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Tide lines show Moon's move

Marks left by tides 3.2 billion years ago suggest the Moon was once much closer

The gravitational pull of the Moon hauls up Earth's oceans into two bulges on opposite sides of the planet. As Earth rotates beneath these twin bulges, sea levels along the coastlines rise and fall, creating the tides. Much of the world's shores, including around the UK, experience two cycles of high and low tides roughly equal in magnitude every day.

 The Sun's gravity also has an effect on the ocean's tides, and roughly twice a month (one lunar orbit), when both the Moon and Sun are in line with Earth, their gravitational effects combine to create a much larger range between high and low water, known as the spring tide. Conversely, when the Moon and Sun are at 90° to each other the tides are weaker, what's called a neap tide.

One factor that has affected the tides over longer timescales is that the rotation of Earth has decelerated over the planet's history, and the Moon has slowly spiralled ever further out in its orbit. Today we know, thanks to measurements taken by a device placed on the Moon by Apollo astronauts, called the Lunar Laser Ranging Experiment, that the Moon is drifting away at a rate of 3.8cm per year.

However, just after the Moon's formation, our satellite circled much, much closer, and each day on Earth was only around four hours long. But what's not clear is exactly how the Earth-Moon system has evolved over the 4.5 billion years since: how have the time Earth takes to rotate and the Moon takes to orbit changed over time? Computer modelling studies widely disagree. What's needed are some actual data points from Earth's deep history.

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