# Orbit of the Moon

Semi-major axis[a] Diagram of the Moon's orbit with respect to the Earth. While angles and relative sizes are to scale, distances are not. 384,748 km (239,071 mi)[1] 385,000 km (239,000 mi)[2] 384,400 km (238,900 mi) 363,228.9 km (225,700.0 mi), avg.(356400–370400 km) 405,400 km (251,900 mi), avg.(404000–406700 km) 0.0549006(0.026–0.077)[3] 6.687°[5] 5.15° (4.99–5.30)[3] 1.543° 27.322 days 29.530 days 18.5996 years 8.8504 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

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. 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

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 ${\displaystyle {\frac {\alpha }{\sin \pi }}}$ 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]