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Slowing the spin of the Earth

The moon, Earth's tides and gravitational friction. Illustration by Patterson Clark Comparing day length. Illustration by Patterson Clark

The moon's gravity deforms the Earth, causing bulges in land and sea (tides). As the Earth turns, it rotates those bulges away from the moon, but the moon's gravity pulls back on the bulges. This is called gravitational friction, which slows down the planet's rotation ever so slightly.

Days grow longer as the minuscule braking adds up over hundreds of millions of years:

How much does the Earth slow down every year?

AN ANALOGY TO PUT THINGS INTO PERSPECTIVE:

Imagine that the 2,445-mile distance between Washington and San Francisco represents today's day length of 24 hours.

200 million years ago, an analagous distance for day length would stretch only from San Francisco to near the West Virginia/Virginia border.

Every 100 years, the length of a day increases by 0.002 seconds — or 3.23 inches farther down the road to Washington. The annual gain is an infinitesimal 0.00002 seconds, which on our analogous cross-country trip would amount to 0.82 millimeters, a little more than the thickness of a thumbnail.

Rotating Earth

Moon's gravity pulls back on bulge displaced by the rotating Earth

Axis of bulge

Gravitational friction

Axis of moon's gravity

Tidal bulge (greatly exaggerated)

Moon

Moon's gravity causes tides

Distances not to scale

Baker, W.V.

D.C.

D.C.

102 miles

S.F.

S.F.

24 hours

23 hours

DAY LENGTH

200 million
years ago

Today

SOURCE: NASA.