1. At sidereal time 20 hours, RA=20 hours is on the meridian, suggesting that it was 4 hours ago that RA=16 hours was on the meridian. Hence an object with RA=16 hours is WEST of the meridian (ie it has already passed by the meridian).
2. On March 21 the sun is at RA=0; on June 21 it is at RA=6h so on July 21 the sun is at RA=8h. Hence at midnight the RA opposite the sun on the meridian (ie the sidereal time) is 8+12=20 hours. So at 10pm on July 21 the sidereal time is 18 hours (ie two hours earlier). A galaxy with coordinates RA=4h20m has an hour angle HA=ST-RA = 18h - 4h20m = 13h40m. Since this is so large, the object is clearly below the horizon.
3. Midnight (recall, the 3rd quarter moon leads the sun by 90 degrees in their westward diurnal motion, so the moon is on the eastern horizon when the sun is directly beneath us at midnight.
4. Waxing crescent of age 3 days has a phase angle of 3/29x360 = 37 degrees (there are 29 days in the synodic month). So 37 degrees separate the moon from the sun, with the sun to the west of the moon (since it is a waxing moon).
5. Precession of the earth's axis refers to the gradual circular motion executed by the Celestial poles as the axis of the earth's rotation also moves in a circle. The polar motion is such that the inclination of the axis to the ecliptic remains 23.5 degrees, so the circle described by the NCP in the sky has a radius of 23.5 degrees and is centered on the pole of the ecliptic. Similarly, the position of the vernal equinox moves around the ecliptic (a great circle) with the same 26000 year period (about 50 arcmin per year). Spring continues to occur on March 21 because our calendar is DEFINED so that March 21 occurs when the sun is on the vernal equinox, whether or not it has moved. The astrological sign of someone born at the end of march is always Aries, even though the constellation in which the sun appears at the end of march continually changes due to precession. In other words, western astrology does not account for the effects of precession (unlike, for example, Indian astrology which does).
6. The precession period is about 26000 years.
7. Since a complete precession cycle is 26000 years, then in 100 years the equinox moves 100/26000 x 360 degrees = 1.384
8. False. Although the sun and moon exert a force which tries to pull the earth's equatorial bulge into alignment with the ecliptic, because the earth is rotating, it instead executes precession, a motion in which the equatorial bulge is always inclined relative to the ecliptic, but the rotation axis slowly describes a circular motion.
9. A partial solar eclipse.
10. c = at new or full moon. Obviously, the shadow of either the moon must fall on the earth or visa versa. Thus, the sun moon and earth must all lie in a line. This will only occur at new or full moon.
11. Some sunlight is scattered as it passes through the earth's atmosphere near the earth's terminator. Some of this scattered light goes into the umbra. Hence the umbra isn't totally dark, and one can see the moon even when it enters the umbra.
12. Moon's orbit is inclined 5 degrees to the ecliptic. The nodes are the points in the moon's orbit where the moon crosses the ecliptic (there are two, the ascending node and the decending node, on opposite sides of the orbit). Obviously, for an eclipse to occurr, both the earth and moon must be in the ecliptic plane (since that is where the shadow is), and so since the earth is (by definition) in the ecliptic, the moon must be at one of its nodes.
13. False. The 26000 year period refers to the precession of the earths rotation axis. The period for the precession of the moon's orbital axis is 18.6 years --- significantly shorter.
14. False. They occur every 5.6 months -- ie in consecutive eclipse seasons. Remember, eclipses must occur when the sun and moon lie on the line of nodes. Although one might expect this to occur every 6 months, the precession of the moon's orbital plane causes the line of nodes to move around (regress) slowly, so that the true interval between eclipse seasons is more like 5.6 months instead of 6.
15. Never. Remember, planets only pass through the zodiac constellations and Orion is not one of them. Note that precession wont help here either, since that only changes the plane of the earth's equator, not the ecliptic plane.
16. True. Since they are inferior planets, they each have a maximum elongation which is less than 90 degrees (which is the elongation needed to make quadrature).
