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Definition: Moon from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

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.

During the 21st century, probes have been sent to the Moon by the European Space Agency, China, Japan, India, and the USA. NASA's Lunar Reconnaissance Orbiter is mapping the Moon from its polar orbit. A pair of GRAIL spacecraft began orbiting the Moon in tandem at the beginning of 2012, travelling just 50 km/30 mi high and about 200 km/120 mi apart. For nearly a year, before being deliberately crashed on the Moon, they measured their varying spacing in order to produce extraordinarily detailed maps of the Moon's gravitational field from which the satellite's inner structure can be deduced.

A joint Indian-Russian mission is planned to include an orbiter, a lander, and a rover, and is due to be launched in 2016. In the longer term, Russia plans an uncrewed lunar ‘colony’, with its own power station, laboratory, long-range rover, and associated satellite in orbit. The Google corporation is offering a prize of $30 million for any privately funded team that can send a rover to the Moon that travels more than 500 m/1,640 ft across the surface. Twenty-two teams registered for the contest and they have until 2015 to accomplish the feat.

In January 2004 US president George W Bush announced a plan to put astronauts back on the Moon by 2020. However, the successor administration of President Barack Obama cancelled the plan, turning its attention to crewed trips to Mars and the asteroids instead. Japan and India have announced their intention to send crewed missions to the Moon.

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.

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moon

From The Columbia Encyclopedia
natural satellite of a planet (see satellite, natural ) or dwarf planet, in particular, the single natural satellite of the earth . The moon is the earth's nearest neighbor in space. In addition to its proximity, the moon is also exceptional in that it is quite massive compared to the earth itself, the ratio of their masses being far larger than the similar ratios of other natural satellites to the planets they orbit (though that of Charon and the dwarf planet Pluto exceeds that of the moon and earth). For this reason, the earth-moon system is sometimes considered a double planet. It is the center of the earth-moon system, rather than the center of the earth itself, that describes an elliptical orbit around the sun in accordance with Kepler's laws . It is also more accurate to say that the earth and moon together revolve about their common center of mass, rather than saying that the moon revolves about the earth. This common center of mass lies beneath the earth's surface, about 3,000…
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Full text Article Moon

From Astronomy Encyclopedia
Moon When it is at its brightest, at full moon,...
Earth's only natural satellite. Because of its proximity, it is the brightest object in the sky, apart from the Sun, being at a mean distance of only 384,000km (239,000mi). Like the Sun, its apparent diameter is about half a degree. With an actual diameter of 3476 km (2160 mi) and a density of…
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Full text Article Moon

From Chambers Dictionary of the Unexplained
The Earth’s only natural satellite, which has long been the subject of speculation and has an important place in folkloric, religious, occult and esoteric traditions from around the world. The Moon is second only to the sun as a feature of the heavens. As the Sun is associated with the daytime, …
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Full text Article Moon

From Philip's Encyclopedia
Natural satellite of a planet; in particular the natural satellite of the planet Earth. Apart from the Sun it is the brightest object in the sky as seen from the Earth, being at a mean distance of only 384,000km (239,000mi). Its diameter is 3,476km (2,160mi). As the Moon orbits the Earth, it goes…
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Full text Article moon

From Astronomy Encyclopedia
Moon Much of the Moon's surface is covered in a...
Natural SATELLITE of one of the larger bodies in the Solar System. Mars, for example, possesses two moons, PHOBOS and DEIMOS . Each of the giant planets is accompanied by a large retinue of moons. Smaller moons may be termed moonlets, and the distinction between a moon/moonlet and a body too small…
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Full text Article moon

From The Columbia Encyclopedia
natural satellite of a planet (see satellite, natural ) or dwarf planet, in particular, the single natural satellite of the earth . The moon is the earth's nearest neighbor in space. In addition to its proximity, the moon is also exceptional in that it is quite massive compared to the earth itself, …
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Buzz Aldrin photographed by Neil...
In the third century BC Aristarchus of Samos estimated the size and distance of the Moon, in terms of Earth diameters. His result was accurate to within about 15%. Galileo started to observe the Moon with one of his first telescopes in late 1609. He noted that the terminator was not the uniform…
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Full text Article MOON

From Gaither's Dictionary of Scientific Quotations
American environmentalist and nature writer The old moon, like a worn and ancient coin, is still hanging in the west when I awake. Desert Solitaire Terra Incognita: Into the Maze (p. 289 ) Ballantine Books. New York New York USA . 1968. English essayist, poet, and statesman ... …
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Full text Article moon

From The Macquarie Dictionary
the body which revolves around the earth monthly at a mean distance of 384 403 km, accompanying the earth in its annual revolution about the sun. It is 3476 km in diameter, and its mass is 0.0123 that of the earth. Plural: moons this heavenly body during a particular lunar month, or during a certain…
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Full text Article moon

From The American Heritage Student Science Dictionary
moon
The moon changes in appearance during its...
Often Moon The natural satellite of Earth, visible by reflection of sunlight and traveling around Earth in a slightly elliptical orbit at an average distance of about 238,855 miles (384,400 kilometers). The moon’s average diameter is 2,159 miles (3,475 kilometers), and its mass is about 8– that of…
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Full text Article moon

From The American Heritage(R) Dictionary of the English Language
moon PhotoDisc, Inc. Photo:
often Moon The natural satellite of Earth, visible by reflection of sunlight and having a slightly elliptical orbit, approximately 363,100 kilometers (225,600 miles) distant at perigee and 405,700 kilometers (252,100 miles) at apogee. Its mean diameter is 3,475 kilometers (2,159 miles), its mass…
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