The Moon, or Earth I, is the only permanent natural satellite of planet Earth. It is the fifth largest natural satellite in the Solar System, and the largest of the planetary satellites relative to the size of the planet around which it orbits. It is the second densest satellite in the Solar System after Io, a satellite of Jupiter.
| Orbital characteristics | |
|---|---|
| Semi-major axis | 384,399 km (0.002 57 au) |
| Apogee | 406,300 km (0.002 7 au) |
| Perigee | 356,700 km (0.002 4 au) |
| Orbital circumference | 2,449,000 km |
| Eccentricity | 0,054 90 |
| Period of revolution | 27,321 582 d (27 days 7 h 43.1 min) |
| Synodic period | 29,530 589 days |
| Average orbital speed | 1.022 km/s |
| Maximum orbital speed | 1.052 km/s |
| Minimum orbital speed | 0.995 km/s |
| Tilt on the ecliptic | 5,145° |
| Known satellites | 0 |
| Satellite of | Earth |
| Systematic designation | Earth I |
| Physical characteristics | |
| Equatorial radius | 1,737.4 km (0.273 Earth) |
| Polar radius | 1,735.97 km (0.273 Earth) |
| Equatorial perimeter | 10,921 km (0.273 Earth) |
| Area | 37,871,220.85 km2 (0.074 Earth) |
| Volume | 2.195 8 × 1010 km3 (0.020 Earth) |
| Mass | 7.347 7 × 1022 kg (0.0123 Earth) |
| Global density | 3.346 4 × 103 kg/m3 |
| Surface gravity | 1.622 m/s2 (0.165 4 g) |
| Release speed | 2.38 km/s |
| Rotation period (sidereal day) |
27,321,582 days |
| Rotational speed (at the equator) |
16.657 2 kph |
| Axis tilt | 6,687° |
| Right ascension of the North Pole | 270,00° |
| Declination of the North Pole | 66,54° |
| Visual geometric albedo | 0,136 |
| Temperature Maximum | 396 K (123 °C) |
| Temperature Average | 200 K (−73 °C) |
| Temperature Minimum | 40 K (−233 °C) |
| Characteristics of the atmosphere | |
| Atmospheric pressure | 10−10 Pa |
The Moon is in synchronous rotation with the Earth, thus constantly showing it the same face. This one, called the visible side, is marked by dark volcanic lunar seas that fill the spaces between the clear highlands and its prominent impact craters. Conversely, it has a hidden side, which has fewer seas but many more craters, including the South Pole-Aitken basin, the largest of the satellite and one of the largest in the Solar System by its diameter of 2,500 km. It lacks a dense atmosphere and magnetic field. Its gravitational influence on the Earth produces ocean tides, land tides, a slight lengthening of the length of the day and stabilization of the inclination of the Earth’s axis.
The average orbital distance of the Moon is 384,402 km, about thirty times the Earth’s diameter, and its period of revolution is worth 27.3 days. The apparent size of the Moon in the sky is approximately the same as that of the Sun, since the diameter of the star is about 400 times that of the satellite, but it is also 400 times farther away. Therefore, the Moon can almost exactly cover the Sun in the sky, allowing the appearance of total solar eclipses.
This correspondence of apparent size will disappear in the distant future due to the increase in the lunar distance of about 3.8 cm per year. The formation of the Moon is thought to date back to about 4.51 billion years ago, shortly after Earth’s. The most widely accepted explanation is that the Moon formed from the remaining debris after a giant impact between a proto-Earth and a protoplanet the size of Mars, called Theia.
The Moon was first flown by the Luna 2 spacecraft in 1959. For more than a decade, it was studied by the Luna and Apollo programs, respectively Soviet and American. This space race culminated in 1969 with the first humans setting foot on the Moon during the Apollo 11 mission carrying Neil Armstrong and Buzz Aldrin. Ten other NASA astronauts then set foot on the lunar soil until Apollo 17 in 1972.
