hw4_solutions - Homework #4: Solutions Astro 10, spring...

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Homework #4: Solutions Astro 10, spring 2010 General Notes to Graders: If numerical answers are roughly correct, do not take marks off. Award part marks if the student has made some progress with the question. Be generous. 1. [5 points] Estimating by eye, the angle covered by the star trails is roughly 30 degrees. In one sidereal day, the stars make one complete revolution around the North Celestial Pole, so 360 degrees corresponds to 23 hours and 56 minutes (one sidereal day). Therefore the exposure time is t exp = (1436 min) · 30 360 = 119 . 7 min This is a rough estimate though, so we will say that the exposure lasted about 120 min = 2 hours. Notes to graders: This is a rough estimate from a photograph, so any angle between 20 and 40 degrees is fine. This will result in answers between 1 hour, 20 min and 2 hours, 40 min. Answers within this range should receive full credit. Also, the students need not use the sidereal day to receive full credit - just 24 hours for 360 degrees is fine. 2. [5 points] If you see the quarter moon around sunset, it must be the first quarter moon. You can see this by looking at Figure 4-2 on page 66 in the textbook: the first quarter moon is directly overhead at 6:00 PM while the third quarter moon is directly overhead at 6:00 AM. Therefore, since you are seeing the first quarter moon, in a week (about one quarter of the lunar cycle) the full moon will be up. Notes to graders: Students must have a discussion equivalent to this in content for full credit. 3. [10 points] The way to think about this is to visualize the whole Moon phase cycle (see page 66 in text- book). If you were on the Moon while it was in its full phase, both the Earth and Sun would be in the same direction. Thus the Earth would be in a new phase from your perspective! . If you were on the Moon during its new phase, the Earth and Sun would be in opposite directions - therefore Earth would be in a full phase
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hw4_solutions - Homework #4: Solutions Astro 10, spring...

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