On Tue, 30 Nov 1999 you wrote:

> What takes a most time?
> - a revolution of the earth
> - a revolution of the moon
> - a rotation of the earth
> - a rotation of the moon

Well, the term "revolution" refers to the amount of time the objet takes
to complete one orbit. The earth orbits the sun in one year, and so the
revolution of the Earth takes one year.

The term rotation refers to the amount of time it takes a body to spin
around on its axis. For the Earth, this is one day.

The neat thing about the moon is that its revolution and rotation are
exactly the same, at nearly a month. This phenomenon is called "tidal
locking". Earth's gravity has interacted with the moon in a manner such
that the same face of the moon is always pointed toward the Earth.
Eventually, the moon's gravity will also cause the Earth's rotatino to
slow so that the same face of the Earth always faces the moon, but this is
billions of years into the future!

Thanks for writing.

Sincerely,
Kurtis Williams

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