What if the Moon disappeared?

Take the Moon away in an instant: what happens to the tides, the nights, Earth's tilt and the length of the day

A what-if: physics allows it on paper, but no one has ever seen one. Everything shown follows the theory as it stands.

Step by step

The Moon today

It drifts because of the tides it raises. Earth spins faster than the Moon goes round it, so Earth carries its tidal bulges a little ahead of the Moon. Their pull speeds the Moon along and lifts it outward, while the Moon's pull on them slowly brakes Earth's spin.

How could it vanish?

So this is a thought experiment, like asking what a bridge does when one pillar is taken out. The Moon is removed in an instant with nothing left behind, and everything else follows the laws of physics as they are. What changes shows how much of Earth's daily life the Moon quietly shapes.

The instant

Where the Moon was, there would be stars. Every night would be like a moonless night today, and by day there would be no pale Moon in the blue. Nothing falls from the sky: the Moon is simply gone, which is exactly what makes this impossible in reality.

Tides shrink

High tide would come twice a day at the same clock times, not about 50 minutes later each day. Real coasts strengthen or weaken tides through the shape of their bays, so each would change in its own way, but everywhere tides would be much smaller, and the shores, mudflats and pools they uncover would shrink.

Darker nights

On the Great Barrier Reef, many kinds of coral release their eggs and sperm together on a few nights after a full Moon in late spring, a mass spawning first described in 1984. The Moon's monthly cycle of light is part of how they keep time; without it, the timing that lets eggs and sperm meet could break down.

Some animals steer by it. Dung beetles in South Africa roll their balls in a straight line by the pattern of polarised moonlight (Dacke and others, 2003). Many night hunters and their prey change their habits with the Moon's phase; all of them would have to adapt.

Earth's tilt

Laskar, Joutel and Robutel calculated in 1993 that a moonless Earth's tilt would wander chaotically anywhere between 0° and about 85° over millions of years. At a large tilt, much of the planet would swing between scorching summers and long, dark, freezing winters.

Later and longer calculations by Lissauer, Barnes and Chambers (2011) found the swings slower and smaller: typically within 10° to 20° of today's tilt over hundreds of millions of years. Either way nothing would change in a human lifetime: the tilt would drift over millions of years.

The day stops slowing

So the day would stay close to 24 hours far longer. With the Moon, tides would keep stretching it for billions of years.

What stays the same

What is gone for good: solar eclipses, when the Moon hides the Sun, and lunar eclipses, when Earth's shadow falls on the Moon. The month, which began as the time from one new Moon to the next, would be only a custom.

Earth alone

Seen from space: Earth alone, the Moon's orbit empty. Mercury and Venus already live like this: neither has a moon.

Our Moon is unusually big for its planet. The best explanation of where it came from is a collision 4.5 billion years ago between the young Earth and a body the size of Mars, whose debris gathered into the Moon. Mars has two tiny moons, probably captured or made from smaller impacts; Mercury and Venus have none.