17. Recall that at greatest elongation, the direction of the earth to the planet is now tangent to its orbit (assumed circular). Hence we have a right angled triangle with the earth sun side as the hypotenuse. Since this has length 1 AU, then the distance from the sun to the planet is just sin 33 = 0.544 AU.
18. False. Although it is 90 degrees for the angle Mars-Earth-Sun, since Mars is on an outer orbit, the angle Earth-Mars-Sun (which defines the phase of the planet) is significantly more than 90 degrees. Hence Mars is in a gibbous phase.
19. b = Saturn. The planet is at opposition, and hence must be superior relative to Mars. Saturn is the only one in the list.
20. False. This is confusing sidereal period with synodic period. Clearly, the sidereal period is 7 months. But in general this will NOT be the synodic period, which is what is needed to find the period between one greatest elongation and the next one.
21. False. They each undergo retrograde motion near opposition, which is approximately once per year (since they are so far away they hardly move much during the year anyway). Hence they go through retrograde motion equally often, and about once per year.
22. Westwards (ie it is in retrograde motion). Planets usually move eastwards, except near opposition when they move "backwards" in the sky, which is westwards.
23. Saturn is big and rotates fast so its equatorial regions experience significant centrifugal force compared to higher latitudes. This pulls Saturn into an oblate spheroid. Since Saturn is not as massive or dense as Jupiter, it has a higher oblateness than Jupiter (though noticing this is tricky with the rings present).
24. False. Recall, as temperature increases the thermal spectrum peaks further into the blue part of the spectrum. Hence star colors following the rainbow sequence (red, orange, yellow, green, blue) are successively hotter.
25. a. luminosity. recall that temperature comes from color which is independent of distance. Apparent magnitude is just the visible brightness. Composition is measured from the spectral absorption lines (bar code) which is independent of distance.
26. 0.2 arcsec. Recall that at 1pc, the parallax angle is 1 arcsec; at 2pc it is 1/2 arcsec. So at 5pc it is 1/5 arcsec.
27. d. Main sequence stars are happily consuming hydrogen in their cores.
28. c. The HR diagram plots luminosity (y) against color (x).
29. b. Via the p-p chain, overall four protons emerge as one helium nucleus (with the ejection of 2 positrons and 2 neutrinos, and energy).
30. c. The sun is a mid-size star, and so it cannot create heavier elements than carbon and oxygen (from helium). Hence it will die in a planetary nebula phase, ejecting its envelope, to leave its cooling intert hot small core -- ie a white dwarf star.
31. a. Iron is the element out of which no more energy can be extracted. Hence, in a massive star, it builds up in the core as the "ultimate ash". This sets the stage for gravitational collapse to make a neutron star or black hole in a supernova explosion.
32. d. Shapley used globular clusters because they were in the halo and so weren't subject to obscuration by dust in the galactic plane.
33. b. The problem with judging the sun's position in the milky way disk is that the disk contains "dust" (tiny particles of carbon and silicates) which prevents us from seeing clear across the galaxy. Hence we are misled into thinking we're at the center.
34. Have a look at the sketch in Lecture 6 web notes, section C.
35. In general, one measures the mass of things using size and rotation. For galaxies, one finds that stars and gas continue to rotate quite quickly irrespective of how far out of the galaxy one goes. This tells us that even outside the visible edge of galaxies (including the milky way) there is yet more mass. Since this is "outside" the visible galaxy, the mass is "dark". Hence (along with other lines of evidence), we conclude galaxies are surrounded by large halos of dark matter.
36. False. Ursa Major is close to the North Celestial Pole, and is therefore permanently below the horizon for viewers in Australia (it is in their northern circumpolar zone).
37. The sun is clearly 15 magnitudes brighter (difference between -26 and -11). Since each 5 magnitudes is a factor of 100 in brightness, then the sun is 100x100x100 = 106 times brighter.
38. Radio, microwave, Infra-red, optical, ultra-violet, xray, gamma-ray.
39. The angular resolution of a telescope refers to its ability to see detail --- and that detail is measured as a (small) angle --- usually in arcseconds. While telescopes on the ground can rarely see better than about 1 arcsecond (due to the atmosphere), in space one can see more clearly, and for the HST its maximum detail is more like 0.1 arcsec.
