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Moon, natural satellite of the earth. The moon is 2,160 mi (3,476 km) in diameter, or about one-fourth the size of the earth, and has a smaller mass than the earth. It would take 82 moons to tip the scales against the earth. The moon is about 239,000 mi (384,623 km) from earth.

The moon takes just under a calendar month, or 27.322 days, to orbit the earth. In fact, the word “month” is derived from the word “moon.” As it orbits the earth, it also rotates on its axis. The result is that the moon always presents the same side toward the earth.

In the course of its orbit, the moon is seen to go through phases. It reflects sunlight and its phases are the result of the progressive increase and decrease of the portions of its surface reflecting sunlight as it orbits the earth. The new moon occurs when the moon's reflecting surface is turned away from the sun and is completely in shadow. The full moon occurs when the whole of the moon's reflecting surface is illuminated by the sun.

With the naked eye, the moon appears to be divided unevenly into bright and dark areas. Through a telescope astronomers are able to identify the bright regions as upland areas and the dark regions as lowlands, plains, or depressions. The plains are called “maria,” from the Latin for “seas,” because they were once thought to be expanses of water. It is not certain whether there are bodies of water on the moon, although in 1972 Apollo 17 did discover possible traces of water. Neither is there a lunar atmosphere. Without an insulating atmosphere the daytime temperature of the lunar surface reaches 200°F (93°C), and at night it falls to −250°F (−157°C).

The “seas” of the moon are lowland areas that appear to have been flooded with volcanic lava. Scientists reason that the lava has obliterated many craters. But for more than a century, scientists argued about the origin of the moon's many craters. One theory was that the moon's features were the result of explosive impacts by giant meteors. Opponents of that view argued that some form of volcanic action had built up the craters. Closeup photographs by orbiting space probes have provided evidence of both processes. On July 20, 1969, the United States succeeded in landing the first man on the moon. In March of 1998, measurements made by the space probe Lunar Prospector indicated that ice caps might be present on the moon.

See also: Solar System.

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