Earth orbits the Moon at a distance of 384,400 km (238,900 mi) on average, which is approximately 30 times Earth’s diameter. Earth’s tides are primarily driven by its gravitational pull, which also slowly extends the day. The sidereal period of the Moon’s orbit around Earth is 27.3 days. The lunar phases that serve as the basis for the months of a lunar calendar are determined by the amount of visible surface illuminated by the Sun during each synodic period, which lasts 29.5 days.
The Moon is tidally locked to Earth, which means that the synodic period and the length of a full Moon rotation on its own axis coincide, and the lunar day is the same length as the synodic period. However, periodic shifts in perspective known as libration allow us to see 59% of the lunar surface from Earth.
The most common explanation for the Moon’s origin is that it was formed 4.51 billion years ago, not long after Earth, from the debris of a huge impact between Earth and a body about the size of Mars called Theia. After that, tidal interactions with the Earth caused it to recede into a wider orbit. Dark volcanic maria, or “seas,” line the Moon’s near side and stand in for prominent impact craters and bright ancient crustal highlands. By the end of the Imbrian period, roughly three billion years ago, the majority of the large impact basins and mare surfaces were in place.
The reflectance of lunar soil is comparable to that of asphalt, indicating that the lunar surface is relatively non-reflective. The full moon, on the other hand, is the brightest celestial object in the night sky because of its large angular diameter. Because the Moon appears to be about the same size as the Sun, it can almost completely obscure the Sun during a total solar eclipse.
Throughout human history, the Moon’s regular cycle of phases and prominence in the sky have served as cultural references and influences. Language, calendar systems, art, and mythology all show these influences. In 1959, the Soviet Union’s Luna 2 uncrewed spacecraft was the first artificial object to reach the Moon. Luna 9 made its first soft landing in 1966, which was a success.
The United States’ Apollo program has carried out the only human lunar mission to date, landing twelve men on the moon’s surface between 1969 and 1972. Moon rocks from these and subsequent uncrewed missions were used to gain a thorough geological understanding of the Moon’s origins, internal structure, and subsequent history. Other than the Earth, humans will only be able to visit the Moon until 2023.