Ancient tidal signs show that the moon once orbited close to the Earth

It is known that the gravitational force of the moon exists What causes tides on Earthwhich results in the cyclical nature of the tidal regime between low and high tides.

But one factor that has affected the tides over longer periods of time is the fact that the Earth’s rotation has slowed over the course of the planet’s history, and the Moon has risen more slowly in its orbit.

Yes, the Moon is slowly moving away from Earth in its orbit, and this has had an effect on Earth’s tides when we compare it over billions of years.

The Moon is drifting away from Earth at a rate of 3.8 cm per year, which we know thanks to measurements made by a device placed on the lunar surface by Apollo astronauts, called the Lunar Laser Range Experiment.

Part of the Lunar Laser Range Experiment, photographed by Apollo 15 astronaut David Scott on the lunar surface. Credit: NASA/Dr. Scott

However, after moon formationour satellite was orbiting much closer, and every day on Earth was only about four hours long.

But what isn’t clear is exactly how the Earth-Moon system has evolved over the 4.5 billion years since then: How has the time it takes the Earth and the Moon in its orbit to change over time?

Computational modeling studies vary widely. What we need are some actual data points from Earth’s deep history.

And this is where geology can provide important insights.

Certain types of rocks formed from submerged sand dunes in shallow coastal waters, and alternate layers of precipitated sand and clay, generated by strong and weak currents respectively, appear at different times of the tidal cycle.

Coastal sandstone.  Credit: Jenny Detrick/Getty Images

Coastal sandstone. Credit: Jenny Detrick/Getty Images

Tom Ullenfeld and Christoph Hobeck, both in Institute of Earth Sciences, Friedrich Schiller University JenaGermany, re-examined the oldest example in the geological record.

Known as the Moodies Group sandstone in South Africa, it dates back to 3.22 billion years.

The thickness of these alternating layers rotates every 15 layers, and this is thought to be due to the changing current forces over the course of the cycle between spring tides over the course of a month.

These geological measurements together with the application Johannes KeplerThe third law of planetary motion, promulgated by Ullenfeld and Huebeck, made it possible to reconstruct the rate of rotation of the Earth and the orbital period of the Moon at the time when these ancient rocks were deposited.

A thin crescent moon displays an atmosphere of Earth's brightness over ESO's Paranal Observatory in Chile, October 27, 2011. Credit: ESO/B. Trichy (twanight.org)

Billions of years ago, the Moon orbited the Earth much closer than it does today. Credit: ESO/B. Trichy (twanight.org)

They estimated that 3.2 billion years ago the distance between the Earth and the Moon was about 70% of the present value, and that the Earth’s rotation rate thereafter resulted in a year of about 700 days, each day lasting about 13 hours.

More like this

Previous measurements of 650 million-year-old rocks from southern Australia put the distance between Earth and the Moon at 97% of today’s distance at that time.

With these points to fill in the gaps, computer models can begin to build a much better picture of how the moon’s dance around the Earth has changed over time.

Louis Dartnell was reading Constraints on the Moon’s Orbit 3.2 billion years ago from tidal packet data by Tom Ullenfeld and Christoph Hobeck. Read it online at: arxiv.org/abs/2207.05464.

This article originally appeared in the October 2022 issue of BBC Sky at Night Magazine.