The Moon is an astronomical body that orbits planet Earth, and is Earth’s only permanent natural satellite. It 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). The Moon is after Jupiter‘s satellite Io as the second-densest satellite in the Solar System among those whose densities are known.
The Moon is thought to have formed about 4.51 billion years ago, not long after Earth. The most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia.
The Moon is in synchronous rotation with Earth, and thus always shows the same side to earth, the near side. The near side is marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. The Moon is after the sun as the second-brightest regularly visible celestial object in Earth’s sky. Its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.
The Moon’s average orbital distance is 384,402 km (238,856 mi), or 1.28 light-seconds. This is about thirty times the diameter of Earth. The Moon’s apparent size in the sky is almost the same as that of the Sun (because it is 400x farther and larger). Therefore, the Moon covers the Sun nearly precisely during a total solar eclipse. This matching of apparent visual size will not continue in the far future, because the Moon’s distance from Earth is slowly increasing.