Earth's Days Are Getting Longer—Thanks to the Moon

6_5_Earth and Moon
More than a billion years ago, a day on Earth lasted 18 hours, a new study reports. The distance from our planet to the moon, scientists say, is one major reason for the extra six hours we have today. Getty Images

More than a billion years ago, a day on Earth lasted just 18 hours, a new study reports. The distance from our planet to the moon, scientists say, is one major reason for the extra six hours we have today.

Our faithful rocky companion used to lie far closer to our planet—near enough to alter the way it moves, researchers reported Monday in Proceedings of the National Academy of Sciences.

"As the moon moves away, the Earth is like a spinning figure skater who slows down as they stretch their arms out," explained Stephen Meyers, study co-author and a professor of geoscience at the University of Wisconsin–Madison, in a statement. Scientists think the moon is now moving about 1.5 inches away from our planet every year, but this wasn't always the case.

As planets and other astronomical bodies travel through space, they influence each other's movements in a complex display of orbital gymnastics. The force exerted by these objects can affect everything from sunlight distribution to climate change over many thousands of years. Rock can offer a record of some of these changes over hundreds of million years.

Related: Jupiter and Venus are shifting Earth's orbit every 405,000 years

But in the history of our solar system—which stretches back billions of years—a few hundred million isn't all that much. This uncertainty is compounded by something called solar system chaos—a theory that predicts small, early changes in the movement of the planets can eventually lead to massive variations.

Meyers and his team used statistics to marry geological observations and astronomical theory. By combining these disciplines, researchers aim to probe our planets—and our solar system's—past. "The geologic record is an astronomical observatory for the early solar system," Meyers said in the statement. "We want to be able to study rocks that are billions of years old in a way that is comparable to how we study modern geologic processes."

Related: Asteroid on collision course with Earth spotted just hours before impact

The researchers integrated a sophisticated statistical method called TimeOpt with tools from astronomy and geology to get a better handle on our planet's uncertain past. Using a 1.4-billion-year-old rock layer from China and a 55-million-year-old layer from the southern Atlantic Ocean, the team were able to test their idea. "We are looking at its pulsing rhythm, preserved in the rock and the history of life," Meyers said in the statement.

The method, the researchers discovered, could reliably evaluate Earth's rotation and orbit around the sun. They used it to estimate the growing space between Earth and the moon over time, and the slow but inevitable stretching of our days. Next, they want to apply their method to other intervals of geologic time, added study co-author and Lamont Research Professor at Columbia, Alberto Malinverno in the statement.