I was reading elsewhere how Aristarchus calculated the relative distance of the sun and the moon.

http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Aristarchus.html


It's surprisingly easy, but it brings up some questions. My astronomy is a little stale.

1) For 1/2 of a month, the moon is closer to the sun than the earth is -- correct?

2) Twice a month the moon is (more or less) the same distance away -- right?

3) At these 2 times, the moon is half-full?

4) For an observer on earth, for half the month the moon is above the horizon during the day -- yes?

5) If 4 is true, then why is the moon visible during the day only occasionally?

6) For Aristarchus to take his sightings, the moon not only had to be visible during the day, but visible when it was half-full. How often does this happen?

Thanks in advance.

Originally posted by postoak

4) For an observer on earth, for half the month the earth is above the horizon during the day -- yes?

Want to edit that bit?

Thanks Adam! I had to read that line 3 times before I saw my error! :D

5) It's true.

When the moon is new, it's up all day (though you can't see it, since it's new).

When the moon is full, it's up all night.

When it's half full, it's up half the day and half the night.

Since the days are longer (> 12 hours) in summertime (in the northern hemisphere), there are more chances to see the nearly-full moon during the day.

- Warren

I have real problems visualizing this sort of thing. It seems to me that the moon is new only for an instant each month -- when it is closest to the sun. So, for the rest of the half month that it is closer to the sun than the earth is, it should be visible during the day -- and yet it is rarely seen. Why is that?