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.
Williams has studied how fast the moon is corkscrewing away by shining lasers from Earth at prism-shaped reflectors placed on the lunar surface in the late 1960s and early 1970s by U.S. astronauts and unmanned Russian probes. Changes in the beam's round-trip time reveal the moon's recession rate—3.8 centimeters per year—which, largely due to the orientation of Earth's landmasses and its effect on oceanic sloshing, is faster now than in previous epochs, Williams says.
Hints of inconsistent Earthly timekeeping come through natural calendars preserved in fossils. Corals, for example, go through daily and seasonal growing cycles that form bands akin to growth rings in trees; counting them shows how many days passed in a year. In the early Carboniferous period some 350 million years ago an Earth year was around 385 days, ancient corals indicate, meaning not that it took longer for the planet to revolve around the sun, but that a day–night cycle was less than 23 hours long.
Sedimentary rocks such as sandstone also testify to the quicker days of yore. As moon-spawned tides wash over rocks they deposit mineral specks, layer upon layer. In southern Australia, for example, these vertically accumulating tidal "rhythmites" have pegged an Earth day at 21.9 hours some 620 million years ago. This equates to a 400-day year, although other estimates suggest even brisker daily rotations then.
"As you start going further back in time, the records get difficult to interpret," says Kurt Lambeck, a geophysicist at the Australian National University in Canberra. Lambeck, who serves as president of the Australian Academy of Science, wrote a book on the subject, The Earth's Variable Rotation: Geophysical Causes and Consequences, in 1980. "But the records have tended to support a general pattern going back that the number of days in the year increases," Lambeck says.