he Moon is Earth's only natural satellite and is the fifth largest
satellite in the Solar System. It is the largest natural satellite in
the Solar System relative to the size of its planet, a quarter the
diameter of Earth and 1/81 its mass, and is the second densest satellite
after Io. It is in synchronous rotation with Earth, always showing the
same face; the near side is marked with dark volcanic maria among the
bright ancient crustal highlands and prominent impact craters. Despite
being the brightest object in the sky after the Sun, its surface is
actually very dark, with a similar reflectance to coal. Its prominence
in the sky and its regular cycle of phases have since ancient times made
the Moon an important cultural influence on language, the calendar, art
and mythology. The Moon's gravitational influence produces the ocean
tides and the minute lengthening of the day. The Moon's current orbital
distance, about thirty times the diameter of the Earth, causes it to be
the same size in the sky as the Sun allowing the Moon to cover the Sun
precisely in total solar eclipses.
The Moon is the only celestial body on which humans have made a manned landing. While the Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft, the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972 the first being Apollo 11 in 1969. These missions returned over 380 kg of lunar rocks, which have been used to develop a detailed geological understanding of the Moon's origins (it is thought to have formed some 4.5 billion years ago in a giant impact), the formation of its internal structure, and its subsequent history.
Moon
Orbital characteristics
Perigee 363,104 km (0.0024 AU)
Apogee 405,696 km (0.0027 AU)
Semi-major axis 384,399 km (0.00257 AU)
Eccentricity 0.0549
Orbital period 27.321582 d (27 d 7 h 43.1 min)
Synodic period 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
(between 18.29° and 28.58° to Earth's equator)
Longitude of ascending node regressing by one revolution in 18.6 years
Argument of perigee progressing by one revolution in 8.85 years
Satellite of Earth
Physical characteristics
Mean radius 1,737.10 km (0.273 Earths)
Equatorial radius 1,738.14 km (0.273 Earths)
Polar radius 1,735.97 km (0.273 Earths)
Flattening 0.00125
Circumference 10,921 km (equatorial)
Surface area 3.793 × 107 km2 (0.074 Earths)
Volume 2.1958 × 1010 km3 (0.020 Earths)
Mass 7.3477 × 1022 kg (0.0123 Earths)
Mean density 3,346.4 kg/m3
Equatorial surface gravity 1.622 m/s2 (0.165 4 g)
Escape velocity 2.38 km/s
Sidereal rotation
period 27.321582 d (synchronous)
Equatorial rotation velocity 4.627 m/s
Axial tilt 1.5424° (to ecliptic), 6.687° (to orbit plane)
Albedo 0.136
Surface temp. min mean max
equator 100 K 220 K 390 K
85°N 70 K 130 K 230 K
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 (day)
10−10 Pa (night)
Composition Ar, He, Na, K, H, Rn
The Moon is the only celestial body on which humans have made a manned landing. While the Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft, the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972 the first being Apollo 11 in 1969. These missions returned over 380 kg of lunar rocks, which have been used to develop a detailed geological understanding of the Moon's origins (it is thought to have formed some 4.5 billion years ago in a giant impact), the formation of its internal structure, and its subsequent history.
Moon
Orbital characteristics
Perigee 363,104 km (0.0024 AU)
Apogee 405,696 km (0.0027 AU)
Semi-major axis 384,399 km (0.00257 AU)
Eccentricity 0.0549
Orbital period 27.321582 d (27 d 7 h 43.1 min)
Synodic period 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
(between 18.29° and 28.58° to Earth's equator)
Longitude of ascending node regressing by one revolution in 18.6 years
Argument of perigee progressing by one revolution in 8.85 years
Satellite of Earth
Physical characteristics
Mean radius 1,737.10 km (0.273 Earths)
Equatorial radius 1,738.14 km (0.273 Earths)
Polar radius 1,735.97 km (0.273 Earths)
Flattening 0.00125
Circumference 10,921 km (equatorial)
Surface area 3.793 × 107 km2 (0.074 Earths)
Volume 2.1958 × 1010 km3 (0.020 Earths)
Mass 7.3477 × 1022 kg (0.0123 Earths)
Mean density 3,346.4 kg/m3
Equatorial surface gravity 1.622 m/s2 (0.165 4 g)
Escape velocity 2.38 km/s
Sidereal rotation
period 27.321582 d (synchronous)
Equatorial rotation velocity 4.627 m/s
Axial tilt 1.5424° (to ecliptic), 6.687° (to orbit plane)
Albedo 0.136
Surface temp. min mean max
equator 100 K 220 K 390 K
85°N 70 K 130 K 230 K
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 (day)
10−10 Pa (night)
Composition Ar, He, Na, K, H, Rn
