Why doesn't the Moon fall to Earth?

There is a way of rephrasing the question that makes it far more interesting: why doesn’t the Moon come down on us, if gravity pulls it towards us without pause? The correct answer is that gravity is working perfectly. The Moon is falling. It is falling right now, and it has been falling for four and a half billion years.

It never arrives because it is doing something else at the same time.

Newton’s cannon thought experiment

Newton explained it with an image that still holds. Picture a cannon on top of a very high mountain, aimed horizontally. Fire it gently and the ball traces an arc and lands nearby. Increase the charge and it lands further away. Harder still, and it lands beyond the horizon.

Increase it again and something new happens: the ball keeps falling at the usual rate, but it moves sideways so fast that the ground curves away beneath it at the same pace as it drops. It never touches down. It goes on falling around the planet forever.

That is an orbit. Not a balance between two opposing forces, not a zone where gravity stops working. It is a falling trajectory that closes on itself.

The numbers, for scale

The Moon is on average 384,400 kilometres away, according to NASA’s fact sheet: about thirty Earth-sized planets would fit in the space between us. It completes one lap in 27.3 days, which works out at roughly a kilometre per second, about 3,700 km/h.

Its light takes a little over a second to reach us. Looking at the Moon means seeing it as it was a second ago.

And the falling? That can be quantified too, and the arithmetic is within anyone’s reach. In one second the Moon drops towards us by a little over a millimetre. In that same second it moves sideways by a kilometre, and at a distance of 384,400 kilometres a kilometre of sideways travel corresponds to exactly that millimetre of curvature. The two motions cancel with a precision that is not a coincidence: it is the very definition of a circular orbit.

Why it always shows the same face

A detail that looks like chance and isn’t: from Earth we always see the same lunar hemisphere. The Moon rotates on its axis in exactly the time it takes to go round us, a condition called synchronous rotation. It is not a lucky accident: it is the consequence of tidal deformation, which over hundreds of millions of years slowed the rotation until it locked to the orbital period.

Watch out for a common misreading: this does not mean the Moon doesn’t rotate. It does, once every 27.3 days. The far side is not a dark side: it gets sunlight like the other one, it is simply never turned towards us.

It is also escaping, slowly

The same tidal effect that synchronised the rotation is still at work, transferring energy from Earth’s spin into the lunar orbit. We measure the result: the Moon moves a few centimetres further away every year, a figure we know precisely because reflectors were left on the lunar surface and the round trip of a laser pulse is timed from the ground.

There is a price: Earth slows down. Our days lengthen, by a tiny amount but relentlessly, and it is the same physics that makes Earth’s rotation imperceptible but not unchanging.

The same holds for everything that orbits

This is not a special rule for the Moon. The Space Station orbits at about 400 kilometres and, to stay up there, has to travel at roughly 28,000 km/h, completing a lap every 90 minutes. If it slowed, drag from the traces of atmosphere still present at that height would bring it down: which is why it is periodically boosted back up.

And that is why the astronauts on board float. Not because there is no gravity up there: at 400 kilometres Earth’s gravity is still about 90 per cent of what you feel now. They float because they are falling along with the station, all at the same rate. Inside a lift in free fall nobody presses on the floor. The right name is not weightlessness, it is continuous free fall, and it applies to the roughly 16,000 active satellites that are falling around us right now without ever touching us.