The Moon is moving away from Earth. Not in theory. We can measure it, to within a few millimeters, every year.
How we know
In July 1969, the Apollo 11 astronauts placed a panel of mirrors, called a retroreflector, on the Moon’s surface. Later, the Apollo 14 and Apollo 15 crews left more, and the Soviet Union’s robotic Lunokhod 1 and Lunokhod 2 rovers carried reflectors too. Scientists on Earth still fire laser pulses at these reflectors and time how long the light takes to come back. Light travels at a known speed, so the timing gives the distance to the Moon to within a few millimeters, NASA says.
How fast
The timing shows the Moon drifts about 3.8 centimeters (roughly 1.5 inches) farther from Earth every year. NASA compares that to the rate at which fingernails grow.
Why it happens
The Moon’s gravity raises tides in Earth’s oceans. Because Earth spins faster than the Moon orbits, those tidal bulges are carried slightly ahead of the Moon, and their gravity tugs the Moon forward along its orbit. That forward pull is what makes the Moon spiral slowly outward. Earth’s spin slows a tiny amount in return, so the length of Earth’s day is slowly increasing.
What it means
Long ago the Moon was much closer to Earth than it is now, so it would have looked much bigger in the sky. Today the Moon and the Sun look almost exactly the same size from Earth. NASA explains why: the Sun is about 400 times wider than the Moon, and also about 400 times farther away. That match is why we get total solar eclipses.
As the Moon keeps drifting away, it will appear slightly smaller. NASA says that once the Moon is about 14,600 miles (23,500 kilometers) farther from Earth than it is today, it will be too small in our sky to cover the Sun completely. If it keeps drifting at today’s rate of about 1.5 inches a year, NASA puts that at more than 600 million years from now. NASA lunar scientist Richard Vondrak has said that about 600 million years from now, Earth will see its last total solar eclipse. Treat that as a rough estimate for today’s rate, not a countdown. Total eclipses will get rarer first, and annular “ring of fire” eclipses already happen when the Moon is farther away in its orbit and looks a little too small to cover the Sun.
Bottom line: if you have ever seen a total solar eclipse, you saw something with an expiration date.
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