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Moon

The Moon (Latin: Luna) is the Earth's only natural satellite. Although not the largest natural satellite in the Solar System, it is the largest relative to the size of the object it orbits (its primary)  and, after Jupiter's satellite Io, it is the second most dense satellite among those whose densities are known.

The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the most luminous object in the sky after the Sun. Although it appears a very bright white, its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art, and mythology. The Moon's gravitational influence produces the ocean tides and the slight lengthening of the day. The Moon's current orbital distance is about thirty times the diameter of Earth, causing it to have an apparent size in the sky almost the same as that of the Sun. This allows the Moon to cover the Sun nearly precisely in total solar eclipse. This matching of apparent visual size is a coincidence. The Moon's linear distance from Earth is currently increasing at a rate of 3.82±0.07 cm per year, but this rate is not constant.


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Natural satellite

A natural satellite, or moon, is a celestial body that orbits another body, e.g. a planet, which is called its primary. There are 173 known natural satellites orbiting planets in the Solar System, as well as at least eight orbiting IAU-listed dwarf planets. As of January 2012, over 200 minor-planet moons have been discovered. There are 76 known objects in the asteroid belt with satellites (five with two satellites each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers. There are also 84 known natural satellites of trans-Neptunian objects. Some 150 additional small bodies have been observed within rings of Saturn, but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, though numerous candidates have been detected to date, none have yet been confirmed.

Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The large gas giants have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four Galilean moons, Saturn's Titan, and Neptune's Triton. Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five. It has been suggested that some satellites may potentially harbour life, though there is currently no direct evidence of life.


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