How It Works

How the Earth-Moon system works


What many people don’t know is the Moon doesn’t just orbit the Earth, but Earth orbits the Moon too. While the Moon is propelled around Earth in an elliptical orbit, the pull of the Moon’s own gravity causes our planet to move slightly off its own centre and around in a small circle.

Think of it like an Olympic hammer thrower swinging the hammer around their body while holding onto the chain: even though the hammer is many times smaller than the thrower, it’s enough to pull the thrower slightly off their mark.

The barycentre marks the centre of mass for this Earth-Moon relationship. The forces involved in Earth-Moon barycentre dynamics are very regular, but even so, tiny variances mean the Moon is gradually moving away from our world.

When the Moon was first formed it was very close and had a powerful effect on the development of the early Earth. At first it moved away from us at a rate of ten kilometres (6.2 miles) per year, slowing down over billions of years to its current rate of just 3.8 centimetres (1.5 inches) per year.