The Moon Is Drifting Away from Earth, and Scientists Can Measure It Down to Inches

The Moon is slowly drifting away from Earth, a movement so subtle that scientists can only measure it with great precision. This gradual shift is a result of the gravitational pull between our planet and its lunar companion, which transfers momentum from Earth to the Moon over time.

Tracking the Moon’s distance from Earth may seem like a minor detail, but it actually holds significant clues about the history of our planet. By studying how the Moon moves, experts can gain insights into the changes that both Earth and its satellite have undergone over billions of years.

Scientists have developed innovative methods to measure this change. They use lasers to bounce off mirrors left on the Moon by astronauts and robots, timing how long the light takes to return. These measurements reveal that the average distance between the Earth and the Moon is approximately 239,000 miles (385,000 kilometers), but it can vary by around 12,400 miles (20,000 kilometers) as the Moon orbits our planet. This variation is why we sometimes witness supermoons, where the Moon appears larger in the sky.

The Tidal Forces at Play

The Moon’s retreat is driven by tidal forces caused by the differences in gravitational pull across Earth. According to NOAA, the Moon’s gravity pulls more strongly on the side of Earth facing it, creating two ocean bulges – one towards the Moon and one on the opposite side. As Earth rotates, friction causes these bulges to lead slightly ahead of the Moon, generating a forward pull that increases the Moon’s orbital momentum. This effect transfers some of Earth’s rotational energy to the Moon, causing it to move away at a rate of 1.5 inches per year, while slowing Earth’s rotation.

Although this process may seem imperceptible in daily life, it illustrates the subtle yet ongoing exchange of energy between a planet and its satellite, shaping the dynamics of the Earth-Moon system. Richard Vondrak, a lunar scientist at NASA’s Goddard Space Flight Center, explained in a NASA release:

“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.”

Looking Back in Time

The Moon hasn’t always been as far away as it is today. A study published in Paleoceanography and Paleoclimatology suggests that it formed approximately 4.5 billion years ago, when a young Earth was struck by a Mars-sized protoplanet. The collision threw debris into space, which eventually clumped together to form the Moon, much closer than it is today.

Fossilized clam shells provide further clues. They show daily growth patterns indicating that 70 million years ago, near the end of the dinosaurs, a day lasted about 23.5 hours. Paleontologists say this matches astronomical models, confirming that Earth’s rotation has slowed over millions of years as it moved outward.

Current Implications for Earth

The Moon’s slow retreat also affects tides and the length of days. The Conversation reported that tidal bulges can change water levels by about 5 feet in coastal cities like New York or Los Angeles. These bulges are also part of the reason it drifts away, transferring momentum bit by bit.

For humans, the changes are tiny, but scientists point out that observing these processes allows researchers to study the mechanics of planetary motion, the evolution of orbital systems, and the long-term consequences of gravitational forces. And even though the Moon will continue to drift away, inch by inch each year, it will never truly break free from Earth’s pull.