2.46 Billion Years ago, a day on Earth was Only 17 Hours and the Moon was Much Closer

As the ages pass the Moon slowly drifts away from the Earth. In conjunction the length of our day gradually gets longer. For the first time astronomers have been able to estimate the length of the day and the distance to the Moon as it was almost two and a half billion years ago. Back then, our day was only 17 hours long.

As the Moon orbits the Earth it raises tides. These bulges stretch in the direction of the Moon and on the opposite side of the Earth. But the Earth is also spinning, and that rotation brings the bulge of the tide forward with respect to the current position of the Moon. This lump gives an extra gravitational tug to the Moon, like an invisible leash pulling on our satellite. That gravitational tug boosts the Moon into a more distant orbit.

But the energy to boost to that orbit has to come from somewhere, and it comes from the rotation of the Earth. In the distant past our day was much shorter as the Earth spun much faster on its axis. But with every day that passes, it gets longer and longer. 

Astronomers have known about this for quite some time and have used geologic records to estimate the length of the day in the past. But there’s a limit to what those records can tell us. New research has turned to a different kind of geologic record to estimate the length of the day.

The researchers studied banded iron formations in sedimentary rocks in Western Australia. In these kinds of formations, iron-rich layers alternate with more clay-like layers. The shape of these patterns is determined by the Earth’s climate. The Earth’s climate goes through several different variations, including one called the Milankovitch cycle, which is determined by the Earth’s orbit and the length of the Earth’s day.

The scientists were able to reconstruct the length of the Earth’s day at the time these formations were laid, 2.46 billion years ago. They found that at the time the dat lasted only 17 hours, much shorter than the current 24.

The current distance to the Moon gives us the very special phenomenon of total solar eclipses. This can only be possible if the apparent size of the Sun and the Moon are the same on our sky. While that’s true for now, in a few hundred million years the Moon will be too far away, and it will be too small to give us total solar eclipses. So enjoy them while they last.