The Earth and the Moon are entangled in a gravitational ballet, with the Moon orbiting the Earth and both of them going around the Sun. These movements have consequences; for example, the movement of the tides and the Moon always showing the same face. Another is the fact that the Moon is slowing down Earth’s days, effectively making them longer.

According to a new study, published in Proceeding of the National Academy of Science, Earth’s day went from around 18 hours about 1.4 billion years ago to the 24 hours that it is today. To work this out, the researchers used a statistical approach called astrochronology that combines astronomical observations and geological analysis to study the history of the Solar System, which they consider a key approach to explaining the ancient climate of Earth.

“One of our ambitions was to use astrochronology to tell time in the most distant past, to develop very ancient geological time scales,” co-author Professor Stephen Meyers, from the University of Wisconsin–Madison, said in a 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.”

The Moon formed in the collision between Earth and a protoplanet the size of Mars 4.5 billion years ago. It was a lot closer to Earth back then than it is today. The system obeys the laws of physics and so changes to one parameter affect the other. As we already know, the Moon is moving further away from our planet at a rate of 3.82 centimeters per year, so as the system interacts, it's resulting in the Earth slowing down.

“As the moon moves away, the Earth is like a spinning figure skater who slows down as they stretch their arms out,” explained Meyers.

Working out this six-hour difference over the last 1.4 billion years was not as simple as one might think, as values change significantly over time, meaning the further back you go the less reliable any conclusion is. If, for example, we take the current receding rate of the Moon and apply it to its whole history, then we would conclude that the Moon only formed 1.5 billion years ago, as before that it would have been too close to our planet that Earth would have ripped it apart. The team had to use statistical analysis to work out how these changes evolved over time and used the geological evidence to strengthen their model.

“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.”

Researchers recently used geological evidence to show how Earth’s orbit has gone from circular to more elliptical over millions of years. In fact, many researchers are working hard to understand how astronomical cycles affect climate on Earth.