The Moon

The Moon

The Moon is Earth's only natural satellite. It orbits around Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, having a sidereal period of 27.3 days and a synodic period of 29.5 days. The Moon faces Earth always with its near side by having a rotation period that equals its orbital period, resulting from being tidally locked to Earth. Its gravitational influence is the main driver of Earth's tides and causes Earth's day to lengthen very slowly. Its diameter is 3,474 km (2,159 mi), which is roughly one-quarter that of Earth or twice the width of Australia, making it by far the largest and most massive satellite in the Solar System in relation to its parent planet and the fifth-largest Solar System satellite overall. The Moon is a satellite planet in geophysical terms and among all planetary-mass objects of the Solar System larger and more massive than all known solar dwarf planets. The Moon's mass, density and surface gravity of about one-sixth of Earth's (at 0.1654 g), are rivaled among Solar System satellites only by Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with no significant atmosphere, hydrosphere, or magnetic field. The most widely accepted origin explanation posits that the Moon formed 4.51 billion years ago, not long after Earth's formation, out of the debris from a giant impact between Earth and a hypothesized Mars-sized body called Theia. It receded to a wider orbit because of tidal interaction with the Earth. 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. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The Moon is the brightest celestial object in Earth's night sky. This is mainly due to its large angular diameter, while the reflectance of the lunar surface is comparable to that of asphalt. The apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse. The Moon is, beside when passing through Earth's shadow during a lunar eclipse, always illuminated by the Sun, but from Earth the visible illumination shifts in the course of its orbit, producing the lunar phases. From Earth about 59% of the lunar surface is visible over time due to cyclical shifts in perspective (libration), making parts of the far side of the Moon visible. 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. Such influences can be found in language, calendar systems, art, and mythology. On September 14, 1959 the Moon was for the first time reached by an artificial object, the Soviet Union's Luna 2 impactor, and in 1966 soft-landing was achieved by Luna 9. In 1969 humans for the first time arrived on the Moon, landing on July 16 at Mare Tranquillitatis with the lander Eagle of Apollo 11 as part of the United States' Apollo program. Until 1972 five more crews were sent, with each time two men staying for up to twelve days. These missions remain the only times humans have visited an extraterrestrial body. Continued exploration of the Moon has allowed a detailed understanding of the Moon and its significance for Earth and life on it.

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