Days on Earth Are Getting Longer. You Can Thank the Moon, Not the Seasons.

For anyone who has ever wished there were more hours in the day, geoscientists have some good news: Days on Earth are getting longer. Very slowly.

A new study that reconstructs the deep history of our planet’s relationship to the moon and other planetary bodies shows that 1.4 billion years ago, a day on Earth lasted about 18.7 hours. Days have since gradually lengthened due to the planet’s interplay with the moon. Moreover, the study shows that other, longer-term changes affecting Earth’s rotation and orbit, and thus its climate, have operated in regular cycles over the same vast stretch of time—the first time that such cycles have been demonstrated to operate so far back.

Coauthor Alberto Malinverno, a research professor at Columbia University’s Lamont-Doherty Earth Observatory, said the results confirm that the record of orbital changes “can be extended to time intervals much older than those explored so far. [This] will allow for reconstructing fundamental characteristics of the Earth-moon system and of other planets in the Solar System in deep time.”

The study describes a statistical method that links astronomical theory with geological observations 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—a specialized field known as astrochronology. The research was published this week in the Proceedings of the National Academy of Sciences.

“One of our ambitions was to use astrochronology to tell time in the most distant past, to develop very ancient geological time scales,” said lead author Stephen Meyers, a professor of geoscience at the University of Wisconsin, Madison. “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.”

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, after the Serbian mathematician who first described them in the 1920s. They determine where and when sunlight is distributed on Earth, driving natural cyclic changes in climate over tens or hundreds of thousands of years. These climate cycles are recorded in ancient sediments; for example, alternating wet and dry climates result in sediments that contain greater or lesser amounts of material eroded from continents.

Some of the shorter cycles can be reliably calculated back only about 50 million years—a relatively short time for geologists. However, last month, another team led by scientists at Lamont-Doherty traced a consistent 405,000-year cycle, driven by the relative motions of Jupiter and Venus, back 215 million years. Going back even further has proved challenging because typical geologic means, like radioisotope dating, do not provide the precision needed to identify the cycles. The task is also complicated by lack of knowledge of the history of the moon, and the movements of the planets. The solar system has so many moving parts that small, initial variations in these moving parts can propagate into big changes millions of years later. Trying to account for it all can be like trying to trace the so-called butterfly effect in reverse.

Last year, Meyers and colleagues cracked part of 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, usually the further back in the rock record he and others 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. As it does so, the Earth reacts like a spinning figure skater who slows down as she stretches her arms out; the rotation slows. That means days have kept getting longer. 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 so close to the Earth that the Moon would be ripped apart. 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 Lamont-Doherty while on sabbatical in 2016. In the audience that day was Malinverno. “I was sitting there when I said to myself, ‘I think I know how to do it. Let’s get together,’” said Malinverno. “It was exciting because, in a way, you dream of this all the time; I had a solution that was looking for a problem.”

The two teamed up to use a statistical method Meyers had designed to deal with uncertainty across time, dubbed TimeOpt. They combined this with astronomical theory, geologic data and a sophisticated statistical approach called Bayesian inversion that allows 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 underwater record from Walvis Ridge, in the southern Atlantic Ocean.

With the approach, they found they could reliably assess from layers of rock in the geologic record variations in the direction of the axis of rotation of Earth and the shape of its orbit both in more recent time and in deep time, while also addressing uncertainty. They were also able to determine the length of day and the distance between the Earth and the moon 1.4 billion years ago—estimated to then be about 341,000 kilometers, or 212,000 miles. (It is now 385,000 kilometers, or close to 240,000 miles.)

“In the future, we want to expand the work into different intervals of geologic time,” said Malinverno.

The study adds to a growing body of recent astrochronology-related findings. The previous team at Lamont-Doherty used a rock formation in Arizona to confirm the remarkable regularity of Earth’s orbital fluctuations from nearly circular to more elliptical on a 405,000 year cycle. Also in May, another team in New Zealand, in collaboration with Meyers, looked at how changes in Earth’s orbit and rotation on its axis have affected cycles of evolution and extinction of marine organisms called graptoloids, going back 450 million years.

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

The study was funded by the U.S. National Science Foundation.

(This story is adapted from a press release by the University of Wisconsin, Madison.)