a half-circle that stretches from the due south point on the horizon, through the zenith, to the due north point on the horizon. D. Answers will vary with latitude; here is a sample answer for 40°N: The celestial equator is a half-circle that stretches from the due east point on the horizon, through an altitude of 50° due south, to the due west point on the horizon. 16) Consider the following statement, and explain whether or not it is sensible: If you lived on the Moon, you'd see full Earth when we see new Moon. Answer: This is true, because at full Moon, Earth lies between the Sun and the Moon. Thus, an observer on the Moon would be looking at the night side of Earth. 17) Suppose you lived on the Moon near the center of the face that we see from Earth. During the phase of full Moon, what phase would you see for Earth? Would it be day or night at your home? Answer: During the full Moon, it would be daytime and you would see the phase of new Earth. 18) Suppose you lived on the Moon near the center of the face that we see from Earth. During the phase of new Moon, what phase would you see for Earth? Would it be day or night at your home? Answer: During the new Moon, it would be nighttime and you would see the phase of full Earth. 19) Suppose you lived on the Moon near the center of the face that we see from Earth. At what phase of the Moon would you see sunset? What phase of Earth would you see at this time? Answer: Sunset would occur at the Moon's first-quarter phase. You would see Earth in thirdquarter phase at this time. 20) Suppose you lived on the Moon near the center of the face that we see from Earth. At what phase of the Moon would you see sunrise? What phase of Earth would you see at this time? Answer: Sunrise would occur at the Moon's third-quarter phase. You would see Earth in firstquarter phase at this time. 21) What would you see if you were on the Moon during a lunar eclipse? Answer: During a lunar eclipse, you would see Earth pass in front of the Sun. It would be completely dark where you were. 14 22) Why is the Moon not completely invisible (it appears as a very deep red color) to the naked eye during a total lunar eclipse? Answer: The Moon shines through reflected light from the Sun and thus it becomes very dark during a lunar eclipse since the Moon lies within Earth's shadow at this time. However, some sunlight still gets through because it is bent (similar to the way a lens works) by Earth's atmosphere. We see the reflection of this faint light and thus the Moon is not completely invisible. (The bending of light is called refraction and the effect is strongest at long wavelengths. Thus it is most pronounced for red light and the eclipsed Moon appears dark red.) 23) What would you see if you were on the Moon during a solar eclipse? Answer: During a solar eclipse, you would see a small circular shadow traveling across a portion of Earth's surface. 24) Suppose the distance to the Moon were twice its actual value. Could we still have solar eclipses? If so, what type(s)? Answer: If the Moon were twice its actual distance from us, we would no longer be able to see total solar eclipses because the Moon would not be able to completely cover the surface of the Sun; however, we would still see partial and annular eclipses, although the Moon would not block as much of the Sun during these times. 25) Consider the following statement, and explain whether or not it is sensible: Last night I saw Mars move westward through the sky in its apparent retrograde motion. Answer: This statement does not makes sense because the apparent retrograde motion is noticeable only over many nights, not during a single night. (Of course, like all celestial objects, Mars moves from east to west over the course of every night.) 26) If, from the point of view of Earth-bound observers, the Moon is in new Moon phase on a particular day, what phase is Earth as seen from the Moon? Answer: full Earth 27) Process of Science: Your friend hypothesizes that the phases of the Moon are produced by Earth's shadow being cast on the Moon's surface. Devise an experiment to prove your friend wrong. Describe an observation you will make (time of day/night, location in sky) and describe what you will see that will clearly demonstrate that your friend's idea cannot be correct. Answer: If you observe any time when the Moon and Sun are both in the sky (e.g., in the daytime during a crescent Moon), you can clearly see that Earth's shadow cannot be cast on the Moon, as it is on the other side of Earth, where the Moon is not. Similarly, an observation of the full Moon shows the opposite: no Earth shadow at all, though the alignment would favor one. If the full Moon occurred when Earth's shadow could not hit it, it should be seen in the daytime, not at night–and then it would have no light source. 28) Process of Science: How could you show that the seasons depend on the tilt of the Earth rather than the distance the Earth is from the Sun? Answer: You could fly across the equator to see that there can be winter in one hemisphere at the same time there is summer in the other. 15