These missions make it possible to bring back to Earth lunar rocks which, together with observations made on site, make it possible to develop geological knowledge of the Moon, its internal structure and the history of its formation. Abandoned from 1974 by the space powers, the star knows a new interest in the 1990s, two NASA missions – Clementine and Lunar Prospector – discovering clues of the presence of water ice, especially at the South Pole. Beginning in the late 1990s, the Moon was the primary destination for space probes from new space nations, including China, Japan, and India. New manned missions to the Moon, or even colonization, are envisaged in the 2020s.
As the second celestial object in the terrestrial sky by its apparent magnitude, after the Sun, and because of its regular cycle of phases corresponding to its synodic period of 29.5 days, the Moon has served as a reference and cultural influence for human societies since time immemorial. These are found in language, calendars, art and mythology.
Physical Characteristics of the Moon
Dimensions and mass
The Moon is a spheroid due to the stretching created by tidal forces, its major axis being displaced 30° relative to the Earth due to gravitational anomalies caused by its impact basins.
Its shape is more elongated than current tidal forces can explain. This “fossil bulge” suggests that the Moon solidified when it orbited at half the distance of its current one with Earth and that it would now be too cold for its shape to adapt to this change of orbit. Its equatorial radius is 1,738.1 km and its polar radius is 1,736.0 km, giving it a flattening of 0.001, three times smaller than that of the Earth. Its average radius is 1,737.4 km, which corresponds to about 27% of the Earth’s radius.
Its mass being 7.346 × 1022 kg, or just over one percent of the Earth’s mass, the surface gravity experienced on the Moon is much weaker than that on Earth: at 1.62 m/s2, it is six times smaller. Thus, even if its mass remains constant, a human being on the Moon sees its weight divided by six; similarly, wearing a 90 kg spacesuit is equivalent to the feeling of wearing a 15 kg suit on Earth. In addition, the escape velocity on the Moon is lower than that of the Earth, at 2.38 km/s against 11.2 km/s.
The gravitational field of the Moon is measured by following the Doppler effect of radio signals emitted by orbiting devices. The main features of lunar gravity are the repletion (or mascons), large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense basaltic lava flows that fill the lunar seas. These anomalies greatly influence the orbit of spacecraft around the Moon. However, lava flows alone cannot explain the entire gravitational signature; Mass concentrations independent of sea volcanism have been identified.
Internal structure and composition
The Moon is a differentiated body, structured in a distinct crust, mantle and core. It is the second densest natural satellite in the Solar System after Io, one of Jupiter’s moons. However, its core (probably made of metallic iron alloyed with a small amount of sulfur and nickel) is only about 350 kilometers in radius at most, or 20% of the radius of the Moon. Analyses of variations in the rotation of the Moon indicate that it is at least partially molten and would thus be solid up to 240 km from the center and liquid up to just over 300 km.
| Chemical composition of the surface of the Moon | |||
|---|---|---|---|
| Component | Chemical formula | composition | |
| Seas | Earths | ||
| Silica | SiO2 | 45,4 % | 45,5 % |
| Alumina | Al2O3 | 14,9 % | 24,0 % |
| Calcium oxide | Cad | 11,8 % | 15,9 % |
| Iron(II) oxide | FeO | 14,1 % | 5,9 % |
| Magnesium oxide | MgO | 9,2 % | 7,5 % |
| Titanium oxide | TiO2 | 3,9 % | 0,6 % |
| Sodium oxide | Na2O | 0,6 % | 0,6 % |
| 99,9 % | 100,0 % | ||
Around the core is a boundary layer of partially molten rock up to about 500 km from the center. Beyond this layer are the mantle and crust, both formed of solid rocks but of different chemical and mineralogical compositions. The crust, about 50 kilometers thick on average, outcrops in the “land”; It is also present in the “seas”, but covered by thick layers of lava.
