The Moon that drifts away: 3.8 centimetres a year

The Moon is not fixed in its orbit: every year it moves a few centimetres further from the Earth. The phenomenon, imperceptible over a human lifetime, has been measured with extraordinary precision in recent decades and has profound consequences, concerning the length of the day, the geological history of our planet and even the future of eclipses.

A tidal brake

Tides are not only an oceanic phenomenon but an exchange of energy between the Earth and the Moon. Since the Earth rotates faster than the Moon orbits it, the tidal bulges are dragged slightly ahead of the direction of the Moon. Their attraction speeds the satellite along its orbit, pushing it onto a wider orbit, while tidal friction slows our planet's rotation.

The result is twofold: the Moon moves away by about 3.8 centimetres a year, and the Earth's day lengthens on average by about two thousandths of a second every century. These are tiny quantities on a human scale, but accumulated over geological time they have deeply transformed the system formed by the Earth and its satellite.

Mirrors on the Moon

Direct measurement of the recession is possible thanks to reflectors placed on the lunar surface. The Apollo 11 astronauts installed one in July 1969, followed by the Apollo 14 and Apollo 15 missions; two more sit on the Soviet robotic Lunokhod rovers. Since then some observatories have been firing laser pulses at the Moon and measuring the time the light takes to return, deriving the distance with an error of a few millimetres.

The evidence of the past

At the end of the seventeenth century Edmond Halley, comparing ancient eclipses with the calculations of his own time, noticed that the Moon's motion seemed to have accelerated over the centuries: it was the first hint of tidal interaction and of the Earth's slowing. In the twentieth century the eclipses recorded in Babylon, China and the Greek world were used to measure how much the day has lengthened over the last three thousand years.

Fossils too preserve traces of the process. Some Devonian corals, which lived about 380 million years ago, show fine daily and annual growth rings from which it can be deduced that the year then had about 400 days, each shorter than today's. Even older tidal sedimentary deposits indicate that the day and the month were once considerably shorter.

The end of total eclipses

In the distant future the recession will make total solar eclipses impossible, because the lunar disc will no longer be large enough to cover the solar one; estimates point to a horizon of a few hundred million years. According to the hypothesis most widely accepted today, moreover, the Moon formed much closer to the Earth, as a result of a gigantic impact that took place about 4.5 billion years ago.

In traditional symbolism the Moon has always been the body of change, set against the supposed immobility of the stars. Modern science confirms this intuition in an unexpected way: not only do the phases change every night, but the whole relationship between the Earth and its companion is slowly transforming, centimetre by centimetre, over the ages.