The Moon

Earth's only moon — and the only other world humans have walked on

The Moon is Earth's closest neighbour. It makes no light of its own — it reflects sunlight like a mirror. That is why it looks like a crescent one week and a full circle the next: these are the Moon's phases. There is no air on the Moon, so its sky is black even in daytime. In 1969 people flew to the Moon and walked on it — their footprints are still there!

The Moon is Earth's natural satellite, with a radius of 1,737 km (27% of Earth's). It orbits at 384,400 km once every 27.3 days and spins on its axis in exactly the same time (synchronous rotation), so from Earth we always see the same face. Its phases cycle every 29.5 days: new moon → waxing crescent → full moon → waning crescent. The Moon's gravity drives the tides on Earth. Its surface is covered with craters (meteorite scars) and dark 'seas' — plains of solidified lava. The Apollo programme (1969–1972) landed 12 astronauts and brought back 382 kg of lunar rock.

Fun facts

• The Moon drifts 3.8 cm away from Earth every year — as fast as your fingernails grow.
• Footprints on the Moon last for millions of years: there is no wind or rain.
• On the Moon you'd weigh 6 times less — and jump 6 times higher!
• Nobody had seen the far side of the Moon until 1959, when the Luna 3 probe photographed it.