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The Question

(Submitted April 21, 1998)

I am reading Sobel's Longitude , and she speaks of the Earth's rotation rate slowing as a result of tidal forces. At what rate is the Earth's rotation slowing down? What will be the result of this slowing, and how long will it take?

The Answer

The interaction of the Moon and the tides is pumping angular momentum out of Earth's spin and into the Moon's orbit.

Currently the day is lengthening by about 1.5-2 milliseconds per century.

This is thought to be higher than normal due to resonance frequencies in the slosh time of the current configuration of oceans (which changes with continental drift). If this were to continue forever, the Earth and Moon would end up tidally locked so that they kept the same faces towards each other throughout each day=month which would be about 50 of our current days long. However, the Sun will expand and incinerate the Earth well before that happens.

David Palmer
for Ask an Astrophysicist

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