Scientists Precisely Track the Moon’s Gradual Drift Away from Earth

The Moon is slowly moving farther from Earth, an extremely gradual change that researchers can detect with high accuracy. This movement results from the gravitational interaction between our planet and its natural satellite, causing a steady transfer of momentum from Earth to the Moon.

Monitoring the Moon’s distance is more than a minor observation; it offers vital insights into the geological and orbital evolution of both bodies over billions of years. Experts highlight that the Moon’s trajectory reveals key aspects of Earth’s past development as well.

To track this subtle shift, scientists use lasers bounced off retroreflectors installed on the lunar surface by astronauts and robotic missions. By calculating the time it takes for light to return, they determine that the Moon's average distance is approximately 239,000 miles (385,000 kilometers), though it varies by roughly 12,400 miles (20,000 kilometers) during its orbit. This variation causes phenomena like supermoons, where the Moon appears significantly larger and brighter in the sky.

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The Role of Tidal Forces

The Moon’s gradual separation is primarily driven by tidal interactions due to gravitational forces unevenly applied across Earth. As explained by NOAA, the Moon’s gravity exerts a stronger pull on the side of Earth facing it, creating two tidal bulges—one facing the Moon and another on the opposite side. Because Earth spins, these bulges are slightly ahead of the Moon’s position, generating a forward pull that boosts the Moon’s orbital momentum. This interaction causes Earth’s rotation to slow down gradually while the Moon moves away at about 1.5 inches per year. For more on Earth's rotation changes, see this article.

A tidal bulge caused by the Moon’s gravity is indicated on the Earth’s side facing the satellite. Credit: NOAA

This subtle but continuous energy exchange defines the dynamic nature of the Earth-Moon relationship. According to NASA’s Goddard Space Flight Center lunar scientist Richard Vondrak, as noted in a NASA announcement:

“Over time, the number and frequency of total solar eclipses will decrease,” he said. “About 600 million years from now, Earth will experience the beauty and drama of a total solar eclipse for the last time.”

Tracing Back Through Time

Scientists believe the Moon wasn’t always located so far away. Research published in Paleoceanography and Paleoclimatology suggests the Moon formed around 4.5 billion years ago when a young Earth collided with a Mars-sized body. This event sent fragments into orbit that coalesced into the Moon, which initially orbited much closer than now.

Additional evidence comes from fossil clam shells, which have growth bands that reveal daily cycle lengths. Around 70 million years ago, near the end of the dinosaur era, a day lasted about 23.5 hours. These findings agree with models showing Earth’s rotation has slowed over hundreds of millions of years as the Moon receded.

Earth’s rotation causes tidal bulges to shift ahead of the Moon’s position, generating tidal lag. Credit: NASA/Vi Nguyen

Effects on Our Planet Today

The Moon’s gradual outward motion influences ocean tides and the length of Earth’s days. According to The Conversation, tidal bulges can cause water levels to fluctuate by about 5 feet in coastal cities such as New York and Los Angeles. These bulges also play a role in transferring momentum that gradually pushes the Moon farther out.

While the changes are imperceptible to daily human experience, they provide valuable opportunities for scientists to examine the principles of orbital mechanics, the evolution of planetary systems, and the long-term impact of gravitational forces. Despite its ongoing drift, the Moon will remain gravitationally tied to Earth indefinitely.