These data demonstrate that today's regular ocean tides also happened deep in the past, says Lambeck, lending support to the prevailing moon formation theory of a collision between a primordial Earth and a Mars-size body 4.5 billion years ago. If the moon were instead formed elsewhere and later captured by Earth's gravity, the tides would not have held so steady throughout history, Lambeck says.
Most computer simulations of this explosive lunar genesis suggest that Earth whirled afterward on its axis every six hours, says Jay Melosh, an Earth and atmospheric scientist at Purdue University. As the moon has migrated out from an initial tighter orbit of perhaps 25,000 kilometersto the modern average distance of 384,000 kilometers, it has teamed up with the sun to protract Earthly days fourfold.
Early human record-keeping of solar eclipses has also bolstered the case for lengthening the planet's rotation time. Researchers have extrapolated orbital paths back 32 centuries, demonstrating that if Earth's rotation rate had not changed, eclipse shadows would have appeared thousands of kilometers from Chinese scribes who etched astronomical observations into animal bones. The math indicates that days now are 0.047 second longer than they were in 1200 B.C.
Arriving at this comparatively recent value required factoring in the changing shape of Earth itself due to the thawing of the last ice age, NASA's Williams says. As ice in northern latitudes began melting about 13,000 years ago, the ground rose underneath, making Earth rotate faster, rather like a spinning figure skater who pulls her arms above her head. This continuing post-glacial rebound shaves about 0.6 second off a day per century—not enough to trump tidal braking caused by the moon and sun.
Accordingly, over the eons separating ice ages, the length of Earth's day can waver. Even on a daily basis, as it were, day length is tweaked millionths of a second by shifting mass in the oceans due to windy weather and geologic shifts, such as the magnitude 8.8 Chilean earthquake that struck in February. And global warming is expected to shorten the day by 0.12 millisecond over the next two centuries by heating the oceans and changing the distribution of its mass.
As days dilate overall, "leap seconds" have to be added to official Universal Time, which is based an Earth's rotation, to keep it in sync with ultraprecise atomic clocks that are far more stable. Twenty-four leap seconds have accrued since 1972; the most recent was tacked on in December 2008.
Those leap seconds will not cease. "The moon will continue to go farther away and Earth will continue to slow down," Lambeck says, until Earth becomes tidally locked, meaning only one hemisphere of our planet will see the moon in the sky. (The moon is already tidally locked in its revolution around Earth, so we see the same lunar hemisphere at all times.) A single day on Earth could eventually exceed 1,000 hours, but a back-of-the-envelope reckoning has this happening in 50 billion years. By then, the oceans—the main source of tidal friction—will have long since evaporated, and Earth and the moon might be toast, gobbled up tens of billions of years earlier by the ballooning red giant sun.