The origin of this internal structure would be the fractional crystallization of a lunar magmatic ocean shortly after the formation of the Moon, 4.5 billion years ago. The cooling of this magmatic ocean would first have produced the precipitation and sedimentation of olivine, clinopyroxene and orthopyroxene crystals forming a mafic mantle then, after about three-quarters of the magmatic ocean has crystallized, the formation and flotation of plagioclase crystals are at the origin of the crust. The last liquids to crystallize, caught between the crust and the mantle, would have been highly enriched with incompatible elements, including radioactive elements KREEP producing heat. However, this model does not fully explain the observed characteristics of surface composition, including asymmetries in the distribution of thorium between the visible and hidden faces.
The geochemical mapping of the lunar surface, carried out from the orbiters, is in agreement with this perspective: the surface of the highlands (“lands”), representative of the crust, consists mainly of anorthosites, igneous rocks mainly composed of plagioclase; That of the “seas”, like that of the samples of lunar rocks collected on-site, are lava of mafic composition, richer in iron than terrestrial basalts.
Magnetic field
Current magnetic field
The MAG magnetometer and electron reflectometer of the Lunar Prospector made it possible in 2008 to obtain the first complete map of lunar magnetic fields. It reveals that impact basins dominate the distribution of these fields, with the weakest (less than 0.2 nT) occurring in the largest and most recent basins, Mare Orientale and Mare Imbrium, while the strongest fields (greater than 40 nT) are measured above the surfaces diametrically opposite these same basins. The strongest fields recorded correspond to less than one-hundredth of the Earth’s magnetic field.
The lunar magnetic field is entirely due to the magnetization of crustal rocks, and today the Moon does not have a dipole planetary magnetic field.
Some of the magnetizations can come from transient magnetic fields generated during large impacts. These impacts create the expansion of a plasma cloud during impact, generating an ambient magnetic field. This is confirmed by the apparent location of the largest magnetizations of the crust near the antipodes of the giant impact basins. However, most of the magnetization is inherited from a time when the Moon had a global magnetic field, like Earth and other planets.
History of the lunar magnetic field
The presence of a global magnetic field shortly after the formation of the Moon is attested by the residual magnetization of its oldest rocks. The detailed study of a 4.25 Ga troctolite sample brought back during the Apollo missions demonstrates the existence of a paleo-field with an intensity of 20 to 40 μT – thus very comparable to that of the current Earth’s magnetic field – which would have gradually declined and ended at least after 2.5 Ga ago. This result confirms the presence of a dynamo effect at this time, but does not allow to know precisely the mechanism (thermal or solutal convection, in particular).
Paleomagnetic studies conducted from 2009 to 2014 show that a lunar dynamo operated between at least 4.25 and 1.92 Ga and that a high field period (with an average field strength of about 77 μT at the surface) lasted between 3.85 and 3.56 Ga, followed by a decrease in surface intensity to below 4 μT to 3.19 Ga. Two subsequent studies, in 2017 and 2020, show that the drop of an order of magnitude in lunar paleointensities between 3.56 and 3.19 Ga was followed by a period of low field (surface field intensities of the order of 5 μT) and then a second and final period of decline between 1.92 and 0.8 Ga, which ended with the shutdown of the lunar dynamo, a sign of complete crystallization of the lunar core.
Two hypotheses are proposed to explain the succession of two stable periods, one with a high field and the following with a low field: (1) two distinct dynamo mechanisms could have worked, the first generating a strong field until its collapse and the second maintaining a weak field, or (2) a single dynamo mechanism was bistable, moving from a high field state to a low field state.
Selenography
The topography of the Moon, also called selenography, is measured by laser altimetry and stereoscopy. Its most visible relief is the South Pole-Aitken basin, with a diameter of about 2,500 km, the largest crater on the Moon and one of the largest impact craters in the Solar System, whose shock would have tipped the axis of rotation of the celestial body by 15° 8.
