The Moon looks like the most dependable thing in the sky. It rises, it sets, it waxes and wanes, just as it did for our ancestors. But it's not standing still. Every year, the Moon is a tiny bit farther away than it was the year before. So what is pushing it away?
The short answer is the tides. The Moon's gravity pulls Earth's oceans into bulges, and because Earth spins faster than the Moon orbits, those bulges get carried slightly ahead of the Moon. Their gravity gives the Moon a gentle forward tug, nudging it into a slightly higher orbit. Measurements show the Moon is spiraling away by about 3.8 centimeters a year, roughly an inch and a half.
A Tug From the Tides
The Moon is big enough and close enough to raise tides on Earth. The oceans bulge out both toward the Moon and away from it. If Earth responded instantly, the bulge would point straight at the Moon. But there's a delay, and Earth's quick spin carries the bulge a little bit forward.
That off-center bulge creates a twisting force between Earth and the Moon. It boosts the Moon into a slightly higher orbit and, at the same time, acts as a brake on Earth's spin. Energy is handed from Earth's rotation to the Moon's orbit. Most of the energy Earth loses actually turns into heat from friction in the oceans, and only a small fraction goes to the Moon.
Mirrors on the Moon
How can anyone measure something so small from so far away? On July 21, 1969, the Apollo 11 crew set up a special array of reflectors on the Moon's surface. Apollo 14 and Apollo 15 left two more. Scientists fire lasers from observatories on Earth, bounce them off these reflectors, and time how long the light takes to return.
The first successful measurements were reported on August 1, 1969, from Lick Observatory in California. Today the data can be fitted to within about a centimeter. That's how we know the Moon is drifting away at about 3.8 centimeters a year, a rate scientists have described as unusually high compared with the long-term average.
Our Days Are Getting Longer
The same process is slowing Earth down. Together with the Sun's tidal influence, it has helped stretch Earth's day from about 6 hours long in the distant past to the 24 hours we have now. Atomic clocks show the day is still getting longer by about 2.3 milliseconds per century. You won't notice it, but over millions of years, it adds up, and it even shows up in the fossil record.
People noticed hints of this long before lasers. In 1695, the astronomer Edmond Halley suggested, from old eclipse records, that the Moon's motion seemed to be changing. In 1749, Richard Dunthorne re-examined ancient records and confirmed Halley's hunch.
Will the Moon Ever Leave?
Not anytime soon. In theory, the process would eventually lock Earth and the Moon so that each always shows the other the same face, with Earth's day stretching to match a much longer month. One estimate puts that tens of billions of years away. Scientists don't expect it to happen before the Sun swells into a red giant billions of years from now.
So enjoy tonight's Moon. It's a tiny bit farther away than it was last year, but it will be keeping us company, and pulling on our tides, for a very long time to come.