40. Light gathering power is proportional to the area of the primary mirror which is itself proportional to the square of its diameter. Since 8 inches is 20cm or 0.2 meters, then the ratio of light gathering powers is (2.5/0.2)2 = 156. So the Hubble is 156 times faster at gathering light than the 8 inch Meades. (Notice that the transparency of the atmosphere is NOT a big factor here --- in the optical bands, the atmosphere doesn't cut down the light by much -- though it does cause image blurring).
41. Since the primary optical element is the same in this question, we simply need to realise that magnification is inversely proportional to the focal length of the eyepiece (bigger focal length eyepieces give smaller magnifications). So the switch between 30 and 20 leads to an INCREASE in the magnifying power by 30/20 = 1.5. Similarly, with the increase in the magnification comes a DECREASE in the field of view by the same factor.
42. TWO. The primary concave mirror reflection followed by the tilted FLAT secondary mirror. Remember, for the Newtonian focus, the image is brought out at the side of the telescope up at the top using a small flat mirror angled at 45 degrees .
43. The Nadir is the point opposite the zenith --- so it is 180 degrees away. Since there are 60 arcminutes in a degree, there are 180x60=10800 arcminutes separating the zenith and the nadir.
44. Counter clockwise. Remember, facing north, east is on your right and so the "rising in the east, setting in the west" means objects on your right are rising, and on your left are setting. Hence counter clockwise. In the southern hemisphere, you are lookin south to the SCP, in which case east is on your left and west on your right, which leads to the OPPOSITE (clockwise) motion when you look at the SCP.
45. 47 degrees (=2x23.5 degrees). Remember that the altitude of the noon sun (up off the southern horizon) is just the altitude of the celestial equator plus the declination of the sun. Since the sun moves from -23.5 degrees dec in midwinter to +23.5 degrees in midsummer, and since the altitude of the celestial equator is fixed for any latitude (and is equal to 90-lat) then the DIFFERENCE between the midwinter and midsummer sun is just 23.5 - (-23.5) = 47 degrees, independent of where you are. [A detail : if you are between the tropics of cancer and capricorn --- ie at latitudes between -23.5 and +23.5 then at some point approaching summer the sun goes through the zenith and so further into the summer the sun actually has LOWER altitude. For example on the equator, the celestial equator goes through the zenith, and so the midwinter sun has altitude +66.5 (on the southern meridian) and in midsummer is has altitude +66.5 (on the northern meridian). ]
46. Since Hour Angle = Sidereal time - right ascension, then we simply have 6hr - 3hr 30m = 2hr 30m. Since this is positive the object is in the west. (this makes sense, remember the sidereal time is equal to the right ascension on the meridian, and since this is 6hr, and since right ascension increases to the east, then an RA of 3hr 30 is WEST of RA 6hr by 2hr 30m. Since the sun is on the vernal equinox (RA=0) on March 21, then we know that on March 21 midnight has RA=12hr on the meridian, so it is sidereal time 12hr on that day. We want sidereal time of 6hr at midnight, so we want 3 months before (since ST advances by 2hr per month), which is dec 21 (close to the winter solstice).
47. First quarter moon is 90 degrees east (left) of the sun, so it is high in the sky when the sun is setting. That is why in evening twighlight, if it is near first quarter, the moon is sitting way high up in the sky.
48. Consider the months starting at New moon. In the current situation, the moon reaches its full orbit one sidereal month later (27.3 days) and needs to move an ADDITIONAL 1/12 of its orbit to get back to new, so the synodic month is longer by a couple of days (29.5 days). Now, if the moon went the other way around the earth, it would arrive at New moon BEFORE it had completed its full sidereal orbit, and it would need to move an additional 1/12 of an orbit more to get back to its initial sidereal alignment. Hence the synodic month would the SHORTER than the sidereal month by a couple of days, say around 25 days. Thus we would have 365/25=14.6 months in the year.