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The Moon is falling sideways
Why it never lands
Earth's gravity pulls on everything, the Moon included, and no opposing force holds the Moon up. What keeps it from crashing is a sideways motion called an orbit. A thrown ball traces a tiny orbit of its own, one that ends when it hits the ground.
Speed is the whole difference. A ball thrown fast enough would bend around the world and come back to the thrower, and with no air to slow it down it could orbit forever. The Moon does exactly that, falling sideways around Earth with no air to slow it down, once every 27 days at 3,600 kilometers an hour.
Why orbits stay stable
Changing an orbit means changing the object's speed, which changes where gravity takes it, and even small changes require enormous forces. That is why the large objects of the solar system are so stable. Igniting billions of rocket engines across the Moon's surface would barely move it, and for the Moon to simply stop and plummet toward Earth would break more laws of physics than there is time to explain.
How fast must a ball go to orbit?
Without air, a ball launched sideways near Earth's surface would need about 7.9 kilometers per second, roughly 28,000 kilometers an hour. Air drag makes that impossible at ground level, so real satellites orbit hundreds of kilometers up, where the atmosphere is almost absent. The International Space Station circles at about 400 kilometers of altitude and travels at roughly 27,600 kilometers an hour. It completes a lap of Earth about every 90 minutes.
Why does the Moon move so much slower?
Gravity weakens with the square of the distance. At the Moon's distance of about 384,000 kilometers, a lower speed is enough to keep a circular path, and the path itself is far longer. Both factors stretch one lap into about 27 days. Earth's pull at the Moon's distance is roughly three thousand times weaker than at the altitude of the International Space Station.
Why do astronauts float if gravity still acts?
At the altitude of the International Space Station, Earth's gravity is still about 90 percent as strong as at the surface. Astronauts float because they and the station fall around Earth together, so nothing pushes back on them. Everything inside, including the station's floor, is in free fall at the same rate. Parabolic aircraft flights used for astronaut training create the same effect for about 20 seconds at a time.
How did Newton picture this?
Isaac Newton imagined a cannon on top of a very tall mountain firing balls horizontally. A slow shot lands nearby, a faster one lands farther away, and at high enough speed the ball's fall follows the curve of the planet, so it never lands. The thought experiment is the standard way orbits are still taught. Newton described it in the 1680s while working on the Principia, which appeared in 1687.
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