Visible Sky – Review for the Test
You may use a calculator, planisphere, star charts, and an atlas or map of the world to assist you as you answer the following:
1. (a) What is the difference between a constellation and an asterism? (b) Name an asterism that is contained within a constellation. (c) Name an asterism consisting of stars that lie in different constellations.
2. (a) What prominent star is located near declination +90°? (b) What is unique about the appearance of this star for an observer located somewhere north of the Earth’s equator?
3. How is the Celestial Equator related to the Earth’s Equator?
4. What is the brightest star that ever passes within a few degrees of the zenith for an observer in Rio de Janeiro (location on earth = 23° S, 44°W)?
5. (a) What fraction of the celestial sphere is visible to an observer on earth at any one time? (b) What factor(s) determine which fraction is visible?
6. Kochab is a circumpolar star in the constellation Ursa Minor. (a) For an observer in Knoxville how will Kochab appear to move in the sky if it is observed for several hours over the course of a single night? (b) What is the real motion that is causing this apparent motion?
7. Alnilam is a star in Orion’s “belt” that is almost exactly on the celestial equator. (a) Determine the precise amount of time that this star is above the horizon during any given calendar date. (b) Using the planisphere, what night(s) of the year could you actually observe Alnilam rise and set if you watched it all night?
8. Suppose you set up a telescope with a rigid mount and it points in a fixed direction. At exactly 11:30 p.m. you observe the star Deneb exactly in the center of the telescope’s view. Assume the telescope is not moved. (a) At what time, the next evening, will Deneb be centered in the telescope’s view? (b) How many days must pass until Deneb is once again closest to the center of view of the telescope at the same time 11:30 p.m.?
9. For an observer at 40° N September 10, 10 p.m. mean solar time, determine the most prominent constellations at (a) the zenith, (b) just above the east horizon, (c) just above the west horizon, (d) just above the north horizon, and (e) just above the south horizon.
10. At what mean solar time for an observer at 40° N does the star Arcturus in Bootes rise on November 25? Will it rise at an earlier or later time on November 26?
11. (a) Determine the approximate sidereal time at midnight mean solar time on June 22. (b) Determine the approximate date on which 6 a.m. mean solar time would be equal to 6h sidereal time?
12. (a) The Sun appears to come closest to what bright star in Leo? (b) On what day of the year is it closest to this star? (c) For an observer at 40°N, determine the approximate mean solar time of sunrise and sunset on this day. (d) Convert these times to standard time for an observer in Knoxville.
13. Astrologers assume that the Sun is in each of the 12 constellations of the zodiac for equal amounts of time. (a) If this were true, how many days would the Sun spend in each “sign”? (b) In reality the Sun does not spend equal amounts of time and there are 13 constellations through which the ecliptic passes. In which constellations does the Sun spend the least and the greatest amount of time?
14. Explain what aspects of the Earth’s actual motion cause the apparent motion of the Sun on the celestial sphere.
15. Explain why our calendars and the leap year system (i.e. the Gregorian calendar) is based on the tropical year instead of the sidereal year.
16. Consider the difference between the tropical year and the sidereal year. (a) What causes these values to be different? (b) During a period of precisely 25800 tropical years, how many times will the Earth have orbited the Sun?
17. Find the star Zubenelgenubi in the constellation Libra – it is the bright star very close to the ecliptic. Judging by its position on the ecliptic, during what year in the future (approximately) will the Vernal Equinox be closest to Zubenelgenubi?
18. Given that a synodic month is 29.53 days, what is the phase of the Moon for each of the following ages: (a) 7.3825 days, (b) 25 days, (c) 14.765 days, (d) 13 days.
19. (a) What type of eclipse(s) can occur only at the New Moon phase? (b) What type of eclipse(s) can occur only at the Full Moon phase?
20. Suppose the Moon reaches a point in its orbit such that it forms a right triangle with the Sun and the Earth. If the Sun and Earth form the hypotenuse of this right triangle, what phase is the Moon as seen from Earth? (A sketch should help you with this one.)
