Full Moon 59

[Gerald Weiß]

Ingeophysical terms the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as Australia.[17]) Within the Solar System, it is the largest and most massive satellite in relation to its parent planet, the fifth largest and most massive moon overall, and larger and more massive than all known dwarf planets.[18] Its surface gravity is about one sixth of Earth's, about half of that of Mars, and the second highest among all Solar System moons, after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with no significant hydrosphere, atmosphere, or magnetic field. It 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.

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 magma. These maria were formed when molten lava flowed into ancient impact basins. The Moon is, except when passing through Earth's shadow during a lunar eclipse, always illuminated by the Sun, but from Earth the visible illumination shifts during its orbit, producing the lunar phases.[19] 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 completely during a total solar eclipse. 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.

The Moon has been an important source of inspiration and knowledge for humans, having been crucial to cosmography, mythology, religion, art, time keeping, natural science, and spaceflight. In 1959 the first human-made objects to leave Earth and reach another body arrived at the Moon, with the flyby of the Soviet Union's Luna 1 and the intentional impact of Luna 2. In 1966, the Moon became the first extraterrestrial body where soft landings and orbital insertions were achieved. On July 20, 1969, humans for the first time landed on the Moon and any extraterrestrial body, at Mare Tranquillitatis with the lander Eagle of the United States' Apollo 11 mission. Five more crews were sent between then and 1972, each with two men landing on the surface. The longest stay was 75 hours by the Apollo 17 crew. Since then, exploration of the Moon has continued robotically, and crewed missions are being planned to return beginning in the late 2020s.

Occasionally, the name Luna /ˈluːnə/ is used in scientific writing[25] and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon.[26] Cynthia /ˈsɪnθiə/ is another poetic name, though rare, for the Moon personified as a goddess,[27] while Selene /səˈliːniː/ (literally "Moon") is the Greek goddess of the Moon.

The English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, lūna. Selenian /səliːniən/[28] is an adjective used to describe the Moon as a world, rather than as a celestial object,[29] but its use is rare. It is derived from σελήνη selēnē, the Greek word for the Moon, and its cognate selenic was originally a rare synonym[30] but now nearly always refers to the chemical element selenium.[31] The element name selenium and the prefix seleno- (as in selenography, the study of the physical features of the Moon) come from this Greek word.[32][33]

The lunar geological periods are named after their characteristic features, from most impact craters outside the dark mare, to the mare and later craters, and finally the young, still bright and therefore readily visible craters with ray systems like Copernicus or Tycho.

While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.[56] Models that have the Moon acquiring a significant amount of the proto-earth are more difficult to reconcile with geochemical data for the isotopes of zirconium, oxygen, silicon, and other elements.[57] Above a high resolution threshold for simulations,[clarify] a study published in 2022 finds that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits.[58]

On November 1, 2023, scientists reported that, according to computer simulations, remnants of a protoplanet, named Theia, could be inside the Earth, left over from a collision with the Earth in ancient times, and afterwards becoming the Moon.[59][60]

The newly formed Moon settled into a much closer Earth orbit than it has today. Each body therefore appeared much larger in the sky of the other, eclipses were more frequent, and tidal effects were stronger.[61]Due to tidal acceleration, the Moon's orbit around Earth has become significantly larger, with a longer period.[62]

Following formation, the Moon has cooled and most of its atmosphere has been stripped.[63] The lunar surface has since been shaped by large impact events and many small ones, forming a landscape featuring craters of all ages.

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 restore hydrostatic equilibrium at its current orbital distance.[67]

The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizable as one of its planetary-mass moons, making it a satellite planet under the geophysical definitions of the term.[18] It is smaller than Mercury and considerably larger than the largest dwarf planet of the Solar System, Pluto. While the minor-planet moon Charon of the Pluto-Charon system is larger relative to Pluto,[f][68] the Moon is the largest natural satellite of the Solar System relative to their primary planets.[g]

The Moon's diameter is about 3,500 km, more than a quarter of Earth's, with the face of the Moon comparable to the width of either Australia,[17] Europe or the US without Alaska.[69] The whole surface area of the Moon is about 38 million square kilometers, almost exactly the area of the whole American landmass.

The Moon is a differentiated body that was initially in hydrostatic equilibrium but has since departed from this condition.[71] It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi).[72][73] This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago.[74]

Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallized, lower-density plagioclase minerals could form and float into a crust atop.[75] The final liquids to crystallize would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements.[1] Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly anorthosite.[16] The Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth.[1] The crust is on average about 50 kilometres (31 mi) thick.[1]

The Moon is the second-densest satellite in the Solar System, after Io.[76] However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less,[1] around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyzes of the Moon's time-variable rotation suggest that it is at least partly molten.[77] The pressure at the lunar core is estimated to be 5 GPa (49,000 atm).[78]

The Moon's gravitational field is not uniform. The details of the gravitational field have been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins.[81][82] The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism.[83]

The Moon has an external magnetic field of less than 0.2 nanoteslas,[84] or less than one hundred thousandth that of Earth. The Moon does not have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating.[85][86] Early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today.[84] This early dynamo field apparently expired by about one billion years ago, after the lunar core had crystallized.[84] Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the antipodes of the giant impact basins.[87]

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