With a depth of 13 km, its floor is the lowest point on the surface of the Moon. The highest elevations of the surface are located directly to the northeast, and it is suggested that these landforms may have been thickened by the slightly oblique impact that formed the basin. Other large impact basins, such as the Rainy, Serenity, Crises, Smythii and Eastern Seas, also have low regional elevations and raised edges. The surface of the far side of the Moon is on average about 1.9 km higher than that of the near side.
The discovery of fault escarpments by the Lunar Reconnaissance Orbiter suggests that the Moon has shrunk by about 90 meters over the past billion years. Similar contraction characteristics exist on Mercury. A 2019 study of more than 12,000 images taken by the orbiter shows that the Mare Frigoris, a vast basin near the lunar North Pole and thought to be geologically dead, is cracking and moving. Since the Moon has no tectonic plates, its tectonic activity is slow and cracks develop as it loses internal heat.
Coordinate system
The reference point for the selenographic coordinates is the small crater Mösting A, defined as having the coordinates (−3.212, −5.211). In general, the prime meridian of the Moon corresponds to the center of the lunar disc as seen from Earth, with the IAU recommending as the axis the mean direction from the center of the Moon to the center of the Earth.
Lunar “seas”
The dark and relatively featureless lunar plains, clearly visible to the naked eye from Earth, are called “seas” because they were once believed to be filled with water. They are now known as vast solidified basins of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts contain more iron and no water-altered minerals. The majority of these lavas erupted or flowed into depressions associated with impact basins. Several geological provinces containing shield volcanoes and volcanic lunar domes lie within the “seas” of the near side.
Almost all seas are on the near side of the Moon and cover 31% of the surface on this face, compared to 2% of the far side. Based on geochemical maps obtained by the Lunar Prospector gamma spectrometer, it is estimated that this is due to a concentration of heat-producing elements — also known as KREEP — under the near-side crust that would have caused warming, partial melting, rise to the surface and eruption of the underlying mantle. Most basalts in the lunar seas erupted during the Late Imbrian, 3.0 to 3.5 billion years ago, although some radiometrically dated samples may be as old as 4.2 billion years.
Dated by crater counts, the most recent eruptions on the Moon have long been estimated to be about 1.2 billion years ago However, in 2006, a study of Ina Crater—a tiny depression of Lacus Felicitatis — showed jagged and relatively dust-free features that, due to the absence of erosion by debris fallout, appeared to be only a few million years old, although there is no consensus on this dating. Moonquakes and gas releases also indicate some continuous lunar activity.
In 2014, NASA announced that it had discovered “ample evidence of recent lunar volcanism” in 70 irregular patches of seas identified by the Lunar Reconnaissance Orbiter, some of which are less than 50 million years old. This raises the possibility that the lunar mantle is much warmer than previously thought, especially with regard to the near side where the deep crust is much warmer because of the greater concentration of radioactive elements. Shortly before, evidence of basaltic volcanism dating back 2 to 10 million years inside the crater Lowell — located in the Mare Orientale, at the level of the transition zone between the visible and hidden faces — is reported. An initially warmer mantle potentially associated with local enrichment of KREEP elements in the mantle could be responsible for prolonged volcanic activity also on the other side of the eastern basin.
The lighter regions of the Moon are called terrae, or more commonly highlands because they have a higher elevation than most seas. They are dated, by radiometry, as having been formed 4.4 billion years ago and could represent cumulates of plagioclases of the lunar magmatic ocean. Unlike Earth, no major lunar mountains would have formed as a result of tectonic events.
The concentration of seas on the near side probably reflects a much thicker crust of the highlands on the far side, which could have formed during the low-speed impact of a second moon of the Earth a few tens of millions of years after the formation of the Moon.
Impact craters
The lunar surface also has many impact craters. The absence of atmosphere, liquid surface flows, weather conditions and recent geological processes to create erosion mean that many of these craters are well preserved. Craters form when asteroids and comets collide with the satellite. There are about 300,000 of them with a width of at least one kilometer on the visible side alone.
