Days on Earth are getting Longer and it’s all thanks to the Moon

For anyone who has ever wished there were more hours in the day, geoscientists have some good news - Days on Earth are getting longer, as the moon slowly moving away from us.

A new study that reconstructs the deep history of our planet’s relationship to the moon and the Solar System shows that 1.4 billion years ago, a day on Earth lasted just over 18 hours. This is at least in part because the moon was closer and changed the way the Earth spun around its axis.

"As the moon moves away, the Earth is like a spinning figure skater who slows down as they stretch their arms out," said Stephen Meyers, professor of geoscience at the University of Wisconsin-Madison and co-author of the study. 

That means as the rotation slows, days have kept getting longer. 

The study describes a statistical method that links astronomical theory with geological observations (called astrochronology) to look back on Earth’s past, reconstruct the history of the Solar System and understand ancient climate change as captured in the rock record.

Earth’s movement in space is influenced by the gravitational forces exerted by both the moon and the other planets. These constantly shifting forces cause variations in the Earth’s rotation on its axis, and in the orbit the planet traces around the sun. 

These variations are collectively known as Milankovitch cycles and they determine where and when sunlight is distributed on Earth, driving natural cyclic changes in climate over tens or hundreds of thousands of years.

This image from the Deep Space Climate Observatory (DSCOVR) satellite captured a unique view of the Moon as it moved in front of the sunlit side of Earth in 2015. It shows a view of the farside of the Moon, which faces the Sun, that is never directly visible to us here on Earth. Image Cridet:NASA/NOAA 

But going back further, on the scale of billions of years has proved challenging because typical geologic means, like radioisotope dating, do not provide the precision needed to identify the cycles. It's also complicated by lack of knowledge of the history of the moon, and by what is known as solar system chaos.

The solar system has many moving parts, including the other planets orbiting the sun. Small, initial variations in these moving parts can propagate into big changes millions of years later, which is known as solar system chaos.

Last year, Meyers and colleagues cracked the code of the solar system’s apparent chaos in a study of sediments from a 90 million-year-old rock formation that captured Earth’s climate cycles. Still, the further back in the rock record they have tried to go, the less reliable their conclusions.

For instance, the moon is currently moving away from the Earth at a rate of 3.82 centimeters per year. Using this present-day rate, scientists extrapolating back through time have calculated that "beyond about 1.5 billion years ago, the moon would have been close enough that its gravitational interactions with the Earth would have ripped the moon apart," Meyers said.

Yet, we know the moon and Earth are about 4.5 billion years old. So the rates of movement must have been different in the past.

Meyers brought this problem with him when he gave a talk at Columbia University's Lamont-Doherty Earth Observatory while on sabbatical in 2016.

In the audience, that day was Alberto Malinverno, Lamont Research Professor at Columbia. "I was sitting there when I said to myself, 'I think I know how to do it! Let's get together!" said Malinverno, the other study co-author. "It was exciting because, in a way, you dream of this all the time; I was a solution looking for a problem."

The two teamed up to combine a statistical method that Meyers developed to deal with uncertainty across time, called TimeOpt - with astronomical theory, geologic data and a sophisticated statistical approach called Bayesian inversion that allows the researchers to get a better handle on the uncertainty of a study system.

They then tested the approach, which they call TimeOptMCMC, on two rock layers - the 1.4 billion-year-old Xiamaling Formation from Northern China and a 55 million-year-old record from Walvis Ridge, in the southern Atlantic Ocean.

Examining the geologic record captured in the rock layers, the results revealed changes in Earth's rotation, orbit and distance from the moon throughout history, as well as how the length of a day on Earth has steadily increased.

1.4 billion years ago, the distance between the Earth and the moon was 341,000 kilometers (212,000 miles) which is now 385,000 kilometers (240,000 miles).

"In the future, we want to expand the work into different intervals of geologic time," says Malinverno.

"The geologic record is an astronomical observatory for the early solar system," says Meyers. "We are looking at its pulsing rhythm, preserved in the rock and the history of life."

Source: University of Wisconsin–Madison
            Earth Institute, Columbia University