Theories of the Origin of the Moon
The Moon is the only natural satellite of Earth. The distance from Earth is about 384,400km with a diameter of 3476km and a mass of 7.35*1022kg. Through history it has had many names: Called Luna by the Romans, Selene and Artemis by the Greeks. And of course, has been known through prehistoric times. It is the second brightest object in the sky after the Sun. Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.<br><br><b>Origin of the Moon</b><br>Before the modern age of space exploration, scientists had three major theories for the origin of the moon: fission from the earth; formation inearth orbit; and formation far from earth. Then, in 1975, having studied moonrocks and close-up pictures of the moon, scientists proposed what has come to be regarded as the most probable of the theories of formation, planetesimalimpact or giant impact theory.<br><br><b>Formation by Fission from the Earth</b><br>The modern version of this theory proposes that the moon was spun off from the earth when the earth was young and rotating rapidly on its axis. This idea gained support partly because the density of the moon is the same as that of the rocks just below the crust, or upper mantle, of the earth. A major difficulty with this theory is that the angular momentum of the earth, in order to achieve rotational instability, would have to have been much greater than the angular momentum of the present earth-moon system.<br><br><b>Formation in Orbit Near the Earth</b><br>This theory proposes that the earth and moon, and all other bodies of the solar system, condensed independently out of the huge cloud of cold gases and solid particles that constituted the primordial solar nebula. Much of this material finally collected at the center to form the sun.<br><br><b>Formation Far from Earth</b><br>According to this theory, independent formation of the earth and moon, as in the above theory, is assumed; but the
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