About 2 to 1 billion years ago, a full day lasted just 19 hours, according to a study published in Nature Geoscience. with The Moon’s position is much closer than it is today.

What scientists have learned

We know that the Moon is slowly moving away from Earth, flying further and further into space. This process is so slow and the distances so great that it is completely imperceptible for a short human lifespan. This seems to have lasted throughout the moon’s entire history, but at some point, about two billion years ago, in the middle of the Proterozoic era, the moon suddenly stopped receding and remained at a constant distance for a billion years. The length of the day is 19 hours. Scientists refer to this period as the “boring billion” because of the relative stability of Earth’s tectonic activity, stable climate, and slow biological evolution. Interestingly, after that the process started again and the day started to rise again.

For their study, the scientists used a relatively new geological method to measure the historical length of the day. Known as cyclostratigraphy, this method focuses on variation in sedimentary rock formations. Cyclostratigraphy helps researchers identify “Milankovitch cycles”—changes in Earth’s orbit and rotation that affect the planet’s climate.

Analyzing the large number of cyclostratigraphic records of Milankovitch cycles allowed the researchers to look into the past and determine why the Moon was so close to Earth during this time period. They found that the answer probably has to do with the tides that affect the planet’s rotation.

The Moon’s gravity controls the tides in Earth’s oceans, slowing the planet’s rotation. However, the Sun also exerts a gravitational effect in the form of “solar atmospheric tides” that occur when sunlight heats the Earth’s surface and accelerates the planet’s rotation. Currently, lunar tides are about twice as strong as the sun’s atmospheric tides, meaning they have a greater impact on Earth’s rotational speed. But during the “boring billion,” Earth spun faster, suggesting that the Moon’s gravitational pull was somehow weaker than it is now. So, back then, the solar and lunar tides were more evenly matched.