Savor it while you can, because the moon is creeping away from Earth — at about 3.8 centimeters (1.5 inches) each year, according to NASA. That’s the rate fingernails grow. When the moon’s average distance from Earth increases by an additional 14,600 miles, it will be too far to fully cover the sun from Earth’s perspective.
“3.8 centimeters a year may not mean a lot to anyone,” said Noah Petro, a lunar scientist at NASA. “When you add it up over a lifetime, over thousands or millions of years, it adds up.”
Total solar eclipses are a celestial marvel. The sun’s diameters is about 400 times as big as the moon’s, but it is coincidentally 400 times farther from Earth than the moon. These eclipses occur when the moon’s angular size (the amount of space it takes up in your view) largely matches the sun’s. As the moon moves farther way, its angular size will appear smaller.
Assuming the moon will continue to pull away at its current rate — which is a big assumption, Petro said — the moon will travel 14,600 miles (23,000 kilometers) in about 600 million years. But given that the sun is expected to grow in that time, total solar eclipses may vanish slightly sooner.
Scientists uncovered this slow drift by experimenting with reflecting panels, made of mirrors, left on the moon from the Apollo missions. Over the course of decades, they aimed a beam of light at the reflectors and timed how long it took for the light to come back to Earth. They found the laser’s journey lengthened slightly each year, meaning the moon was inching farther into space.
As it turns out, Earth is pushing the moon away.
The rotation of Earth, the orbit of the moon and the rotation of the moon are all linked, making up the angular momentum of the Earth-moon system, said physicist Richard Olenick. Angular momentum remains constant for such fairly isolated systems as Earth and moon; therefore, if the rotation of Earth changes, then the orbit of the moon will also change. (Think of how the angular momentum of a figure skater is conserved when she pulls her arms in and spins faster.)
Earth’s rotation is slowing down because of the moon’s gravitational forces (although it’s decreasing only at about 1.4 milliseconds per century). That’s because the moon yanks on Earth’s tides and causes the planet’s oceans to bulge out toward it. These tidal bulges create drag and slow down Earth’s spin.
To compensate for the slowing down of Earth’s rotation, the moon’s orbit is becoming larger, said Olenick, a professor at the University of Dallas. The moon also casts a smaller shadow on Earth.
“Because of the tides pulling on the moon a little bit differently, one of the ways that energy goes into the moon is by it slowly moving further away,” Petro said.
The moon has probably been drifting from the beginning of its existence. The moon is popularly thought to have formed from debris after a large Mars-size object collided with a young Earth about 4.5 billion years ago. Modeling predicts that the moon could have been as close as 15,000 miles (24,000 kilometers) after it formed, Petro said. Ever since that moment, the moon has been slowly moving away, although the rate probably fluctuated throughout history as Earth and its gravitational pull changed.
“It’s happening so gradually. It wouldn’t be like someone wakes up one day and is like, ‘Whoa, wait a minute. What happened here?’” Petro said.
If there are human generations still on Earth 600 million years later, they will see only “ring of fire” or annular eclipses, when the moon passes the sun but doesn’t completely cover it. The moon may also appear slightly smaller, probably how we see the moon now at its furthest point in orbit.
As the moon moves away, the “length” of an Earth day will also change slowly over time, Petro said. Olenick said the angular velocity of the moon should also slow down, implying the moon will take longer to go through its phases. Earth’s tides will also be smaller because the moon’s gravitational pull will be weaker.
And maybe objects would look a little different from the moon, too.
“I’d like to think that 600 million years from now, we’ll have people on the moon, as well, looking back at the Earth,” Petro said. “And so the Earth will be ever so slightly smaller.”
This article is part of Hidden Planet, a column that explores wondrous, unexpected and offbeat science of our planet and beyond.
