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Moon, the only natural satellite of the Earth, which is also the nearest celestial object, is an opaque body that shines by reflected light of the Sun.
The Moon is, after the Sun, the brightest object in the sky: its magnitude is -12.7. In size, it ranks fifth among the solar system satellites: the Moon are bigger only 3 satellites of Jupiter (Ganymede, Callisto, and Io), a satellite of Saturn (Titan) and one of Neptune (Triton). Its average diameter is 3476 km, ie ~27 / 100 than the Earth; its shape is almost exactly spherical.
The average density is about 60% that of Earth.
The moon’s closest point to Earth is called perigee, the furthest peak. Due to the gravitational perturbations due to the Sun and the planets, the distances of the Moon at perigee and apogee are not constant over time. The average Earth-Moon distance is ~384.400 km. The Moon has been observed with great interest from the remotest antiquity. The Greeks were among the first to investigate the nature of this celestial body. Already the philosophers of the Eleatic school and maybe even before them, Anaximenes of Miletus had realized that the moon does not shine by its own light, but simply reflects that of the sun. It was the observations at the telescope, started by Galileo in 1609, to unambiguously reveal the characteristics of the lunar surface.
Galileo himself shall spot mountain ranges, valleys, craters and vast dark regions, which he called seas, for their apparent resemblance to terrestrial oceans. The lunar surface is studded with craters of all sizes. In the past about their origin two hypotheses were formulated: the volcanic hypothesis that the craters would have formed (such as terrestrial calderas) following the sinking of extinct volcanoes; and the hypothesis of the impacts, according to which they would be excavated by meteorites. The entire surface of the moon is covered with a thick layer of stones and dust, called regolith. The regolith is derived from the fragmentation of the original rocks at the hands of meteorites. Although inconsistent, this material is durable enough (the boots of the astronauts will sink only a few centimeters).
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