Moon

The far side of the Moon photographed from the Apollo 13 spacecraft in 1970. The lunar landscape has been fashioned by the impact of countless meteorites. The bright-rayed crater seen here was caused by a relatively recent impact (still many millions of years ago). The darker areas are thought to be far older craters which have filled with lava welling up from the Moon's interior.

US astronauts landed on the Moon in July and November 1969. They collected samples of Moon rock for analysis and set up experiments to send data back to Earth. During moon walks the astronauts wore protective suits incorporating an oxygen supply, and weighted boots to compensate for the low gravity on the Moon.

The Moon's south pole. This mosaic is made up from around 1,500 pictures, taken by the US probe Clementine and pieced together to create an orthographic image map. Parts of the south polar Aitken Basin are permanently shadowed by crater walls. It is speculated that ice could persist on the lunar surface here.

A full Moon photographed from the Apollo 11 spacecraft as it made the three-day, 386,000-km/240,000-mi journey back to Earth. This photograph was taken about 18,520 km/10,000 mi from the Moon.

The phases of the Moon as viewed from the Earth. The Moon takes a lunar month (about 29 days) to orbit the Earth. When the Earth is between the Sun and the Moon, the Moon appears to be ‘full’ because we can see all the light from the Sun reflected by the Moon.

Natural satellite of Earth, 3,476 km/2,160 mi in diameter, with a mass 0.012 (approximately one-eightieth) that of Earth.

Its surface gravity is only 0.16 (one-sixth) that of Earth. Its average distance from Earth is 384,400 km/238,855 mi, and it orbits in a west-to-east direction every 27.32 days (the sidereal month). It spins on its axis with one side permanently turned towards Earth. The Moon has no atmosphere and was thought to have no water until ice was discovered on its surface in 1998.

Phases

The Moon is illuminated by sunlight, and goes through a cycle of phases of shadow, waxing from new (dark) via first quarter (half Moon) to full, and waning back again to new every 29.53 days (the synodic month, also known as a lunation). On its sunlit side, temperatures reach 110°C/230°F, but during the two-week lunar night the surface temperature drops to −170°C/−274°F.

Origins

The origin of the Moon is still open to debate. Scientists suggest the following theories: that it split from the Earth; that it was a separate body captured by Earth's gravity; that it formed in orbit around Earth; or - the theory most widely favoured currently - that it was formed from debris thrown off when a body the size of Mars struck Earth.

Research

70% of the far side of the Moon was photographed from the Soviet Lunik 3 in October 1959. Much of our information about the Moon has been derived from this and other photographs and measurements taken by US and Soviet Moon probes, from geological samples brought back by US Apollo astronauts and by Soviet Luna probes, and from experiments set up by US astronauts 1969-72. The US probe Lunar Prospector, launched in January 1998, examined the composition of the lunar crust, recorded gamma rays, and mapped the lunar magnetic field. It also discovered the ice on the moon in March 1998. In January 2004, US president George W Bush announced a plan to put astronauts back on the Moon by 2020.

Composition

The Moon is rocky, with a surface heavily scarred by meteorite impacts that have formed craters up to 240 km/150 mi across. Seismic observations indicate that the Moon's surface extends downwards for tens of kilometres; below this crust is a solid mantle about 1,100 km/688 mi thick, and below that a silicate core, part of which may be molten. Rocks brought back by astronauts show that the Moon is 4.6 billion years old, the same age as Earth. It is made up of the same chemical elements as Earth, but in different proportions, and differs from Earth in that most of the Moon's surface features were formed within the first billion years of its history when it was hit repeatedly by meteorites.

The youngest craters are surrounded by bright rays of ejected rock. The largest scars have been filled by dark lava to produce the lowland plains called seas, or maria (plural of mare). These dark patches form the so-called ‘man-in-the-Moon’ pattern. Inside some craters that are permanently in shadow is up to 300 million tonnes/330 million tons of ice existing as a thin layer of crystals.

One of the Moon's easiest features to observe is the crater Plato, which is about 100 km/62 mi in diameter and 2,700 m/8,860 ft deep, and at times is visible with the naked eye.

The US lunar probe Clementine discovered an enormous crater on the far side of the Moon in 1994. The South Pole-Aitken crater is 2,500 km/1,563 mi across and 13 km/8 mi deep, making it the largest known crater in the Solar System.

moon

In astronomy, any natural satellite that orbits a planet. Mercury and Venus are the only planets in the Solar System not known to have moons.