Sole natural satellite of Earth, which it orbits from west to east at a mean distance of about 238,900 mi (384,400 km). It is less than one-third the size of Earth (diameter about 2,160 mi, or 3,476 km, at its equator), about one-eightieth as massive, and about two-thirds as dense. Its surface gravity is about one-sixth that of Earth, and its gravitational pull is largely responsible for Earth's tides. The Moon shines by reflected sunlight, but its albedo is only 7.3%. It rotates on its axis in about 29.5 days, in exactly the time it takes to orbit Earth, and it therefore always presents the same face to Earth. However, that face is lit by the Sun at different angles as the Moon revolves around Earth, causing it to display different phases over the month, from new to full.
Most astronomers believe the Moon formed from a cloud of fragments ejected into Earth orbit when a Mars-sized body struck the proto-Earth early in the solar system's history. Its surface has been studied by telescope since Galileo first observed it in 1609 and firsthand by a total of 12 U.S. astronauts during the six successful lunar landing missions of the Apollo program. The dominant process affecting the surface has been impacts, both from micrometeorite bombardment, which grinds rock fragments into fine dust, and from meteorite strikes, which produced the craters profusely scattered over its surface mostly early in its history, over four billion years ago. The maria (&see; mare) are huge, ancient lava flows. In the late 1990s unmanned spacecraft found possible signs of water ice near the Moon's poles. More generally, a moon is any natural satellite orbiting a planet or other nonstellar body.
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![As the Moon revolves around Earth, the amount of its illuminated half seen from Earth slowly &elipsis; [Credit: © Merriam-Webster Inc.]](http://images.bookrags.com/images/eb/72198.gif)
