Orbit of the Moon

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Orbit of the Moon
Diagram of the Moon's orbit with respect to the Earth. While angles and relative sizes are to scale, distances are not.
Semi-major axis[a]384,748 km (239,071 mi)[1]
Mean distance[b]385,000 km (239,000 mi)[2]
Inverse sine parallax[c]384,400 km (238,900 mi)
Perigee363,228.9 km (225,700.0 mi), avg.
(356400370400 km)
Apogee405,400 km (251,900 mi), avg.
(404000406700 km)
Mean eccentricity0.0549006
Mean obliquity6.687°[5]
Mean inclination
of orbit to ecliptic5.15° (4.99–5.30)[3]
of lunar equator to ecliptic1.543°
Period of
orbit around Earth (sidereal)27.322 days
orbit around Earth (synodic)29.530 days
precession of nodes18.5996 years
precession of line of apsides8.8504 years

The Moon takes 27.3 days (3.9 weeks) to orbit around the Earth. One side of the Moon faces the Earth at all times, so the moon is tidally locked to Earth.

The phases of the Moon come from the position of the Moon relative to the Earth and Sun. If the Moon and Sun are on opposite sides of the Earth, there is a full moon. If they are on the same side of Earth, there is a new moon.

The Moon makes one circle around Earth, called the sidereal month, in 27.3 days. But in this time, the Earth continues to move forward, so the Moon will be in a different place relative to the Sun, and the phase of the Moon will be different. To reach the same phase, the Moon must travel a bit farther. The time for the Moon to reach the same phase, called the synodic month, is 29.5 days.

References[change | change source]

  1. M. Chapront-Touzé; J. Chapront (1983). "The lunar ephemeris ELP-2000". Astronomy & Astrophysics. 124: 54. Bibcode:1983A&A...124...50C.
  2. M. Chapront-Touzé; J. Chapront (1988). "ELP2000-85: a semi-analytical lunar ephemeris adequate for historical times". Astronomy & Astrophysics. 190: 351. Bibcode:1988A&A...190..342C.
  3. 3.0 3.1 3.2 Meeus, Jean (1997), Mathematical Astronomy Morsels, Richmond, VA: Willmann-Bell, pp. 11–12, 22–23, ISBN 0-943396-51-4
  4. Seidelmann, P. Kenneth, ed. (1992), Explanatory Supplement to the Astronomical Almanac, University Science Books, pp. 696, 701, ISBN 0-935702-68-7
  5. Lang, Kenneth R. (2011), The Cambridge Guide to the Solar System, 2nd ed., Cambridge University Press.

Notes[change | change source]

  1. The geometric mean distance in the orbit (of ELP) which is the semimajor axis of the Moon's elliptical orbit via Kepler's laws.
  2. The constant in the ELP expressions for the distance, which is the mean distance averaged over time.
  3. The inverse sine parallax ɑ/sin π is traditionally the Moon's mean distance from Earth (center to center), where ɑ is Earth's equatorial radius, and π is the Moon's parallax between the ends of ɑ.[3] Three of the IAU 1976 Astronomical Constants were "mean distance of Moon from Earth" 384,400 km, "equatorial horizontal parallax at mean distance" 3422.608″, and "equatorial radius for Earth" 6,378.14 km.[4]