The Moon is getting slightly farther away from the Earth each year − a physicist explains why

Curious Kids^[1] is a series for children of all ages. If you have a question you’d like an expert to answer, send it to CuriousKidsUS@theconversation.com^[2]. “Is the Moon getting farther away from Earth?” – Judah, 9, Broken Arrow, Oklahoma The Moon is getting 1½ inches (3.8 centimeters) farther away^[3] from the Earth every year. Scientists measure the distance to the Moon by bouncing lasers off mirrors^[4] placed there by space probes and astronauts. By measuring the amount of time it takes light to travel to the Moon and back, scientists can very precisely measure the distance to the Moon and how the distance changes. The distance to the Moon actually changes over a single month as it goes around the Earth. The Moon is typically 239,000 miles (385,000 km) away from the Earth^[5], but its orbit is not a perfect circle^[6] and changes by about 12,400 miles (20,000 km) as it orbits the Earth. This change is why some full moons are a bit bigger than others; these are called supermoons^[7]. As an astrophysics researcher^[8], I’m interested in the motion and interaction of objects such as planets, stars and galaxies. The motions of the Earth and Moon have many interesting consequences, and studying how they move over time can help researchers better understand how each has changed over the 4½ billion years since the Earth and Moon formed. Tidal forces So, why is the Moon getting farther away? It’s all because of tides. Tides come from a difference in gravity^[9] across an object. The force of gravity exerted by the Moon is about 4% stronger on the side of Earth that faces toward the Moon, compared to the opposite side of the Earth facing away, because gravity gets weaker with distance. This tidal force causes the oceans to slosh around in two bulges that point toward and away from the Moon. They do this because the gravitational force pulling on Earth by the Moon isn’t just an average force that’s the same strength everywhere. The Moon’s gravity is strongest on the closer side of the Earth, creating a bulge of water pointing toward the Moon. It’s weaker on the opposite side of the Earth, which leaves another bulge of water that lags behind the rest of the Earth.

As the Earth rotates, these bulges move around and keep pointing at the Moon because of its gravitational pull. In New York City or Los Angeles, the water level can change by about 5 feet^[11] due to these tidal bulges.

These liquid bulges do not quite line up with the Moon – they “lead” it a little bit because the Earth is rotating and dragging them forward. These bulges also exert a gravitational pull back on the Moon. The bulge closer to the Moon isn’t just pulling the Moon toward the center of the Earth, but also a little bit ahead in its orbit – like the boost a sports car gets as it goes around a curve.

This forward pull from the closer tidal bulge causes the Moon to speed up, which causes the size of its orbit^[13] to increase. Think of a baseball player hitting a home run. If the player hits the ball faster at home plate, it’ll zoom higher up into the sky.

So the bottom line is that the gravity of the closer tidal bulge on the Earth is pulling the Moon forward, which increases the size of the Moon’s orbit. This means that the Moon gets slightly farther away from the Earth. This effect is very gradual and only detectable on average over years.

Does the Moon’s increasing distance affect Earth?

The Moon gains momentum^[14] as its orbit gets bigger. Think about spinning a weight attached to a string. The longer the string, the more momentum the weight has, and the harder it is to stop.

Because the Earth is doing the work of increasing the Moon’s momentum, the Earth’s rotation slows down^[15] in turn, as its momentum goes to the Moon. To put it another way, as the Moon’s orbital momentum increases, the Earth’s rotational momentum decreases in exchange. This exchange makes a day get very slightly longer.

But don’t worry, these effects are so small: 1.5 inches per year compared to a distance of 239,000 miles (384,000 km) is just 0.00000001% per year. We’ll keep having eclipses^[16], tides and days that last 24 hours for millions of years.

Was the Moon closer to us in the past?

The Earth’s days were shorter in the past.

The Moon probably formed^[17] around 4.5 billion years ago, when a young Earth was hit by a Mars-size protoplanet^[18], causing a lot of material to get knocked off into space.

Eventually, that material formed the Moon, and it was initially much closer to the Earth. Back then, you’d see the Moon much bigger in the sky.

By examining fossilized clam shells for material showing their daily growth patterns, paleontologists found evidence that 70 million years ago^[19] – near the end of the time of dinosaurs – the day was only 23.5 hours long, just as predicted by astronomical data.

What will happen in the future?

So, will the Moon eventually escape from the Earth’s gravitational pull as it moves away?

If we fast-forward tens of billions of years into the future, eventually the Earth’s rotation could slow down until it is tidally locked^[20] with the Moon. That means that it would take just as long for the Earth to rotate as the Moon does to orbit. At this point, the Moon would stop getting more distant, and you would see the Moon only from one side of the Earth.

But two things will stop that from happening. First, in a billion years or so, the Sun will get brighter^[21] and boil away the oceans. Then, there won’t be large tidal bulges of water to cause the Moon to get more distant. A few billion years later, the Sun will expand into a red giant^[22], probably destroying the Earth and the Moon.

But these events are so far in the future that you don’t need to worry about them. You just get to enjoy tides on the beach, solar eclipses and our beautiful Moon.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com^[23]. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

References

1. ^{^} Curious Kids (theconversation.com)
2. ^{^} CuriousKidsUS@theconversation.com (theconversation.com)
3. ^{^} 1½ inches (3.8 centimeters) farther away (www.astronomy.com)
4. ^{^} bouncing lasers off mirrors (www.nasa.gov)
5. ^{^} 239,000 miles (385,000 km) away from the Earth (science.nasa.gov)
6. ^{^} not a perfect circle (www.americanscientist.org)
7. ^{^} called supermoons (theconversation.com)
8. ^{^} As an astrophysics researcher (astrokerby.altervista.org)
9. ^{^} difference in gravity (oceanservice.noaa.gov)
10. ^{^} NASA/Vi Nguyen (assets.science.nasa.gov)
11. ^{^} about 5 feet (www.tide-forecast.com)
12. ^{^} NASA/Vi Nguyen (assets.science.nasa.gov)
13. ^{^} the size of its orbit (www.youtube.com)
14. ^{^} gains momentum (www.britannica.com)
15. ^{^} rotation slows down (www.youtube.com)
16. ^{^} eclipses (theconversation.com)
17. ^{^} Moon probably formed (theconversation.com)
18. ^{^} a Mars-size protoplanet (science.nasa.gov)
19. ^{^} paleontologists found evidence that 70 million years ago (doi.org)
20. ^{^} tidally locked (science.nasa.gov)
21. ^{^} Sun will get brighter (www.space.com)
22. ^{^} a red giant (www.space.com)
23. ^{^} CuriousKidsUS@theconversation.com (theconversation.com)