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.
