Moon

The Moon is the only natural satellite of Earth. It orbits Earth in an elliptical path, at a distance that varies over the year, averaging around . It completes an orbit (lunar month) in relation to Earth and the Sun (synodically) every 29.5 days. The Moon and Earth are bound by gravitational attraction, which is stronger on the sides facing each other. The resulting tidal forces are the main driver of Earth's tides, and have pulled the Moon to always face Earth with the same near side. This tidal locking effectively synchronizes the Moon's rotation period (lunar day) to its orbital period (lunar month). 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 larger and more massive than any known dwarf planet, and the fifth-largest and fifth–most massive moon, as well as the largest and most massive in relation to its parent planet. 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 only a minuscule hydrosphere, atmosphere, and magnetic field.…

The Moon is the only natural satellite of Earth. It orbits Earth in an elliptical path, at a distance that varies over the year, averaging around . It completes an orbit (lunar month) in relation to Earth and the Sun (synodically) every 29.5 days. The Moon and Earth are bound by gravitational attraction, which is stronger on the sides facing each other. The resulting tidal forces are the main driver of Earth's tides, and have pulled the Moon to always face Earth with the same near side. This tidal locking effectively synchronizes the Moon's rotation period (lunar day) to its orbital period (lunar month). 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 larger and more massive than any known dwarf planet, and the fifth-largest and fifth–most massive moon, as well as the largest and most massive in relation to its parent planet. 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 only a minuscule hydrosphere, atmosphere, and magnetic field. The lunar surface is covered in regolith dust, which mainly consists of the fine material ejected from the lunar crust by impact events. The lunar crust is marked by impact craters; some younger ones feature bright ray-like streaks. The Moon was volcanically active until 1.2 billion years ago, surfacing lava mostly on the thinner near side of the Moon, filling ancient craters, which through cooling formed the today prominently visible dark plains of basalt called maria ('seas'). The origin of the Moon is not clear, although it is thought to have been formed out of material from Earth that was ejected by a giant impact of a Mars-sized body 4.51 billion years ago, not long after Earth's formation. 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 spaceflights to an extraterrestrial body were to the Moon, starting in 1959 with the flyby of Luna 1 (sent by the Soviet Union), and the intentional impact of Luna 2, followed in 1966 by the first soft landing (by Luna 9) and orbital insertion (by Luna 10). Humans first arrived in orbit with Apollo 8 (sent by the United States) on December 24, 1968, and then on the surface with Apollo 11 on July 20, 1969, making the Moon the only celestial body beyond Earth that humans have visited. By 1972, six Apollo missions had landed twelve people on the Moon and stayed up to three days. Renewed robotic exploration of the Moon, in particular to confirm the presence of water on the Moon, has fueled plans to return humans to the Moon. These plans include the Artemis program, which included the Artemis II crewed lunar flyby in 2026. ===Classification=== The International Astronomical Union (IAU) calls Earth's satellite "the Moon", with a capital "M". Other natural satellites of planets are called 'moons', with a lower-case "m". The Moon has historically been identified as a classical planet, in the original sense of a wandering object in the sky. When Galileo discovered satellites orbiting Jupiter, he named them 'moons' because, like the Moon, they can be considered planets orbiting another planet. Some experts hold to a geophysical definition that classifies moons as planets that orbit the Sun while incidentally orbiting another planet. In a heliocentric frame of reference the Moon orbits the Sun, perturbed by Earth. This has led some scientists to argue that the Moon could be identified as a planet, both historically and qualitatively, adding that its mass would be enough to clear its orbit around the Sun if it were on its own. Some have suggested that this implies that the Earth–Moon system is a double planet. The International Astronomical Union standards organization has no definition for planets in a binary system, or for what constitutes a double planet system, but most scientists agree that this would require the Moon-Earth barycenter to be outside of Earth, which it is not. == Earth–Moon system == == Observation == == History of scientific understanding and exploration == , by NASA astronaut William Anders, the first color image of Earth taken by a human from the Moon, during Apollo 8 (1968), the first time a crewed spacecraft left Earth orbit and reached another astronomical body|alt=The small blue-white semicircle of Earth, almost glowing with color in the blackness of space, rising over the limb of the desolate, cratered surface of the Moon. Following President John F. Kennedy's 1961 commitment to a crewed Moon landing before the end of the decade, the United States, under NASA leadership, launched a series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the Jet Propulsion Laboratory's Ranger program, the Lunar Orbiter program and the Surveyor program. The crewed Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing, in 1968 Apollo 8 made the first human mission to lunar orbit. The first Earthlings, two tortoises, had circled the Moon three months earlier on the Soviet Union's Zond 5, followed by turtles on Zond 6. The first time a person landed on the Moon and any extraterrestrial body was when Neil Armstrong, the commander of the American mission Apollo 11, set foot on the Moon at 02:56 UTC on July 21, 1969. Considered the culmination of the Space Race, an estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time. While at the same time another mission, the robotic sample return mission Luna 15 by the Soviet Union had been in orbit around the Moon, becoming together with Apollo 11 the first ever case of two extraterrestrial missions being conducted at the same time. The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) removed of lunar rock and soil in 2,196 separate samples. Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations, but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used. Apollo 17 in 1972 was the last crewed mission to the Moon's orbit or surface, and the last to the Moon's vicinity until the Artemis II lunar flyby in 2026. Explorer 49 in 1973 was the last dedicated U.S. lunar probe until 1994. beyond Low Earth Orbit (1969), Apollo 12 astronaut Pete Conrad checking on Surveyor 3 at their shared lunar base, with the Apollo 12 Intrepid lander and S-band antenna in the distant background The Soviet Union continued sending robotic missions to the Moon until 1976, deploying in 1970 with Luna 17 the first remote controlled rover Lunokhod 1 on an extraterrestrial surface, and collecting and returning 0.3 kg of rock and soil samples with three Luna sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976).

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