Moon

The Moon is Earth's only natural satellite. It orbits around Earth at an average distance of (; about 30 times Earth's diameter). Its orbital period (lunar month) and its rotation period (lunar day) are synchronized at 29.5 days by Earth's gravity pulling on the Moon. This makes the Moon tidally locked to Earth, always facing it with the same side. The Moon's gravitational pull produces tidal forces on Earth, which are the main driver of Earth's tides.

In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is , roughly one-quarter of Earth's (about as wide as the contiguous United States). Within the Solar System, it is the largest and most massive satellite in relation to its parent planet. It is the fifth-largest and fifth-most massive moon overall, and is larger and more massive than all known dwarf planets. Its surface gravity is about one-sixth of Earth's, about half that of Mars, and the second-highest among all moons in the Solar System after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. The lunar surface is covered in lunar dust and marked by mountains, impact craters, their ejecta, ray-like streaks, rilles and, mostly on the near side of the Moon, by dark maria ('seas'), which are plains of cooled lava. These maria are believed to have formed when molten lava flowed into ancient impact basins. The Moon was 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 known as Theia.

From a distance, the day and night phases of the lunar day are visible as the lunar phases, and when the Moon passes through Earth's shadow a lunar eclipse is observable. The Moon's apparent size in Earth's sky is about the same as that of the Sun, which causes it to cover the Sun completely during a total solar eclipse. The Moon is the brightest celestial object in Earth's night sky because of its large apparent size, while the reflectance (albedo) of its surface is comparable to that of asphalt. About 59% of the surface of the Moon is visible from Earth, owing to the different angles at which the Moon can appear in Earth's sky (libration), making parts of the far side of the Moon visible.

The Moon has been an important source of inspiration and knowledge in human history, having been crucial to cosmography, mythology, religion, art, time keeping, natural science, and spaceflight. The first human-made objects to fly to an extraterrestrial body were sent to the Moon starting in 1959 with the flyby of the Soviet Union's ''Luna 1'' and the intentional impact of ''Luna 2''. In 1966, the first soft landing (by ''Luna 9'') and orbital insertion (by ''Luna 10'') followed. On July 20, 1969, during the United State's Apollo 11 mission, humans stepped on to an extraterrestrial body for the first time at Mare Tranquillitatis in the lander ''Eagle''. Five more crews were sent between 1970 and 1972, each landing two men on the Moon's surface. The longest stay was 75 hours by the Apollo 17 crew, after which exploration of the Moon was continued using robots. Crewed missions were planned to return in the late 2020s. Provided by Wikipedia