Moon
The Moon is the only natural satellite of Earth. It orbits around Earth at an average distance of, a distance roughly 30 times the width of Earth. It completes an orbit in relation to Earth and the Sun 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 to its orbital period.
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. 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, with some younger ones featuring 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. The origin of the Moon is not clear, although it has been hypothesized to have formed out of material from Earth, ejected by a giant impact into Earth of a Mars-sized body named Theia 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 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, 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, and the intentional impact of Luna 2, followed in 1966 by the first soft landing and orbital insertion. Humans first arrived in orbit on December 24, 1968, with Apollo 8, and then on the surface on July 20, 1969, with Apollo 11. By 1972, six Apollo missions had landed twelve humans 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, starting with the Artemis program scheduled for the late 2020s.
Names and etymology
The English proper name for Earth's natural satellite is typically written as Moon, with a capital M. The noun moon is derived from Old English mōna, which stems from, which in turn comes from Proto-Indo-European mēnsis from earlier mēnōt, which may be related to a verb meaning 'to measure '.The Latin name for the Moon is lūna. The English adjective lunar was ultimately borrowed from Latin, likely through French. In scientific writing and science fiction, the Moon is sometimes referred to as Luna to distinguish it from other moons. In poetry, Luna may also refer to the personification of the Moon as a woman.
The Ancient Greek word referred to the Moon as a celestial body, and also to the moon goddess Selene. The rare English adjective selenian is used to describe the Moon as a world, as opposed to a celestial object. Its cognate selenic, originally a rare synonym, now almost always refers to the chemical element selenium. The corresponding prefix seleno- appears in terms including selenography.
Artemis, the Greek goddess of the wilderness and the hunt, also came to be identified with Selene, and was sometimes called Cynthia after her birthplace on Mount Cynthus. Her Roman equivalent is Diana.
The astronomical symbols for the Moon are the crescent and decrescent, for example in M☾ 'lunar mass'.
Classification
The International Astronomical Union 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. Some have suggested that the Earth and Moon form a double planet system, but most scientists agree that this would require the orbital barycenter to lie outside of Earth, which it does not.
Natural history
Formation
of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System. Historically, several formation mechanisms have been proposed, but none satisfactorily explains the features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force would require too great an initial rotation rate of Earth. Gravitational capture of a pre-formed Moon depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon. A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon. None of these hypotheses can account for the high angular momentum of the Earth–Moon system.The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body with the proto-Earth. The oblique impact blasted material into orbit about the Earth and the material accreted and formed the Moon just beyond the Earth's Roche limit of ~.
Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of the Moon derived from the impactor, rather than the proto-Earth. However, models from 2007 and later suggest a larger fraction of the Moon derived from the proto-Earth. Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth–Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, although this is debated.
The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system. The newly formed Moon would have had its own magma ocean; its depth is estimated from about to.
While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. 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. A study published in 2022, using high-resolution simulations, found 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.
On November 1, 2023, scientists reported that, according to computer simulations, remnants of Theia could still be present inside the Earth.
Natural development
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.Due to tidal acceleration, the Moon's orbit around Earth has become significantly larger, with a longer period.
Following formation, the Moon has cooled and most of its atmosphere has been stripped. 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 was volcanically active until 1.2 billion years ago, which laid down the prominent lunar maria. Most of the mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some are as young as 1.2 billion years and some as old as 4.2 billion years. The distribution of the mare basalts is uneven, with the basalts predominantly appearing on the Moon's near-side hemisphere. The reasons for this are not yet known, although the relative thinness of the crust on the near side of the Moon is hypothesized to be a factor. Causes of the distribution of the lunar highlands on the far side are also not well understood. Topological measurements show the crust on the near side to be thinner than on the far side. One possible explanation for this is that large impacts on the near side may have made it easier for lava to flow onto the surface.