physics question #3056



Matt, a 16 year old male from the Internet asks on November 21, 2005,

Q:

Is the orbit of the moon around earth a perfect circle? If not, when is the moon closest to us? What about farthest from us?

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the answer

Donald J. Barry answered on November 23, 2005, A:

The moon has a nearly circular orbit. The eccentricity (or the amount the orbit deviates from a perfect circle) is 0.05, so that is 5% off circular.

An orbital period of the moon occurs every month. At some point during the year a full moon occurs near the close approach (perigee) - approximately six months later a full moon occurs near the distant approach (apogee). The eccentricity is 5% but the change in distance is a little more - around 15%. There is a good illustration of this in this picture taken from the archives of October 8, 2004 spaceweather.com where the photographer took a picture with the same optics of the moon at the beginning and the third quarter in which the image of the first quarter was at perigee, and the image of the third quarter was close to climax. You can see the difference. If we could do this with the naked eye, two weeks apart - well, that's another story!

The other major consequence is that because the orbit is not perfectly circular, the moon moves about the earth faster and slower in its way - and its rotation (which is like the wheel of clock) does not match its revolution (which speeds up and slows down). Therefore, we on Earth see a Moon that switches back and forth over a month - it also switches the north and south slightly for reasons that are more distantly related. This is really brought home in an animated film of the moon at the Naval Observatory of the US.

Monthly you see the perigee / apogee height variation and also switches in the lunar libration. We actually see something like 64% of the lunar surface, everything said,  not 50% as we would see if we just looked at an exact hemisphere.

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