Moon

Orbit: 384,400 km (238,866 miles) from Earth
Diameter: 3476 km or 2160 miles
Mass: 7.35 x 10²² kg or 14.847 x 10²² lbs.

Earth has only one natural satellite, the Moon. Thousands of small artificial satellites have also been placed in orbit around Earth.

The Moon is called Luna by the Romans, Selene and Artemis by the Greeks.

The Moon has been known since prehistoric times, and is the second brightest object in the sky after the Sun.

As the Moon orbits around Earth once per month, the angle between Earth, the Moon and the Sun changes. This is seen as the cycle of the Moon's phases.

The time between each new moon is 29.5 days.

Due to its size and composition, the Moon is sometimes classified as a terrestrial planet along with Mercury, Venus, Earth and Mars.

The Soviet spacecraft Luna 2 visited the Moon in 1959. The first landing was on July 20, 1969. The last landing was in Dec. 1972. The Moon is the only body from which samples have been returned to Earth.

In 1994, the Moon was extensively mapped by the spacecraft Clementine. Lunar Prospector is now in orbit around the Moon.

Gravitational forces between Earth and the Moon cause the tides.

The Moon does not have a dark side because all of its parts get sunlight half the time.

The Moon has no atmosphere, but there may be water ice in some deep craters near the Moon's south pole. This was confirmed by Lunar Prospector. Apparently there is ice at the north pole as well.

The Moon's crust averages 68 km thick.

There are two primary types of terrain on the Moon - the old highlands that are heavily cratered and the younger maria that are relatively smooth. Maria are huge impact craters that were later flooded by molten lava. Most of the surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts.

A total of 382 kg of rock samples were returned to Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, 20 years after the last Moon landing, scientists still study these samples.

New and detailed information from the Moon rocks led to the impact theory that Earth collided with a very large object and the Moon formed from the ejected material. The impact theory is now widely accepted.