The periods of the lunar geological time scale are named after the most important impact events that took place there, such as the Nectarian after the Mare Nectaris or the Imbrium after the Mare Imbrium. Like the Mare Orientale, these structures are characterized by multiple rings of material raised over a diameter of several hundred or even thousands of kilometers and associated with a large deck of ejecta deposits that form a regional stratigraphy. Other smaller craters such as Eratosthenes and Copernicus are characteristic of later periods and thus gave their name to the Eratosthenian and the Copernican.
Although only a few basins have been dated with certainty, they are useful for assigning relative ages. As impact craters accumulate at an almost constant rate, counting the number of craters per unit area is used to estimate the age of the surface. In addition, the radiometric ages of the molten rocks collected at impact collected during the Apollo missions are between 3.8 and 4.1 billion years: they are one of the main arguments for the existence of a great late bombardment.
The lunar crust is covered by a highly fragmented surface layer ploughed by impacts, called regolith, formed by impact processes. The finest regolith, constituting the silicon dioxide glass lunar soil, has a snow-like texture and a black powder-like scent. Regolith on older surfaces is generally thicker than that on younger surfaces: its thickness varies from 10 to 20 km in the highlands and from 3 to 5 km in the seas. Beneath the finely chopped regolith layer is mega regolith, a layer of highly fractured bedrock several kilometers thick.
A comparison of high-resolution images obtained by the Lunar Reconnaissance Orbiter shows a significantly higher rate of crater appearance than previously estimated. Thus, it is assumed that a secondary cratering process caused by ejecta projected at each impact stirs the first two centimeters of the regolith a hundred times faster than previous models suggested, with a time scale of the order of 81,000 years.
Lunar whirlpools
Lunar vortices are enigmatic bright formations observed on the surface of the Moon. They have a high albedo, optical characteristics similar to those of a relatively young regolith and mostly a sinuous shape. This shape is often accentuated by regions of low albedo that wind between bright vortices.
Presence of water
Liquid water cannot persist on the surface of the Moon. When exposed to solar radiation, water dissociates rapidly by photolysis and is then carried into space. However, since the 1960s, scientists have hypothesized that water ice could be deposited by comets or even be produced by the reaction of lunar rocks rich in oxygen and hydrogen from the solar wind, leaving traces of water that could possibly persist in craters of eternal darkness at the two lunar poles. Numerical simulations suggest that up to 14,000 km2 of the satellite’s surface would be constantly in shadow. The presence of usable quantities of water on the satellite is an important factor in considering colonization of the Moon in a cost-effective way. Indeed, the alternative of transporting water from Earth would be prohibitively expensive.
In 1994, the radar experiment carried out aboard the Clementine orbiter reported the existence of small pockets of frozen water near the surface. However, subsequent radar observations from the Arecibo radio telescope suggest that these discoveries would rather be rocks ejected during the formation of young impact craters. In 1998, the Lunar Prospector neutron spectrometer revealed the presence of high concentrations of hydrogen in the first meter of regolith depth near the polar regions. Volcanic lava beads, brought back to Earth during the Apollo 15 mission, present after searching for small quantities of water inside them.
The Chandrayaan-1 probe, launched in 2008, confirms the existence of water ice on the surface thanks to its onboard module Moon Mineralogy Mapper. The spectrometer observes absorption lines corresponding to hydroxyl in reflected sunlight, indicating the presence of large amounts of water ice on the lunar surface. Data indicate concentrations in the range of 1,000 ppm.
In 2009, LCROSS sent a 2,300 kg impactor into a crater of eternal darkness and detected at least 100 kg of water in a plume of ejected material. Another review of the LCROSS data reveals that the amount of water detected is closer to 155 ± 12 kg. In May 2011, the detection of 615 to 1,410 ppm of water in the magmatic inclusions of lunar sample No 74220 was announced. This is the “orange soil” with a high titanium content of volcanic origin collected during the Apollo 17 mission in 1972. This concentration is comparable to that of magma in the Earth’s upper mantle.
