PHYS 1051/1061 Intro Astronomy Astronomy Today, 9th (and 8th) Ed. Week 4-5. Ch. 1 Disc 10-13 ) Ch. 1 Prob 4,6,7 Ch. 1 M-C 1-7 (2nd assignment from Ch. 1) Ch. 1 Discussion 10. How much of the Moon's surface is lit by the Sun at any one time? Why do we see different phases of the Moon? One-half of the Moon's surface is lit by the Sun at all times except during a lunar eclipse when the Earth blocks the Sun's light. We see phases because, as the Moon orbits the Earth, the lit portion moves into, and out of view. The lit hemisphere faces the Earth during a full Moon, but it is facing away from Earth during a new Moon. 11. What causes a lunar eclipse? A solar eclipse? Why aren't there lunar and solar eclipses every month? A lunar eclipse is caused by the shadow of the Earth falling on the Moon, while solar eclipses are caused by the shadow of the Moon falling on the Earth. The Moon's orbital plane is tilted by about 5 degrees relative to the Earth's orbital plane around the Sun. Consequently, some months the Moon appears to pass up to 5 degrees above or below the Sun instead of covering up the Sun. Similarly, the Moon can pass above or below the shadow of the Earth, thereby avoiding a lunar eclipse. 12. Do you think an observer on another planet might see an eclipse? Why or why not? Yes they could because other planets have moons which can cast shadows on the planets surface, or move into the shadow of that planet. We have images of the moon Phobos eclipsing the Sun as seen from Mars (by a rover). 13. What is parallax? Give an everyday example. Parallax is the apparent motion or shifting of an object cause by the motion or shifting of the observer. Driving by trees in front mountains, the trees will appear to shift relative to the mountains. Staring at your thumb with your right eye and then shifting to the left eye, the thumb will "jump" to the right relative to the background. Ch. 1 Problems [ NOT ASSIGNED: 3. Ophiuchus. Scorpius, Libra, and Sagittarius are also acceptable given the low precision of using Fig 1.15. If Stellarium is set to 10,000 AD, the VE is in Ophiuchus about 3.75 degrees from Scorpius. 8000yrs/26000 yrs = 0.31 of way around zodiac. 0.3x12~ 3.6 constellations. ] 4. Relative to the stars, through how many degrees, arc minutes, or arcseconds does the Moon move in a) 1 hour, b) 1 minute, c) 1 second? How long does it take for the Moon to move d) its own diameter? The Moon moves at 360/27.3 = 13.2 deg/day relative to the stars. This converts to a) 0.508 deg/hour, b) 0.0085 deg/min, c) 0.00014 deg/sec. But it's more convenient to use: a) 0.508 deg/hour, b) 0.508 arcmin/min, c) 0.508 arcsec/sec d) The Moon's angular diameter is 0.518 deg, so .518/.508 = 1.02 hrs. [ NOT ASSIGNED: 5 At what distance is an object if its parallax, as measured from either end of a 1000-km baseline, is a) 1 deg ANS: D = 1000 km * 360 / 2*pi*1 deg = 57,296 km (using book equation, assuming curved baseline) D = 57,294 km (using tan f'n, assuming straight baseline) Or let baseline B=LD in my small-angle formula A(deg)=57.3 LD/D. Then 1 deg = 57.3 B/D so D = 57.3 B/1 = 57.3 (1000km) = 57,300 km b) 1' ANS: 3.44x10^6 km (60x last answer) c) 1" ANS: 2.06x10^8 km (60x last answer) Using correct def of parallax you get: a) 28,700 km b) 1,719,000 km c) 1.03x10^8 km ] 6. Given the angular size of Venus is 55" when the planet is 45,000,000 km from Earth, calculate Venus's diameter (in km). A(deg) = 57.3 LD/D A('') = 206265 LD/D, so (55*D)/206265 = LD = 11999~12000 km. 7. The Moon lies 384,000 km from Earth, and the Sun lies 150,000,000 km away. If both have the same angular size as seen from Earth, how many times larger than the Moon is the Sun? Using similar triangles: 150,000,000/384,000 = LD_Sun/LD_Moon= 391 times ~ 400. Or, use the class notes which mention the magic number 400. Chap 1 Multiple Choice 1. (b) 2. (b) 3. (d) (actually, none of the above. Sun is in Sag and Capric in Jan.) 4. (a) 5. (c) waning phase 6. (a) 7. (a) (The book's answer, c, is wrong. Best answer is a: if the Moon's speed doubled, it's period would be halved, and the frequency of new moons would double. So during the 34day eclipse season the number of solar eclipses would approx double.) [NOT ASSIGNED: 8. (b and/or c) 9. (a) smaller parallax angle 10.(d) geometry ]