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Monday, June 1, 2009

How Fast Is The Moon?


After I compared a hydrogen atom to the solar system, I got to wondering about how fast the Moon is traveling. I mean it takes a month for it to go all the way around the Earth, and whenever someone talks about the moon they say it is just hanging in the sky, like it isn't moving at all. But in that month of traveling, it has a long way to go. So, can you see it move?

I did some calculations (the Moon is about 240,000 miles from Earth and about 2,000 miles in diameter) and the Moon occupies about 1/2 of a degree of your field of vision, which is about the same as the width of your thumb at the end of your outstretched arm. The Moon travels about 12 degrees a day (360/30), or about 1/2 degree per hour. So the Moon should appear to move the width of your thumb (or its' own diameter) in about an hour.

So if you watch carefully for a few minutes you should be able to see some stars being occluded by the Moon, or you would if the light from the Moon wasn't overpowering the nearby stars. Comparing it to some fixed Earthbound landmark, like a tall building or an antennae wouldn't work because the velocity of the landmark across the sky is going to be much higher than the Moon's velocity. The picture at the top is a trick photograph. What you can actually see is probably very different.

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