physics question #4854



Amanda, a 14 year old female from New Lebanon, Ohio asks on December 17, 2009,

Q:

How are tides and eclipses related?

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the answer

Donald J. Barry answered on December 21, 2009, A:

There's no particular relation, apart from the normal moon/earth/tide relationship, but since solar eclipses occur in a narrow path and are vivid enough events that they tend to get recorded by civilizations with written languages which have survived over thousands of years, we have records of eclipses going back 2500 years. And because we know *where* the eclipses were observed to be total (a narrow path), we can figure out quite precisely how the moon and earth were aligned at that moment.

The relation to the tides is that in raising them, the moon both slows the earth's rotation, and also gets a little tug that transfers energy to its orbit, raising it. The measurement of that evolution of the lunar orbit and the rotation rate of the earth was first done over long timescales through historical records of eclipses. (e.g., F. Richard Stephenson, "Historical Eclipses and the Earth's Rotation, Cambridge University Press, 1997), or the earlier article that I read as an undergraduate, "Historical Eclipses", Scientific American v247, Oct 1982, p. 170-176.

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