What If the Moon Disappeared?

No more moonlight, smaller tides, and a planet whose tilt could wander over millions of years. What Earth would be like without its Moon — and what the Moon has already done for us.

The Moon is about a quarter of Earth's width — unusually large compared with its planet. It has been our companion for about 4.5 billion years. What would change if it vanished overnight?

Right away: darker nights

A full Moon is by far the brightest object in the night sky. Without it, every night would be as dark as a new-Moon night. Many animals time their behaviour — feeding, hunting, mating — to the lunar cycle. Some coral species spawn by moonlight. These rhythms would be disrupted. Solar eclipses would disappear too.

Within hours: smaller tides

Tides are caused mainly by the Moon's gravity, with the Sun adding a smaller share. Without the Moon, only the solar tides would remain — roughly a third of the size of today's typical tides. Coastlines would change, and the intertidal zones that support a huge variety of life would shrink.

Over millions of years: a wobbling planet

Earth's axis is tilted about 23.4°, and today that tilt only varies by a couple of degrees over tens of thousands of years. The Moon's gravity helps hold it steady. Without it, calculations suggest the tilt could vary much more widely over long periods. Early studies suggested swings could be enormous; later work suggests the range might be smaller, though still larger than today. Either way, seasons and climate zones would become less stable over geological time — a challenge for life.

What the Moon has already done

Slowed our day. Tidal friction from the Moon has been slowing Earth's rotation for billions of years. Days were much shorter in the distant past. The Moon also drifts away from Earth by about 3.8 cm per year as a result.
Steadied our seasons. Its stabilising effect may have helped give life a steady climate over long timescales.
Shaped coastal life. Some scientists suggest strong tides helped early life explore the boundary between sea and land, though this is debated.

Do habitable planets need a big moon?

Not necessarily — but it may help. A planet without one could still be habitable, perhaps with a less stable climate. Large moons like ours seem to form through giant impacts, so their frequency around other planets is an open question.

How to build it

To give your planet a large moon, include a late giant impact in step 3 and let the debris collect in orbit.