Earth's Moon

Scientists theorize that the Moon was formed billions of years ago when a planet-sized object collided with the newly forming Earth. A large piece of Earth's mantle and debris material was blasted into space. Over millions of years the molten material accumulated and crystallized to form the lunar crust.

The Moon is Earth's only natural satellite, making one complete revolution every 27.3 days. The Moon's rotation and revolution periods are the same, which is called synchronous rotation. This means that the same side of the Moon faces the Earth at all times. Humans did not see the dark or far side of the Moon until the space flights of the 1960s and 1970s.

The Moon is composed of a solid ball of basalt covered by rocks and dust. Since the Moon's formation billions of years ago, the surface has been struck by asteroids, meteoroids and comets. These impacts have created dust, rocks, boulders and basins. There is no tectonic activity on the Moon, so the rock composition has remained unchanged since its formation.

The Moon is the fifth largest known moon in the solar system. Its diameter is 3,474 km. The Moon's volume is about 1/50th that of the Earth. Its gravitational pull is about 1/6th that of the Earth.