Scientists have discovered that the Moon is moving approximately 1½ inches (3.8 centimetres) farther away from the Earth every year. This gradual change in distance is measured with remarkable precision by bouncing lasers off mirrors placed on the Moon by astronauts and space probes. By timing how long it takes light to travel to the Moon and back, researchers can calculate not only the current distance but also how it changes over time.

The Moon does not maintain a constant distance from Earth. Its orbit is not a perfect circle, so the distance varies by about 12,400 miles (20,000 km) each month. Typically, the Moon is about 239,000 miles (385,000 km) away. This variation explains phenomena like supermoons, where the Moon appears slightly larger in the sky.

Why is the moon moving away?

Astrophysics researcher Stephen DiKerby explains that the Moon’s increasing distance is caused by tides. “Tides come from a difference in gravity across an object,” he says. The Moon’s gravity is about 4% stronger on the side of Earth facing the Moon compared to the opposite side. This uneven pull creates tidal bulges in Earth’s oceans that point toward and away from the Moon.

As the Earth rotates, these bulges move, and their gravitational pull slightly leads the Moon. “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,” DiKerby explains. He compares the effect to 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.”

How this affects earth

The Moon gains momentum as its orbit expands. Because the Earth is doing the work of increasing the Moon’s momentum, the planet’s rotation slows slightly. This exchange means that days on Earth become longer over millions of years. However, the effect is minimal:1.5 inches per year compared to a distance of 239,000 miles is only 0.00000001% per year. Daily life, eclipses, and tides will remain largely unaffected for millions of years.

A look into the past

The Moon was much closer to Earth in the past. Scientists believe it formed around 4.5 billion years ago after a young Earth was struck by a Mars-sized protoplanet, sending material into space that eventually formed the Moon. At that time, the Moon appeared much larger in the sky.

Evidence from fossilised clam shells indicates that 70 million years ago, near the end of the dinosaurs’ era, Earth’s day was only 23.5 hours long. This matches predictions from astronomical data about Earth’s rotation slowing over time.

What will happen in the future?

Could the Moon eventually escape Earth’s gravity entirely? DiKerby explains that over tens of billions of years, Earth’s rotation could slow enough to become tidally locked with the Moon, meaning one side of Earth would always face the Moon. At this point, the Moon would stop moving away.

However, other cosmic events will prevent this scenario. In about a billion years, the Sun will become hotter, evaporating Earth’s oceans and eliminating the large tidal bulges that drive the Moon’s recession. Later, the Sun will expand into a red giant, likely destroying both Earth and the Moon.

“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,” DiKerby concludes.

Author: Stephen DiKerby, Postdoctoral Researcher in Physics and Astronomy, Michigan State University

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