The Moon has hung in the sky your whole life without ever dropping. Earth's gravity plainly reaches it — so why doesn't it come down?

It is coming down

Start by throwing out two tempting answers. The Moon is not beyond gravity's reach, and it is not propped up by some balancing force. Earth's gravity pulls on it hard — that pull is the entire reason the Moon stays near us instead of drifting off into the dark. The truth is stranger: the Moon is falling toward the Earth, constantly, right now.

The catch is that it is also moving sideways, and fast — about a kilometre every second. So in the moment it falls toward the Earth, the round Earth curves away beneath it by very nearly the same amount. It drops, and the ground it was falling toward is no longer under it. It drops again, and misses again. An orbit is nothing grander than that: falling, and forever missing the floor.

Newton's cannonball

Isaac Newton saw it first, with a thought experiment. Picture a cannon on a mountaintop so tall it pokes above the air. Fire the ball gently and it arcs down to the ground a mile away. Fire it harder and it lands farther off. Fire it hard enough and the Earth's surface curves away as fast as the ball falls — so it never lands at all. It circles. It is the same gravity that binds any moon to its planet, and the same pull that stretches our seas into tides. The Moon is simply Newton's cannonball, launched long ago and moving quickly enough that it has been missing the Earth for four and a half billion years.

Astronauts on the Space Station don't float because gravity has run out up there — at that altitude it is still nearly as strong as at the ground. They float because they, and the station around them, are in constant free fall toward the Earth. Weightlessness isn't the absence of gravity. It's falling, with nothing to land on.

The Moon isn't defying gravity. It is obeying it utterly — and travelling sideways fast enough that "down" never quite arrives. Einstein would later recast the whole picture as coasting along the curve of spacetime, but the older image still holds: the Moon is a stone thrown so well that it never comes back.