The analysis of the results of the Moon Mineralogy Mapper (M3) brings in August 2018 for the first time the confirmation of the presence of water ice on the surface of the Moon. The data reveal the distinct reflective signatures of water ice, as opposed to those of dust and other reflective substances. Ice deposits are found. on the North and South Poles, although they are more abundant in the South, where craters of eternal darkness are more widespread.
In October 2020, astronomers reported detecting water on the Sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (SOFIA).
The volume of water present on the Moon is estimated in 2018 by Paul Spudis at between 100 million and one billion cubic meters at each pole.
The surface temperature of the Moon
The inclination of the Moon’s axis relative to the ecliptic is only 1.5424°, much less than the 23.44° of the Earth. For this reason, the solar irradiance of the former varies much less with the seasons, and topographical details play a crucial role in seasonal effects.
According to images taken by Clementine in 1994, four mountainous regions at the edge of Peary Crater, near the Moon’s North Pole, could remain illuminated throughout the lunar day, creating peaks of eternal light. Such regions do not exist at the South Pole. Similarly, there are places that remain in permanent shadow at the bottom of many polar craters, implying that these “craters of eternal darkness” are extremely cold. The Lunar Reconnaissance Orbiter measures the lowest summer temperatures in craters at the South Pole at 35 K (−238°C) and only 26 K (−247°C) around the winter solstice in Hermite crater at the North Pole. This is the lowest temperature in the Solar System ever measured by a spacecraft, lower even than that of Pluto’s surface.
The average temperatures of the surface of the Moon differ greatly depending on the time of day for the regions considered: up to about 400 K (127 °C) when exposed to sunlight at the equator and up to 100 K (−173 °C) when in the shade.
Atmosphere
Current composition
The atmosphere of the Moon is so thin that its total mass is less than 10 tons, a density almost corresponding to a vacuum: the surface pressure of this small mass is about 3 × 10−15atm (0.3 nPa), this one varying with the lunar day. Its sources include outgassing and sputtering, a product of ground bombardment by solar wind ions. Among the elements detected are sodium and potassium, produced by sputtering and also present in the atmospheres of Mercury and Io; helium-4 and neon from the solar wind; and argon-40, radon-222 and polonium-210, degassed after creation by radioactive decay in the crust and mantle. The absence of neutral species (atoms or molecules) such as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not explained.
Water vapor is present in varying amounts depending on the latitude, with a maximum at about 60-70°. It is probably produced by the sublimation of water ice from the regolith. These gases return to the surface due to the gravity of the Moon or are lost in space, either by the pressure of solar radiation, or — if ionized — by being carried away by the magnetic field of the solar wind.
Dust
A permanent asymmetric lunar dust cloud exists around the Moon, created by small comet particles. It is estimated that 5 tons of these hit the surface every 24 hours and eject this dust. It remains in suspension for about 10 minutes, taking 5 minutes to get up and 5 minutes to fall. On average, 120 kilograms of dust are permanently present above the Moon, rising up to 100 kilometers from the surface.
Dust measurements are carried out by LADEE’s Lunar Dust EXperiment (LDEX) experiment, between 20 and 100 kilometers above the surface for a period of six months. LDEX detects an average of 0.3 micrometers per minute of lunar dust particles. Dust particle counting peaks during the Geminid, Quadrantid and Taurid meteor showers in particular, when the Earth and Moon pass through comet debris. Clouds are asymmetrical, denser near the boundary between the day and night sides of the Moon.
Past thick atmosphere
In October 2017, scientists from the Marshall Space Flight Center and the Lunar and Planetary Institute in Houston announced that they had discovered from studies of magma samples from the Moon, taken during the Apollo missions, that the Moon would have had a relatively thick atmosphere for a period of 70 million years three or four billion years ago. This atmosphere, derived from gases ejected during lunar volcanic eruptions, was twice as thick as that currently found on the planet Mars. The ancient lunar atmosphere would have been gradually stripped by the solar wind and then dissipated in space.
