The moon is slowly drifting away

Did you know that every year the moon moves a tiny bit farther from Earth? This is because the oceans' tidal forces tug on it, slowly changing its orbit and even Earth's rotation over billions of years. By Stephen DiKerby

- Stephen DiKerby

The motions of the Earth and moon have many interesting consequences.

(Photo: iStock)

The moon is getting 3.8cm farther away from the Earth every year. Scientists measure the distance to the moon by bouncing lasers off mirrors placed there by space probes and astronauts. By measuring the amount of time it takes light to travel to the moon and back, scientists can very precisely measure the distance to the moon and how it changes.

The distance to the moon actually changes over a single month as it goes around the Earth. The moon is typically 385,000km away from the Earth, but its orbit is not a perfect circle and changes by about 20,000km as it orbits the Earth. This change is why some full moons are a bit bigger than others; they are called supermoons.

As an astrophysics researcher, I'm interested in the motion and interaction of objects such as planets, stars and galaxies. The motions of the Earth and moon have many interesting consequences, and studying how they move over time can help researchers better understand how each has changed over the four and a half billion years since the Earth and moon formed.

Tidal forces

So, why is the moon getting farther away? It’s all because of tides.

Tides come from a difference in gravity across an object. The force of gravity exerted by the moon is about 4% stronger on the side of Earth that faces towards the moon, compared with the opposite side of the Earth facing away, because gravity gets weaker with distance.