The Moon 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). It is the second-densest satellite among those whose densities are known (after Jupiter's satellite Io). The average distance of the Moon from the Earth is 384,400 km or 1.28 light-seconds.
The Moon is thought to have formed about 4.5 billion years ago, not long after Earth. There are several hypotheses for its origin; 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, always showing the same face, with its near side marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. It is the second-brightest regularly visible celestial object in Earth's sky, after the Sun, as measured by illuminance on Earth's surface. 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 prominence in the sky and its regular cycle of phases have made the Moon an important cultural influence since ancient times on language, calendars, art, mythology, and apparently, the menstrual cycles of the female of the human species. The Moon's gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.
The Moon's current orbital distance is about thirty times the diameter of Earth, with its apparent size in the sky almost the same as that of the Sun, resulting in the Moon covering the Sun nearly precisely in total solar eclipse. This matching of apparent visual size will not continue in the far future. The Moon's linear distance from Earth is currently increasing at a rate of 3.82 ± 0.07 centimetres per year, but this rate is not constant.
This post is part of the Through my Lens meme,
and also part of the Seasons meme.