|Discovered by||Always Known|
|Discovery date||Always Known|
|Periapsis||363,295 km (0.0024 AU)|
|Apoapsis||405,503 km (0.0027 AU)|
|Semimajor axis||384,399 km (0.00257 AU)|
|Orbital Period||27.321582 d (27 d 7 h 43.1 min)|
|Average orbital speed||1.022 km/s (2,286 mph, 3,678 kph)|
|Volume||2.1958 × 1010 km3 (0.020 Earths)|
|Mass||7.3477 × 1022 kg (0.012300 Earths)|
|Axial tilt|| 1.5424° (to ecliptic)
6.687° (to orbit plane)
|Mean density||3.3464 g/cm3|
|Surface temperature (min)||100 K (-279 °F, -173 °C)|
|Surface temperature (avg)||220 K (-63 °F, -53 °C)|
|Surface temperature (mean)||220 K (-63 °F, -53 °C)|
|Surface temperature (max)||390 K (242 °F, 116 °C)|
|Apparent magnitude|| −2.5 to −12.9
−12.74 (mean full moon)
The Moon is the only natural satellite of Earth. It is the fifth largest moon in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary. Among other satellites with known densities, the Moon is the second densest, after Io, which is a satellite of Jupiter.
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 brightest object in the sky after the Sun, although its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt.
The Moon is thought to have formed nearly 4.5 billion years ago, not long after the Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body.
After the Apollo 17 mission in 1972, the Moon has only been visited by unmanned spacecraft. Of these, orbital missions have dominated. Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters, which have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. The post-Apollo era has also seen two rover missions: the final Soviet Lunokhod mission in 1973, and China's ongoing Chang'e 3 mission, which deployed its Yutu rover on 14 December 2013.
Future manned missions to the Moon have been planned. For example, China plans to have a manned mission to the moon in a currently unknown date, though it is planned to occur before, or during 2025.
Name and etymologyEdit
The English proper name for Earth's natural satellite is "the Moon". The noun moon derives from the word moone, from 1380, which is derived from mone, from 1135, which derives from the Old English word mōna (which is dated before 725), which, like all Germanic language cognates, ultimately stems from Proto-Germanic *mǣnōn.
The principal modern English adjective pertaining to the Moon is lunar, which is derived from the latin word Luna.
Several mechanisms have proposed that the Moon formed about 4.527 billion years ago, about 30 or 50 million years after the origin of the Solar System.