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The U.S. Naval Observatory has announced that it will add an extra second to the nation’s atomic clocks this New Year’s Eve—the first time a “leap second” has been deployed since 1998. The adjustment is necessary because the length of an atomic day—i.e., 86,400 seconds ticked off an atomic clock—was set according to observations made around 1900. Back then, the Earth rotated a tiny bit faster than it does in 2005. Scientists say the rotation slows about 2 milliseconds every 100 years. What could make this happen?

The moon’s gravitational pull, over the long term. When the moon tugs on the Earth’s surface, it stretches the planet into a slightly oblong shape. (Both land and water bulge out toward the moon on one side, and away from the moon on the other.) These tidal bulges don’t form instantly. Friction—between, say, the ocean and the ocean floor—slows the process down. By the time the bulges do form, the Earth has spun a bit on its axis. That means the tidal bulge isn’t exactly underneath the moon.

What does that mean? Since the bulge comprises a large mass of material, it exerts its own gravitational attraction on the moon, and vice-versa. As the moon pulls, it draws the tidal bulge toward itself—in the opposite direction of the Earth’s rotation. That slows the rotation down by a small amount.

In theory, this deceleration should end when the Earth and the moon reach a synchronous rotation—when the Earth is spinning perfectly in time with the moon’s orbit. (In other words, when each day lasts a month.) It will take so long for that to happen that the sun will likely flare out first.

The moon isn’t the only thing that affects the length of a day. The Earth’s rate of rotation also depends on its overall shape (and not just on the shape of the tidal bulges). An oblong Earth that swells out at the equator would turn more slowly than a spherical Earth—astronomers use the analogy of a figure skater who turns faster as he draws in his arms. So, anything that serves to deform the shape of the Earth will affect the speed with which it spins.

The end of the last ice age slowed down the Earth as large masses of ice at the poles melted and flowed toward the equator. (Seasonal cycles of melting snow have a similar effect on a much shorter time span.) But as the ice disappeared, it removed some of the weight on the Earth’s crust, which in turn allowed for more mass to move back to the poles over thousands of years. This is called the “post-glacial rebound“; scientists estimate that it affects the Earth’s rotation about a quarter as much as the moon does.

Tectonic shifts—like those that correspond to large earthquakes—can have very small effects on the Earth’s rotation. So can meteorological phenomena, like El Niño, that push around large volumes of cold water. Finally, fluid currents in the Earth’s inner core can change the rotation of the planet.

Bonus Explainer: If the Earth’s rotation keeps getting slower, does that mean days were much shorter in the past? Ancient eclipse records confirm that the Earth has gradually slowed over many centuries. But figuring out the lengths of days of yore isn’t as easy as subtracting two milliseconds for every century. Scientists think the effect of lunar gravity on the Earth’s rotation has increased over the millennia.

Got a question about today’s news? Ask the Explainer.

Explainer thanks Geoff Chester of the U.S. Naval Observatory and John Wahr of the University of Colorado.