Moon

The Moon is Earth's only natural satellite. It orbits around Earth at an average distance of 384400 km (238900 mi; 1.28 ls), or about 30 times Earth's diameter. The Moon faces Earth always with its near side by having a rotation period that equals its orbital period, resulting from being tidally locked to Earth. This period is in relation to the Sun 29.5 days long and visible as the cycle of the lunar phases. For Earth, the Moon has been aside from the Sun the most influential celestial object, particularly with its gravitational influence it has been the main driver of Earth's tides.

Quick facts: Designations, Designation, Alternative names,... ▼

Moon

The near side of the Moon (north at top) as seen from Earth
Designations
Designation	Earth I
Alternative names	Luna Selene (poetic) Cynthia (poetic)
Adjectives	Lunar Selenian (poetic) Cynthian (poetic) Moonly (poetic)
Symbol	or
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)[1]
Eccentricity	0.0549[1]
Orbital period (sidereal)	27.321661 d (27 d 7 h 43 min 11.5 s[1])
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[2][lower-alpha 1]
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[lower-alpha 2][3]
Physical characteristics
Mean radius	1737.4 km   (0.2727 of Earth's)[1][4][5]
Equatorial radius	1738.1 km   (0.2725 of Earth's)[4]
Polar radius	1736.0 km   (0.2731 of Earth's)[4]
Flattening	0.0012[4]
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)[4]
Mass	7.342×1022 kg   (0.0123 of Earth's)[1][4][6]
Mean density	3.344 g/cm3[1][4] 0.606 × Earth
Surface gravity	1.622 m/s2  (0.1654 g; 5.318 ft/s2)[4]
Moment of inertia factor	0.3929±0.0009[7]
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[8] 6.687° to orbit plane[2] 24° to Earth's equator [9]
North pole right ascension	17h 47m 26s 266.86°[10]
North pole declination	65.64°[10]
Albedo	0.136[11]
Surface temp. min mean max Equator 100 K[12] 250 K 390 K[12] 85°N  150 K 230 K[13]
Surface absorbed dose rate	13.2 μGy/h (during lunar daytime)[14]
Surface equivalent dose rate	57.0 μSv/h (during lunar daytime)[14]
Apparent magnitude	−2.5 to −12.9[lower-alpha 3] −12.74  (mean full moon)[4]
Absolute magnitude (H)	0.2[15]
Angular diameter	29.3 to 34.1 arcminutes[4][lower-alpha 4]
Atmosphere[16]
Surface pressure	10−7 Pa (1 picobar)  (day) 10−10 Pa (1 femtobar)    (night)[lower-alpha 5]
Composition by volume	He Ar Ne Na K H Rn

The Moon's mass, density and surface gravity of about one-sixth of Earth's (at 0.1654 g), are rivaled among Solar System satellites only by Jupiter's moon Io. It is a satellite planet in geophysical terms and among all planetary-mass objects of the Solar System larger and more massive than all of its known dwarf planets.[17] Its diameter is 3,474 km (2,159 mi), which is roughly one-quarter that of Earth or about the width of Australia,[18] making it by far the largest and most massive satellite in the Solar System in relation to its parent planet and the fifth-largest Solar System satellite overall. The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. It formed 4.51 billion years ago, not long after Earth's formation, out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called Theia.

The lunar surface is covered in impact craters and is marked, mostly on the near side of the Moon, by dark maria ("seas"), which are plains of cooled magma. These maria formed when large impacts on the far side of the Moon heated up low lying layers of its crust on the near side. The Moon is, beside when passing through Earth's shadow during a lunar eclipse, always illuminated by the Sun, but from Earth the visible illumination shifts during its orbit, producing the lunar phases.[19] The Moon is the brightest celestial object in Earth's night sky. This is mainly due to its large angular diameter, while the reflectance of the lunar surface is comparable to that of asphalt. The apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse. From Earth about 59% of the lunar surface is visible over time due to cyclical shifts in perspective (libration), making parts of the far side of the Moon visible.

For humans the Moon has been an important source of inspiration and knowledge, having been crucial to cosmography, mythology, religion, art, time keeping, natural science, and spaceflight. On September 13, 1959 the Soviet Union's Luna 2 was the first human-made object to reach the Moon and any extraterrestrial body, and the first soft landing was achieved in 1966 by Luna 9. On July 20, 1969 the United States' Apollo 11 mission landed humans for the first time on the Moon, and any extraterrestrial body, at Mare Tranquillitatis with the lander Eagle. Five more crews were sent between then and 1972, each with two men landing on the surface. The longest stay was 75 hours by the Apollo 17 crew. Thereafter exploration of the Moon has continued robotically, with crewed missions being planned to return starting in the late 2020s.