Moon Over Mon Ami

"Hmmm...." I replied. "The Moon was new at Halloween, just over two weeks ago, so no."

She looked at me, exasperated. "How do you do that?" she asked.

I looked back at her. I'm used to it, I thought. I think about things like that a lot, so I get practice. But my wife, like most people, isn't used to thinking about how and when the Moon appears in the sky. But it's easy, once you know how.

The Earth spins on its axis about once every 24 hours. Now imagine the Moon did not orbit the Earth, but were somehow suspended above it, motionless. Someone standing on the Earth would see it go once around the sky every 24 hours, a reflection of the observer's own motion. They would see the stars moving at the same rate as well.

But the Moon does revolve around the Earth, taking about 29 days to make one orbit. This means that during the one day it takes the Earth to spin once, the Moon moves about 1/29 of the way around the Earth. To an observer on the Earth, this means the Moon is moving a bit faster than the apparent motion of the stars. This means the Earth has to spin just a bit more to "catch up" with the Moon. This in turn means the Moon rises by a bit less than an hour later every night.

So I knew that the Moon set just after sunset on Halloween. I also knew that more than two weeks had passed. So therefore the Moon would not set until about 14 hours later than sunset; in other words, it would not yet have risen at sunset! Sure enough, when we put our daughter in her bed, the Moon had not yet risen even though it had been dark out for a couple of hours.

One of the amazing things about astronomy is its predictability. Knowing just a little bit about a cycle can let you predict it for some time to come! Check out a newspaper to se when the Moon rises tonight, then check it again every day for a few days to see how much later it rises. At some point you'll see it rises with the Sun. That's when the Moon is new, and the cycle will start again.

The animation of the moon phases at the top of this page is courtesy of Ed Stephan who has done lots of interesting animations.