Training
The Moon began to form 4.51 billion years ago, 30 to 60 million years after the formation of the Solar System. Several mechanisms of formation are proposed, among which the separation of the Moon from the Earth’s crust by centrifugal force (which would require too high an initial rotation speed of the Earth), the gravitational capture of a preformed Moon (which would, however, require an unrealistic extended Earth atmosphere to dissipate the energy of the Moon of passage) and the co-formation of the Earth and the Moon in the primordial accretion disk (which cannot explain the disappearance of metals in the Moon). Nor can these assumptions explain the high angular momentum of the Earth-Moon system.
For the dominant hypothesis, the Earth-Moon system was formed after the impact of a protoplanet of a size similar to that of Mars (named Theia, the mother of Selene in Greek mythology) with the proto-Earth; It is called the giant impact hypothesis. The impactor, the crust and part of the Earth’s mantle break up and throw a large amount of debris into orbit around the Earth. The Moon then forms by accretion of part of this cloud of debris in a very short time, on the order of a century. The impact would have released a lot of energy, melting the outer layer of the Earth, and thus formed an ocean of magma. Similarly, the newly formed Moon would have possessed a lunar magmatic ocean with an estimated depth of at least several hundred kilometers.
Although the giant impact hypothesis can explain many parameters, some elements are not explained, especially with regard to the isotopic compositions close to the Moon and Earth, its relatively recent volcanism, or the past existence of a planetary magnetic field. Indeed, the measurement in 2001 of the isotopic signatures of the lunar rocks of the Apollo program reveals that they have the same isotopic signature as terrestrial rocks, thus distinguishing them from almost all other bodies in the Solar System.
This observation is unexpected because it was assumed that most of the material that formed the Moon came from Theia, but it was then announced in 2007 that there was less than a one percent chance that Theia and Earth would have identical isotopic signatures in this way. Other Apollo lunar samples studied in 2012 have the same titanium isotope composition as Earth, which conflicts with what is expected if the Moon had formed far from Earth or originated from Theia.
These discrepancies can be explained by variations in the giant impact hypothesis. Alternative models have proposed a series of less cataclysmic impacts or the formation of a synestia — a toric cloud of gas and rock fragments.
Earth-Moon System and Sun-Earth-Moon System
Orbit
The Moon completes a full orbit around the Earth relative to the fixed stars about once every 27.3 days — its period of revolution or sidereal period. However, since the Earth moves simultaneously in its orbit around the Sun, it takes about two more days before the Moon shows the same phase to Earth, 29.5 days — its synodic period.
Unlike most natural satellites of other planets, it orbits closer to the plane of the ecliptic than to the equatorial plane of the planet. Its orbit is subtly disturbed by the Sun and Earth in many different ways. For example, the plane of the Moon’s orbit gradually rotates every 18.61 years, affecting other aspects of lunar motion. These consecutive effects are mathematically described by Cassini’s laws.
Moreover, the Moon is the only permanent natural satellite of the Earth. There are a number of NEOs such as 3753 Cruithne that co-orbit with Earth: their orbits bring them closer to Earth at a regular interval, but deteriorate over the long term. They are quasi-satellites and not natural satellites because they do not orbit the Earth but around the Sun, the existence of other moons of the Earth is not confirmed. However, some of these asteroids can sometimes become temporary satellites of Earth for a few months or even years. Only 2006 RH120 is known to have been in this case, between 2006 and 2007.
Heliocentric trajectory
The path of the Moon in a reference frame linked to the Sun has the particularity of being entirely concave, without loops or inflection points. This is the only case among all known satellites, natural or artificial.
