The 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.
Orbit and relationship to Earth:
The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days (its sidereal period). However, since the Earth is moving in its orbit about the Sun at the same time, it takes slightly longer for the
Moon to show the same phase to Earth, which is about 29.5 days (its synodic period). Unlike most satellites of other planets, the
Moon orbits near the ecliptic and not the Earth's equatorial plane.
The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbital motion gradually rotates, which affects other aspects of lunar motion. These follow-on effects are mathematically described by
Cassini's laws. There are several known near-Earth asteroids that have unusual Earth-associated horseshoe orbits:
3753 Cruithne,
54509 YORP, (85770)
1998 UP1 and
2002 AA29.They are co-orbital with the Earth, so that their orbits bring them close to Earth for periods of time but then alter in the long term, and they are not natural satellites of Earth.
Seasons:
Although the Moon's minute axial tilt (1.54 degrees) means that seasonal variation is minimal, it is just enough to create a 3-degree variation in the Sun's elevation at the poles, resulting in a very slight "summer" and "winter". From images taken by
Clementine in
1994, it appears that four mountainous regions on the rim of
Peary crater at the Moon's north pole remain illuminated for the entire lunar day, creating peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters, and these dark craters are extremely cold:
Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−
238 °C), and just 26 K close to the winter solstice in north polar
Hermite Crater. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of
Pluto
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
- published: 05 Jun 2010
- views: 11481