Earth's Moon is unusually large compared with the planet it orbits, and the leading explanation begins with catastrophe: another world roughly the size of Mars slammed into the young Earth and blasted material into orbit.

Four and a half billion years ago, Earth was barely a planet.

It was a half-molten ball of rock, still cooling from its own violent formation, when something roughly the size of Mars came out of the dark and hit it. Not a graze. A direct hit, hard enough to melt rock into vapor and throw a ring of debris into orbit around what was left of our planet. That debris never went anywhere. It is still up there, and you can see it tonight if the sky is clear.

It sounds like something you’d dismiss as a bad science fiction plot, except it happens to be the leading scientific explanation for a genuine puzzle: why our moon is so strangely large for the planet it orbits. Most rocky planets in this solar system have no moon at all, or moons so small they barely register. Ours is different, and the reason behind that difference is a lot more violent than “the moon has just always been there.”

How did the moon get so big in the first place?

Mercury and Venus have no moons at all. Mars has two, Phobos and Deimos, both small enough and lumpy enough to look more like potatoes caught in orbit than proper moons, most likely asteroids the planet’s gravity grabbed in passing. Earth’s moon is a different category of object. It measures a little over a quarter of Earth’s own diameter, which makes it the largest moon relative to its planet found anywhere in the solar system, according to Space.com’s breakdown of lunar size data.

If Earth were the size of a nickel, the moon would sit next to it roughly the size of a coffee bean, not a grain of sand.

For a long time, three competing ideas tried to explain that ratio. One guessed the moon formed somewhere else entirely and simply wandered close enough for Earth’s gravity to catch it. Another proposed that a young, fast-spinning Earth flung off a piece of itself. A third suggested Earth and the moon simply formed side by side, out of the same cloud of dust, at the same time. None of the three held up once scientists compared the chemistry of moon rocks the Apollo astronauts carried home to rock from Earth’s own mantle. The two were too similar for a captured object passing through, and too specific to be a coincidence.

What is the giant-impact hypothesis, exactly?

The explanation that survived all that testing is also the most dramatic one. A Mars-sized world, one researchers nicknamed Theia after the Greek titan said to have given birth to the moon goddess Selene, struck the early Earth at an angle rather than head on.

The collision vaporized part of both worlds and threw the wreckage into orbit, where it eventually pulled itself together into the moon. Scientists running detailed impact simulations at NASA’s Ames Research Center have found the process may have happened startlingly fast, with the debris clumping into a moon-sized body within hours of the original impact rather than the millions of years researchers once assumed.

“This opens up a whole new range of possible starting places for the Moon’s evolution,” said Jacob Kegerreis, a postdoctoral researcher at NASA’s Ames Research Center who worked on those simulations, in a 2022 NASA release on the findings. A moon born out of a direct hit to our own planet is a strange thing to look up at on an ordinary evening.

We put the fuller version of this story into a video, tracing what a collision like that would have actually looked like and then following the trail forward to something you can watch happen at any beach: the tide. It walks through how NASA has used laser reflectors the Apollo astronauts left sitting on the lunar dust to measure the moon inching away from us by about the width of a fingernail every year, and how researchers matched that number against eclipse timings ancient Babylonian astronomers pressed into clay tablets nearly three thousand years ago.

How do we know the moon used to sit so much closer to Earth?

Ancient rock holds a surprising amount of the answer. Certain layers of sediment, laid down one thin band at a time by tides pushing in and out over millions of years, work like a diary of how fast Earth used to spin and how close the moon used to sit. Reading bands like that in South Australia, geologists have pegged a day at 21.9 hours some 620 million years ago, which works out to a 400-day year, according to Scientific American’s rundown of the tidal rhythmite research. The year itself hadn’t changed length. It simply held more days, because the planet spun faster and a nearer moon hadn’t yet dragged that speed away.

Norman Murray, a theoretical astrophysicist at the University of Toronto’s Canadian Institute for Theoretical Astrophysics, put the scale of the whole process plainly: “When the moon first formed some 4.5 billion years ago, the day was less than 10 hours long. But since then, the moon’s gravitational pull on the Earth has been slowing our planet’s rotation, resulting in an increasingly longer day,” he said, as reported by ScienceDaily in 2023. Babylonian eclipse tablets going back to 750 BC back up the same trend on a shorter timescale. Researchers who reconciled those ancient observations against modern atomic clocks found the length of a day has been growing by about 1.8 milliseconds every century, a small enough number that it took clay tablets and laser beams three millennia apart to confirm it, per a 2016 write-up of the study.

Why is the moon still drifting away today?

Gravity’s ledger always balances eventually. Earth’s oceans bulge slightly toward the moon and slightly away from it on the opposite side, and friction from that water dragging across the seafloor acts as a brake on Earth’s spin. The energy that brake removes from Earth doesn’t just disappear. Because Earth spins faster than the moon orbits, our planet’s tidal bulge sits slightly ahead of the moon rather than lined up underneath it, and that extra bit of mass tugs the moon forward, feeding it energy. Feed an orbiting object energy and it climbs into a wider orbit, which is the mechanism behind the same 3.8 centimeters a year of drift NASA and JPL have tracked using those same Apollo-era mirrors since the 1970s.

I stood under a genuinely dark sky once, in the Atacama Desert in Chile, drinking a pisco sour mixed with a wild local herb called rica-rica and watching more stars than I’d ever seen appear at once. It felt, for a few minutes, like standing on a different planet entirely. Picturing an Earth spinning under a moon four times closer than it is now, filling half the sky and hauling brutal tides across young, empty coastlines, makes that desert night feel almost tame by comparison.

What does a moon that keeps leaving actually mean for us?

Nothing about it will matter within any of our lifetimes, or our children’s, or several hundred thousand generations after that. But it does eventually run out. As the moon drifts farther from Earth, it will appear smaller in our sky, and the near-perfect coincidence that lets it currently cover the sun exactly during a solar eclipse won’t hold forever. “About 600 million years from now, Earth will experience the beauty and drama of a total solar eclipse for the last time,” said Richard Vondrak, a lunar scientist at NASA’s Goddard Space Flight Center, in comments reported by Space.com.

Every fingernail’s width the moon retreats, and every fraction of a millisecond added to your day, traces back to one violent collision billions of years before anything alive was around to notice it. We happen to exist in the narrow window where the math still works and the sky still puts on the show. Whatever you make of that timing, it is worth an occasional look up.