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I encountered the following question in a previous year paper of a graduation-based test.

On a certain night the moon in its waning phase was a half-moon. At midnight the moon will be (choose one of the following)

  1. on the eastern horizon.
  2. at 45 degrees angular height above the eastern horizon.
  3. at the zenith.
  4. on the western horizon.

I remember looking out of my window and finding the moon on left and sometimes on right. I never imagined that it would be having a fixed pattern. This question is making me think otherwise. But I am unable to decide what that pattern is. Is the position of the moon really determine-able?

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Hi @Ramit. While this may not be real homework, according to our homework policy, it is to be tagged as such, because it is "homework-like". ' –  Dimensio1n0 Sep 14 '13 at 10:31
    
Ok, sure. That makes sense. –  Ramit Sep 14 '13 at 10:39

1 Answer 1

up vote 3 down vote accepted

The waning half moon is 3 weeks old, it rises at about midnight. All celestial objects appear to rise in the east and set in the west because of the rotation of the earth. Therefore, from your choices, the answer would be 1, on the eastern horizon.

I guess that doesn't fully answer your question, which is different from the question you reference. The moon rises about 48 minutes later each day. When the moon is full it rises as the sun sets because it is... full. 3 weeks later you have 21 days * 48 minutes = 1008 minutes or about 17 hours. If the sun sets at 6pm then the moon will rise about 17 hours later, and about an hour later, at midnight, it will be just above the eastern horizon. Hope that makes more sense :)

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I guess, instead of 'If the sun sets at 6pm,' you want to say it rises at 6am. or is it implied? If yes, then it would mean that we see sun for 12 hours each day. that's an new information if it's true. –  Ramit Sep 14 '13 at 10:30
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Hrm, I don't think so, did I get that wrong? I was saying that if the sun sets at 6pm, then if the moon were full it would be rising at the same time. 3 weeks later it would be starting to rise 17 hours later, and be just above the eastern horizon at midnight. –  Jessie Boone Sep 14 '13 at 10:33
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A new moon is when the sun and the moon are on the same side of the earth. Until it reaches a full moon it is waxing (getting bigger). At 1 week it is a waxing half moon. At 2 weeks it is a full moon. In 2 more weeks it will be a new moon again, until then it is waning (getting smaller) and at 3 weeks, halfway between the full and new moon, it is a waning half moon. –  Jessie Boone Sep 14 '13 at 10:45
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The relationship is that the earth rotates approximately once every 24 hours, as such, fixed objects in the sky would rise and set every 12 hours. Since none of them are actually fixed you have to account for their individual motions, as well as the precession of the earths rotation. It is a rule of thumb that the moon rises "about" 48 minutes later every 24 hours. You could get mathematically accurate here, but this is observational astronomy :) –  Jessie Boone Sep 14 '13 at 10:54
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patterns make the world go 'round - literally! –  Jessie Boone Sep 14 '13 at 10:58

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