The moon (Part II)

What: As a follow up to my last post, I wanted to post about why the moon cycles at all from new to quarter to full and back to new in 28 days. To get a sense of this myself I went out to Centennial Field the past three nights at 10pm, 11pm, and 12am to take photos. Below are the shots from 11pm on Tuesday, Wednesday, and Thursday nights. In New England, about half the days are good for viewing stars, so I was happy to get at least some shots of the moon between cloud breaks (Wednesday was easily the best, but it was super cold, last night the moon wasn't clear until the clouds broke around 12:20am).

Taken on Tuesday night at 11:03pm

Taken on Wednesday night at 10:58pm

Taken on Thursday at 11:00pm, moon hadn't risen yet

Two things come up in this. Apart from the lighting being totally different in each photo, note the different positions of the moon. This is true in two ways. First, the moon appears at a different bearing (direction on a compass) in each photo even though they were all taken within 5 minutes of each other on each night. Put another way, it takes a little longer than 24 hours for the moon to rotate a full 360o around the earth.

I remember that my calculations over a long period of time in Santa Barbara indicated that the time difference for the moon to get to the same bearing (not height, or altitude, but compass direction) in the sky was offset by about 52 minutes later each night. So each night the moon rises about 52 minutes later than the previous night.

For the moon to return to the same bearing in the sky at the same time of day/night, which is the same as completing a full revolution around the earth, it would take 24 hours (a full cycle) / 52 minutes, or approximately 28 days.