- The Moon is the only natural satellite of Earth. It is also called Luna by the Romans, Selene by the Greeks, and many other names in other mythologies. The average distance from the Moon to the Earth is 384,401 kilometres (238,857 mi). The Moon's diameter is 3,476 kilometres (2,160 mi). Reflected sunlight from the Moon's surface reaches Earth in 1.3 seconds (at the speed of light). The Moon is the Solar System's fifth largest moon, both by diameter and mass, ranked behind Ganymede, Titan, Callisto and Io.
- The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.
- Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.
- The first man-made object to land on the Moon was Luna 2 in 1959; the first photographs of the normally occluded far side of the Moon were made by Luna 3 in the same year. The first manned mission to orbit the Moon was Apollo 8, and the first people to land on the Moon came aboard Apollo 11 in 1969. It is the only celestial body other than the Earth upon which humans have set foot.
- The Moon is in synchronous rotation, meaning that it keeps nearly the same face turned toward Earth at all times (there is a small variation, called libration). The side of the Moon that faces Earth is called the near side, and the opposite side is called the far side. The far side is also sometimes called the dark side, which means unknown and hidden, and not lacking light as might seem to be implied by the name; in fact, the far side receives (on average) as much sunlight as the near side. Spacecraft are cut off from direct radio communication with Earth when on the far side of the Moon due to line of sight. One distinguishing feature of the far side is its almost complete lack of maria (singular: mare), which are the dark albedo features. (An albedo feature is a large area on the surface of a planet (or other solar system body) which shows a contrast in brightness or darkness (albedo) with adjacent areas.)
- The Earth and Moon have many physical effects upon one another, including the tides. Most of the tidal effects seen on the Earth are caused by the Moon's gravitational pull, with a smaller contribution from the Sun. Tidal effects result in an increase of the mean Earth-Moon distance, over long periods of time, of about 4 metres per century.
- The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgassing — the release of gases, for instance radon, which originate deep within the Moon's interior. Another important source of gases is the solar wind, which is briefly captured by the Moon's gravity
- Compared to that of Earth, the Moon has a very weak magnetic field. While some of the Moon's magnetism is thought to be intrinsic (such as a strip of the lunar crust called the Rima Sirsalis), collision with other celestial bodies might have imparted some of the Moon's magnetic properties. Indeed, a long-standing question in planetary science is whether an airless solar system body, such as the Moon, can obtain magnetism from impact processes such as comets and asteroids. Magnetic measurements can also supply information about the size and electrical conductivity of the lunar core - evidence that will help scientists better understand the Moon's origins. For instance, if the core contains more magnetic elements (such as iron) than Earth, then the impact theory loses some credibility (although there are alternate explanations for why the lunar core might contain less iron).
- The Moon is continuously blocking our view of the sky directly behind it. The Moon blocks about a 1/2 degree wide circular area. When a bright star or planet passes behind the Moon it is occulted or hidden from view. A solar eclipse is an occultation of the Sun. Because the Moon is close to Earth, occultations of stars are not visible everywhere. Because of the moving nodes of the lunar orbit, each year different stars are occulted.
- The first leap in lunar observation was caused by the invention of the telescope. Galileo Galilei made especially good use of this new instrument and observed mountains and craters on the Moon's surface.
- The Cold War-inspired space race between the Soviet Union and the United States of America led to an acceleration of interest in the Moon. What was the next big step depends on the political viewpoint: In the US (and the West in general) the landing of the first humans on the Moon in 1969 is seen as the culmination of the space race. Neil Armstrong, representing the human race, became the first person to walk on the Moon as the commander of the American mission Apollo 11, first setting foot on the Moon at 02:56 UTC on July 21, 1969. The last person (as of 2006) to stand on the Moon was Eugene Cernan, who as part of the mission Apollo 17 walked on the Moon in December 1972.
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