Earth To Timekeepers: Hey, Wait Just A Second

Most people have wished for a day longer than 24 hours, and today that wish will come true in a very small way.

The extra time, added to help keep the world’s most accurate atomic clocks in sync with the rotation of the Earth, will total 1 second, added by timekeeping institutions such as the U.S. Naval Observatory in Washington between 7:59:59 and 8:00:00 tonight.

The “leap second” is needed because the Earth’s spin is slowing.

Very, very slightly, but enough that every once in a while a gap occurs between a day as measured by the cesium atomic clock and a day as measured by the sun in the sky.

To make sure that noon on the world’s atomic clocks means the same as noon in the sky, every year or so since 1972 the people who keep the world’s standard time pause for a second to let the Earth catch up.

Scientists have several theories about why the Earth’s spin is slowing down. One most commonly cited involves the gravitational force of the moon, which drives the ocean tides and creates “tidal friction” that acts like a brake on the Earth’s spin.

Another factor is that the North American continent continues to slowly rebound from the weight of the heavy glaciers that began melting 100,000 years ago. The slight bulging slows down the Earth’s spin in the same way a figure skater can slow her spin by extending her arms.

But there are some forces that cause the Earth to speed up.

One theory holds that because prevailing winds generally go from west to east, the same direction that the Earth spins, they blow on mountain ranges and turn them into sails.