Astronomy: The Moon Is Leaving — Slowly

Every year, the Moon drifts a little farther from Earth. The movement is small — about 3.8 centimeters annually, roughly the rate at which fingernails grow — but it is measured with remarkable precision. Panels of retroreflectors left on the lunar surface by Apollo astronauts return laser pulses fired from observatories on Earth, and by timing each pulse's round trip, astronomers can calculate the Earth–Moon distance to within a few millimeters. After five decades of such measurements, the conclusion is unambiguous: the Moon is receding, and apparently has been for most of its history.

The engine of this retreat is tidal friction. The Moon's gravity raises two bulges in Earth's oceans, on the sides facing toward and away from the Moon. Because Earth rotates faster than the Moon orbits, friction drags the nearer bulge slightly ahead of the Earth–Moon line. That displaced mass of water exerts a gravitational pull on the Moon, tugging it forward in its orbit. A forward tug adds energy to the orbit, and an orbit with more energy is a larger orbit — so the Moon moves outward. The energy is not free: it is drawn from Earth's rotation, which slows in compensation. The day lengthens by roughly 2.3 milliseconds per century, imperceptible in a human lifetime but enormous over geological time.

Remarkably, some of the strongest evidence for this process comes not from telescopes but from fossils. Certain Devonian corals, around 380 million years old, laid down fine growth ridges on their skeletons — one ridge per day, grouped into annual bands by the seasons. Counting these ridges reveals about 400 daily ridges within each yearly band, meaning that a year then contained roughly 400 days. Since the length of the year has barely changed, each day must have lasted only about 22 hours: Earth was spinning considerably faster than it does today. Layered tidal sediments called rhythmites, which record the cycle of tides in stone, tell a consistent story for other periods, confirming that days have lengthened and, by implication, that the Moon was once much closer.

One might assume the retreat has proceeded at today's measured rate, but that assumption leads to a contradiction. If the Moon had always receded at 3.8 centimeters per year, then just 1.5 billion years ago it would have been so close to Earth that it should have been torn apart by Earth's gravity — yet geological evidence shows the Moon is far older and was never destroyed. The resolution is that the rate varies. Tidal friction depends strongly on the shape and arrangement of the ocean basins, which change as continents drift. At present, the Atlantic Ocean happens to be near a resonant size, sloshing in a way that amplifies tidal friction, so today's rate is probably unusually high; in eras when the oceans were configured differently, the retreat was slower.

The long-term consequences are profound. As the Moon's distance grows, its apparent size in the sky shrinks, and in roughly 600 million years it will no longer appear large enough to cover the Sun completely: total solar eclipses, a temporary coincidence of the present era, will cease to occur. Meanwhile, days will continue to lengthen. In principle, tidal friction would eventually slow Earth's rotation until one side of Earth faced the Moon permanently, with day and month both lasting about 47 present days — but the Sun should expand and engulf the inner planets long before that equilibrium, cutting the process short.