Fact or Fiction: The Days (And Nights) Are Getting Longer

Fossilized corals, lasers beamed at a receding moon, Chinese artifacts, and other evidence have revealed that over the ages the length of time it takes Earth to spin once on its axis has increased significantly.

The summer solstice that falls this year on June 21 marks the longest day of the year in the Northern Hemisphere, sunlight-wise. Almost imperceptibly, however, Earth's day–night cycle—one rotation on its axis—is growing longer year by year, and has been for most of the planet's history.

Forces from afar conspire to put the brakes on our spinning world—ocean tides generated by both the moon and sun's gravity add 1.7 milliseconds to the length of a day each century, although that figure changes on geologic timescales. The moon is slowly spiraling away from Earth as it drives day-stretching tides, a phenomenon recorded in rocks and fossils that provides clues to the satellite's origin and ultimate fate. "You're putting energy into the moon's orbit and taking it out of the Earth's spin," says James Williams, a senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The moon's gravity generates tides by pulling hardest on the side of Earth facing it. This attraction causes the planet to bulge, especially in its malleable oceans. (The sun affects tides in the same way, although in comparison due to its great distance they amount to only about a fifth of the lunar influence on our planetary pirouette.) Earth rotates faster than the moon orbits it, so the watery tidal bulge travels ahead of the moon's relative position. This displaced mass gravitationally tugs the moon forward, imparting energy and giving the satellite an orbital boost, whereas friction along the seafloor curbs Earth's rotation.

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