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Name: Unknown
Status: Other
Age: N/A
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Date: Prior to 1993

I have heard that the moon and earth are slowing in their respective rotations due to the gravitational attraction between them. Does this mean that an earth day was shorter in the past?

Yes, in fact the angular momentum of the earth-moon system remains constant, but tidal forces cause a kind of friction that slowly slows the earth's rotation. In order to conserve angular momentum, the moon's orbit is slowing increasing its diameter. It is interesting that we (humans) exist right now because some millions of years ago the moon would have been closer and looked larger and total solar eclipses would not have been as spectacular as they are now. Similarly, some millions of years in the future, the moon will be further away, will look smaller, and there will never again be a total solar eclipse, only annular eclipses. We just happen to be living during the relatively small period of time when the moon and sun have the same apparent size as seen from the earth!

John Hawley

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