Moon

Moon
	The near side of the Moon (north is at top)
Designations
Designation	Earth I
Alternative names	Luna; Selene (poetic); Cynthia (poetic);
Adjectives	Lunar; Selenian (poetic); Cynthian (poetic); Moonly (poetic);
Orbital characteristics
	Epoch J2000
Perigee	362600 km; (356400–370400 km)
Apogee	405400 km; (404000–406700 km)
Semi-major axis	384399 km (1.28 ls, 0.00257 AU)
Eccentricity	0.0549
Orbital period (sidereal)	27.321661 d; (27 d 7 h 43 min 11.5 s)
Orbital period (synodic)	29.530589 d; (29 d 12 h 44 min 2.9 s)
Average orbital speed	1.022 km/s
Inclination	5.145° to the ecliptic
Longitude of ascending node	Regressing by one revolution in 18.61 years
Argument of perigee	Progressing by one; revolution in 8.85 years
Satellite of	Earth
Physical characteristics
Mean radius	1737.4 km ; (0.2727 of Earth's)
Equatorial radius	1738.1 km ; (0.2725 of Earth's)
Polar radius	1736.0 km ; (0.2731 of Earth's)
Flattening	0.0012
Circumference	10921 km (equatorial)
Surface area	3.793×107 km2 ; (0.074 of Earth's)
Volume	2.1958×1010 km3 ; (0.02 of Earth's)
Mass	7.342×1022 kg ; (0.0123 of Earth's)
Mean density	3.344 g/cm3; 0.606 × Earth
Surface gravity	1.622 m/s2 (0.1654 g; 5.318 ft/s2)
Moment of inertia factor	0.3929±0.0009
Escape velocity	2.38 km/s; (8600 km/h; 5300 mph)
Synodic rotation period	29.530589 d; (29 d 12 h 44 min 2.9 s; synodic; solar day) (spin-orbit locked)
Sidereal rotation period	27.321661 d (spin-orbit locked)
Equatorial rotation velocity	4.627 m/s
Axial tilt	1.5424° to ecliptic; 6.687° to orbit plane; 24° to Earth's equator ;
North pole right ascension	17h 47m 26s; 266.86°;
North pole declination	65.64°
Albedo	0.136
Surface absorbed dose rate	13.2 μGy/h
Surface equivalent dose rate	57.0 μSv/h
Apparent magnitude	−2.5 to −12.9; −12.74 (mean full moon);
Angular diameter	29.3 to 34.1 arcminutes
Atmosphere
Surface pressure	10−7 Pa (1 picobar) (day); 10−10 Pa (1 femtobar) ; (night);
Composition by volume	He; Ar; Ne; Na; K; H; Rn;

The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia), it is the fifth largest satellite in the Solar System, the largest satellite in the Solar System relative to its major planet, and larger than any known dwarf planet. The Moon is a planetary-mass object that formed a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term. It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's (0.1654 g). Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.

Orbiting Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, its gravitational influence very slowly lengthens Earth's day and is the main driver of Earth's tides. The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. That said, 59% of the total lunar surface can be seen from Earth through shifts in perspective due to libration.

The most widely accepted origin explanation posits that the Moon formed 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic maria ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. However, because it has a large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.

Both the Moon's prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union's Luna 2 uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history.