FARAI CHIDEYA, host:
This is DAY TO DAY. I'm Farai Chideya.
The days are getting longer, and we don't just mean that the winter solstice has passed and we're heading towards spring. The time it takes for the Earth to make one rotation is actually lengthening. In fact, on New Year's Eve, the US Naval Observatory will add an extra leap second to the nation's atomic clocks. The last leap second was added in 1998. So why is this happening? That's a question for the explainer team at the online magazine Slate. Here with the answer is Slate's Andy Bowers.
ANDY BOWERS reporting:
Over the long term, it's due to the moon's gravitational pull. When the moon tugs on the Earth's surface, it stretches the planet, both the water and the land itself, into a slightly oblong shape. What does that mean? Since the bulge on Earth 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. 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; that is, when the Earth is spinning perfectly in time with the moon's orbit and each Earth day lasts an entire month. However, 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 beyond just 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 is spins.
Things that can affect the shape of the Earth include the melting of polar ice caps and the movement of water toward the equator, large earthquakes and meteorological events, such as El Nino, which pushes around large volumes of cold water. And even fluid currents in the Earth's inner core can change the rotation of the planet.
CHIDEYA: Andy Bowers is a Slate senior editor, and that explainer was compiled by Daniel Engber. Transcript provided by NPR, Copyright NPR.