Discover Why the Moon is Drifting Away from Earth: Insights from a Physicist

The moon is drifting away from Earth at about 1.5 inches (3.8 centimeters) each year. That might not sound like much, but it adds up. Over time, this gradual movement can have interesting effects.

Scientists track the moon’s distance by bouncing lasers off special mirrors placed there by astronauts during the Apollo missions. By timing how long it takes for the light to return, they can figure out exactly how far the moon is from us, which usually hovers around 239,000 miles (385,000 kilometers). But as it orbits, the moon’s distance fluctuates. Its path isn’t a perfect circle; it varies by about 12,400 miles (20,000 kilometers), resulting in some full moons appearing larger, known as supermoons.

You might be curious: why is the moon drifting away? The answer lies in the ocean tides. The moon’s gravitational pull creates tidal bulges on Earth. These bulges are slightly ahead of the moon because of Earth’s rotation. As the moon is pulled by these bulges, it picks up speed, causing its orbit to expand gradually. This tiny change affects Earth too—our rotation slows down as the moon gains momentum. It might seem trivial, but it means that days are getting a little longer over millions of years.

In the past, the moon was much closer to Earth. Paleontologists, studying ancient clam shells, discovered that 70 million years ago, a day lasted just 23.5 hours, suggesting that the moon was nearer then. Around 4.5 billion years ago, when the moon formed after a massive impact, it was significantly closer. Back then, it might have looked much larger in the sky.

Looking ahead, scientists expect that in billions of years, the moon’s gradual drift will stop as Earth and the moon become tidally locked. This means they would rotate in sync—showing only one side to each other. At some point, the sun will become a red giant, potentially swallowing up both Earth and the moon.

For now, we can enjoy the wonders of the night sky, tides, and the occasional solar eclipse without worrying about these distant events. The moon, with its slow but steady retreat, remains a beautiful part of our universe.



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