21. When the Moon is in the gibbous phase is it nearer or farther the Sun than the Earth? Explain.
22. Suppose you observe the Moon one night and it is in a waxing crescent phase. On the following night at the same time: (a) How will its appearance (i.e. phase) be different? (b) How will its location relative to the horizon be different?
23. Explain why an eclipse does not occur every time the Moon completes an orbit of the Earth.
24. Does an eclipse occur every time the Moon crosses the ecliptic? Explain
25. Suppose a person viewing a solar eclipse happens to be on the line joining the center of the Sun and the center of the Moon. In such a circumstance the viewer can see either a total eclipse or an annular eclipse. What determines which type will be seen?
26. A solar eclipse is usually followed by a lunar eclipse that occurs during the same orbit of the Moon. Based on this fact and your knowledge of phases and ages, approximately how many days will pass between successive solar and lunar eclipses?
Answers:
1. (a) constellation is officially recognized by astronomers and is a well defined region – an asterism is an informal pattern of stars without boundaries. (b) The “teapot” is contained in Sagittarius. (c) The “Summer Triangle” spans Cygnus, Aquila, and Lyra.
2. (a) Polaris (the North Star) (b) It always appears in the same spot in the sky and the other stars appear to rotate about it. Also its altitude will equal the observer’s latitude.
3. The celestial equator is the contained in the same plane as the Earth’s equator.
4. Antares
5. (a) half (b) The date, time, latitude, and (to a lesser extent) the longitude determine what part of the celestial sphere is visible.
6. (a) Kochab will appear to circle Polaris in a counterclockwise direction. (b) This appearance is due to the Earth’s rotation about its axis.
7. (a) 11 hours, 58 minutes (b) December 18th would be the best time.
8. (a) 11:26 p.m. (b) 365 days later
9. (a) Cygnus, (b) Cetus, (c) Serpens (Caput), (d) Ursa Major, (e) Microscopium
10. (a) 3 a.m. (b) It rises a few minutes earlier on the next day.
11. (a) 18h (b) September 22
12. (a) Regulus, (b) August 25, (c) 5:15 a.m. to 6:45 p.m., (d) 6:51 a.m. to 8:21 p.m.
13. (a) 30.4 days, (b) Scorpius (least) and Virgo (greates)
14. Earths motion in its orbit around the Sun make it appear to move west to east relative to the stars. The tilt of the Earth’s axis explains why the ecliptic curves north and south relative to the celestial equator.
15. The tropical year is the time for a cycle of seasons and it is desirable for the seasons to repeat during the same dates each year. If the sidereal year was used the seasons would “move” through the months.
16. (a) The Earth’s precession causes the point of intersection of the ecliptic and the equator to move westward relative to the stars. (b) 25799
17. 11700 A.D.
18. (a) First Quarter, (b) Waning Crescent, (c) Full Moon, (d) Waxing Gibbous
19. (a) Partial, Total, or Annular Solar Eclipse (b) Partial, Penumbral, or Total Lunar Eclipse
20. It is first or last quarter.
21. At gibbous it is farther than the Earth from the Sun because it is nearing the full phase at which point it is on the opposite side of the Earth from the Sun.
22. (a) The crescent will have grown in width. (b) It will be higher in altitude and farther east.
23. The Moon’s orbit is tilted relative to the orbit of the Earth and so in most cases the Moon does not align perfectly with the Earth and the Sun.
24. No, an eclipse does not occur every time the Moon crosses the ecliptic – it must “meet” the Sun at that point or be 180° on the opposite side of the ecliptic. In other words it must be New or Full at the time it crosses the ecliptic in order for an eclipse to occur.
25. If the Moon is far enough away and the Earth is close enough to the Sun then the Sun’s apparent diameter is greater than the Moon’s apparent diameter and an annular eclipse occurs – otherwise it will be a total eclipse.
26. About 14 days (the time between New and Full or vice versa)