The Moon is drifting away from Earth, a tiny shift that scientists can measure with remarkable precision. This slow drift comes from the way Earth and its lunar companion pull on each other, gradually transferring momentum from our planet to the satellite.
Keeping track of the lunar body’s distance might sound like a small detail, but it actually tells us a lot about the history of our planet. Experts say that studying how the Moon moves helps us understand how both Earth and its satellite have changed over billions of years.
Scientists have clever ways to measure this change. They bounce lasers off mirrors left on our satellite by astronauts and robots, timing how long the light takes to come back. These measurements show the average distance to the Moon is about 239,000 miles (385,000 kilometers), but it can swing by around 12,400 miles (20,000 kilometers) as it goes around Earth. That’s why we sometimes see supermoons, when the lunar body looks noticeably bigger in the sky.
Tidal Forces At Work
Its retreat is driven by tidal forces caused by gravitational differences across Earth. According to NOAA, the Moon’s gravity pulls more strongly on the side of Earth facing it, creating two ocean bulges; one toward the Moon and one on the opposite side. As the planet rotates, friction causes these bulges to lead slightly ahead of the Moon, generating a forward pull that increases its orbital momentum. This effect transfers some of Earth’s rotational energy to the Moon. The celestial body moves away at 1.5 inches per year, while Earth’s rotation slows very gradually.
The bulge on the side facing the Moon is labeled “Tidal bulge due to gravity,” indicating it is caused by the Moon’s gravitational pull. Credit: NOAA
Though imperceptible in daily life, this process illustrates the subtle but ongoing exchange of energy between a planet and its satellite, shaping the dynamics of the Earth-Moon system. As lunar scientist at NASA’s Goddard Space Flight Center Richard Vondrak explained in a in 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 celestial body hasn’t always been so far away. A study published in Paleoceanography and Paleoclimatology suggest that it formed 4.5 billion years ago, when a young Earth was hit 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 give more 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.
Earth’s rotation causes tidal bulges to lead the Moon slightly, creating tidal lag. Credit: NASA/Vi Nguyen
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 keep drifting away, inch by inch each year, it will never really break free from Earth’s pull.