The Moon is an astronomical body that orbits Earth, being Earth’s only permanent natural satellite. The Moon is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits (its primary). Among satellites with reasonably circular orbits, far larger than those of typical asteroids or trans-Neptunian objects, only Ganymede (Jupiter III) and Titan (Saturn VI) are larger and more massive than the Moon. Although it is not perfectly spherical, it is close enough to a sphere for practical purposes. Its equatorial radius is about 3,475 km (2,159 mi), and its polar radius is about 3,355 km (2,077 mi), resulting in an equatorial circumference of about 10,921 km (6,784 mi) and a polar circumference of about 10,751 km (6692 mi).
The solid body of the Moon has been imaged extensively since 1959 by unmanned spacecraft launched by both the Soviet Union and United States. From these images we know that: The bright highlands on the near side were formed largely by impacts from space after most craters had already formed; dark areas consist mostly of lava flows; several large impact basins filled with dark lava date from this same era; at various times in its history magma oceans may have covered part or all of the lunar surface; finally any remaining magma was drawn down into the interior by gravity where it cooled to form a central core surrounded by a mantle layer.
The average distance from Earth to moon is 384 403 kilometers or 1 mean lunar distance which equals 3 686 400 kilometers . This means that light takes 1.3 seconds to travel between Earth and moon . The sidereal month , defined as the time needed for one complete orbit around Earth as measured against the background stars , averages 27.32 days . In contrast , synodic months or lunation s occur when Full moon occurs again relative to Sun – new moon – first quarter – full moon – third quarter … etc When viewed from Earth , phases of moon result because different illuminated fraction becomes visible as moons position changes relative to Sun . For example when orbiting bodies such as planets or moons cast their shadows onto other bodies we see eclipses e g solar eclipse Lunar eclipse Transits also can be seen when one body moves across disk another e g Venus transit Mercury transit Finally libration s rocking motion manifests itself as slight variation angle view over course year giving impression observer on earth actually seeing more 50% surface area from particular location vantage point .