the angular momentum of the earth, moon system is conserved. due to friction caused by the tides, earth is slowing down, so the angular momentum of the moon has to inrease. But how? The gravitational field of the earth shows (nearly) a radial symmetry, so where are the forces that accelerate the moon?


Date: Tue Aug 17 10:58:41 1999
Posted By: Jeff Robertson, Faculty, Physical Sciences, Arkansas Tech University
Area of science: Astronomy
ID: 934481747.As---- Message:

The key is what you wrote yourself.

"The earth shows (nearly) a radial symmetry"

Do not underestimate the word nearly.  Since both the earth and
the moon are not spherically symmetric, neither are their
gravitational fields.  This is where the torques come from
to drive angular momentum changes. The rotation of the earth 
and moon early in their formation caused them to become oblate,
not perfect spheres.

Millions of years ago, total solar eclipses were longer, 
millions of years from now we may not have any total eclipses.
The moon will be too far away and its angular size smaller,
providing only annular eclipses.

Is the Moon moving away from the Earth? Edit

Yes it is, but very slowly. The current rate at which the Earth day is increasing is 0.0018 seconds/century. The semi-major axis of the lunar orbit is increasing by 3.8 centimeters/year according to laser ranging measurements made since the 1970's using the Apollo 'corner cube reflectors' deposited on the surface by the astronauts.

It is expected that in 15 billion years, the orbit will stabilize at 1.6 times its present size, and the Earth day will be 55 days long equal to the time it will take the Moon to orbit the Earth. Of course, in less than 7 billion years, the Sun will have evolved into a red giant star and engulfed the Earth-Moon system, thereby incinerating it!


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