Moon

 MOON:

Such influences can be found in language, calendar systems, art, and mythology. The lunar surface is fairly non-reflective, with the reflectance of lunar soil being comparable to that of asphalt. The near side of the Moon is marked by dark volcanic maria ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. It then receded to a wider orbit because of tidal interaction with the Earth. It lacks any significant atmosphere, hydrosphere, or magnetic field. The Moon is a planetary-mass object with a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term and larger than all known dwarf planets of the Solar System. The Moon is the only celestial body visited by humans. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of Australia). However, due to its large angular diameter, the full moon is the brightest celestial object in the night sky. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The first artificial object to reach the Moon was the Soviet Union's uncrewed Luna 2 spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. The Moon is Earth's only natural satellite. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.

Both the Moon's prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Its surface gravity is about one-sixth of Earth's at 0.1654 g, with Jupiter's moon Io being the only satellite in the Solar System known to have a higher surface gravity and density.

The Moon orbits Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter. However, 59% of the total lunar surface can be seen from Earth through cyclical shifts in perspective known as liberation.

The most widely accepted origin explanation posits that the Moon formed 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history. The Moon's orbit around Earth has a sidereal period of 27.3 days. Most of the large impact basins and mare surfaces were in place by the end of the Imbriani period, some three billion years ago. Its gravitational influence is the main driver of Earth's tides and very slowly lengthens Earth's Day. One explanation suggests that large meteorites were hitting the Moon in its early history leaving large craters which then were filled with lava. After the Moon's formation the Moon settled in orbit around Earth much closer than today, making both bodies appear much larger in each's sky and causing on both more frequent and stronger eclipses and tidal effects. The causes for the eruption of mare basalts, particularly their uneven occurrence on mainly the near side, like the lunar highlands on the far side, has been an unresolved issue due to differing explanations. Other explanations suggest processes of lunar volcanism. Volcanically active until 1.2 billion years ago, most of the Moon's mare basalts erupted during the Imbriani period, 3.3–3.7 billion years ago, though some being as young as 1.2 billion years and some as old as 4.2 billion years. Since then, due to tidal acceleration, the Moon's orbit around Earth has become significantly larger as well as longer, tidally locking the so-called lunar near side, always facing Earth with this same side.

The post formation cooled lunar surface has been shaped by large and many small impact events, retaining a broadly cratered landscape of all ages, as well as by volcanic activity, producing the prominent lunar maria. The Moon is a very slightly scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to adjust to its orbit.