Is the earth's rotation slowing down?

Asked by: Mallory Thomson


Yes it is, due to a transfer of Earth's rotational momentum to the Moon's orbital momentum as tidal friction slows the Earth's rotation. That increase in the Moon's speed is causing it to slowly recede from Earth (about 4 cm per year), increasing its orbital period and the length of a month as well.

To picture what is happening, imagine yourself riding a bicycle on a track built around a Merry-go-Round. You are riding in the same direction that it is turning. If you have a lasso and rope one of the horses, you would gain speed and the Merry-Go-Round would lose some. In this analogy, you and your bike represent the Moon, the Merry-Go-Round is the rotating Earth, and your lasso is gravity. In orbital mechanics, a gain in speed results in a higher orbit.

The slowing rotation of the Earth results in a longer day as well as a longer month. Once the length of a day equals the length of a month, the tidal friction mechanism will cease. (ie. Once your speed on the track matches the speed of the horses, you can't gain any more speed with your lasso trick.) That's been projected to happen once the day and month both equal about 47 (current) days, billions of years in the future. If the Earth and Moon still exist, the Moon's distance will have increased to about 135% of its current value.

Answered by: Paul Walorski, B.A., Part-time Physics Instructor


The Earth's rotation is slowing down from rotational energy transfer to the moon's orbit through tides. The moon is very slowly increasing its orbital radius.

Every 18 months on the average, with variation, a leap second is added to planetary time keeping to keep the day consistent with atomic clocks and astronomical observations.

The next leap second will be at the end of December 2005. The one before that was December 1998.

Answered by: Uncle Al, Irvine, CA



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