Chapter 15: Restless Realm: Oceans and Coasts
Feature Articles
Tides and the Length of the Day
by Stephen Marshak
Rocky Intertidal Zone
Credit: Kip Evans, NOAA
Because of friction in the water of the ocean, the sublunar bulge can't quite keep up with the movement of the Moon across the Earth. In fact, it lags behind the Moon by about 1degree. The Moon exerts a slight pull on the side of the bulge. This effect, along with the twice-daily collision between the tidal bulges and the edges of continents, acts like a brake that gradually slows the Earth's spin. As a result, throughout Earth history the length of a day has steadily increased by a rate or about 2% per hundred million years (0.002 seconds per century). Since the slowing of Earth's spin has no effect on the planet's rotation around the sun, however, the length of a year remains unchanged.
This slowdown may not seem like much, but remember that the Earth has been around for 4.6 billion years, and given long periods of time, the slowdown adds up. In the middle Devonian period, about 375 million years ago, days were about 21.9 hours long, and there were about 400 days in a year. This estimate has been confirmed by counting daily growth rings in fossil corals and clams.
As a spinning ice skater stretches out his arms, he slows down. Similarly, as the spinning Earth slows in its orbit, the Moon moves farther away, at about 4 cm per year (4 km every million years). In middle Devonian time, the Moon was about 1500 km closer, and at the beginning of the Archean (3.9 Ga), it may have been over 15000 km (about 10%) closer. This change would mean that the tidal reach was much larger earlier in Earth history, as has been confirmed by studying characteristics or ancient strata deposited in the intertidal zone.
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