Rotation
The Moon is in synchronous rotation around the Earth: its rotation period is equal to its period of revolution. It therefore always presents the same hemisphere called “near side of the Moon” to a terrestrial observer, the opposite hemisphere being consequently called “far side of the Moon”. However, due to the effect of libration, about 59% of the Moon’s surface can in practice be seen from Earth. The far side is sometimes mistakenly called the “dark side”, but it is fully illuminated as often as the visible side: once every 29.5 Earth days, at the new moon.
This synchronous rotation results from friction created by the tidal forces of the Earth on the Moon, the rotational energy having dissipated in the form of heat. Previously, the Moon had a faster rotational speed but, quite quickly in its history, it gradually slowed down until the period of this movement coincided with that of the revolution of the satellite around the Earth.
In 2016, using data collected during the Lunar Prospector mission, planetary scientists detected two hydrogen-rich areas (probably ancient water ice) at two opposite points on the Moon. It is assumed that these areas were billions of years ago the lunar poles before it was locked with Earth.
Relative sizes
The Moon is an exceptionally large natural satellite compared to the Earth: it is more than a quarter of the diameter and 1/81th of the mass of the planet. It is also the largest moon in the Solar System relative to the size of its planet, although Charon is larger compared to the dwarf planet Pluto, making 50% of its diameter and 1/9th of its mass. The area of the Moon is slightly smaller than that of Asia.
The barycenter of the Earth-Moon system, their common center of mass, is located about 1,700 km (about a quarter of Earth’s radius) below the Earth’s surface. The Earth revolves around this barycenter once a sidereal month, at 1/81th of the speed of the Moon, or about 41 kilometers per hour. This motion is superimposed on the much faster revolution of the Earth around the Sun — at a speed of about 30km/s — and is therefore generally negligible.
Tidal effects
The gravitational attraction between celestial bodies decreases inversely squared by the distance of these masses from each other. As a result, the attraction exerted by the Moon is slightly greater for the side of the Earth closest to it than for the opposite side. This results in a tidal force that affects both the oceans and the Earth’s crust.
The most obvious effect of tidal forces is to cause two bulges in Earth’s oceans, one on the side facing the Moon and the other on the opposite side. This results in variations in sea levels, called ocean tides. When the Earth rotates on its axis, one of the ocean bulges (high tide) is locally held in place “under” the Moon, while another such tide is opposite. As a result, there are about two high tides and two low tides in one day. Since the Moon orbits the Earth in the same direction as the Earth’s rotation on itself, high tides occur approximately every 12 hours and 25 minutes, with the 25 minutes being due to the time it takes for the Moon to orbit the Earth.
The Sun also produces tides but of lower amplitude, 40% of that due to the Moon. During syzygy, when the Moon and Sun are aligned with the Earth, the sum of the Moon-Earth and Sun-Earth interactions is responsible for the high tides at the time of the spring and autumn equinoxes.
If the Earth had no continents, the tide produced would be only one meter wide and would be very predictable. In reality, ocean tides are greatly affected by other factors: the friction of water at the level of the ocean floor, the inertia of the movement of water or the sloshing of water between the different ocean basins.
While gravity causes Earth’s fluid oceans to accelerate and move, the gravitational coupling between the Moon and the Earth’s solid body is mostly elastic and plastic. The result is another tidal effect of the Moon on the Earth that causes a bulge of the solid part of the Earth closest to the Moon that acts as a moment in opposition to the rotation of the Earth: a solid tide, or terrestrial tide. This “drains” the angular momentum and kinetic energy of the Earth’s rotation, gradually slowing it down. This angular momentum, lost from Earth, is transferred to the Moon in a process known as tidal acceleration that raises the Moon to a higher orbit.
Thus, the distance between the Earth and the Moon increases—the Moon was about ten times closer to the Earth at the time of its formation than in contemporary times—and the Earth’s rotation slows down in reaction. Measurements of lunar reflectors left during the Apollo missions reveal that the Earth-Moon distance increases by an average of 3.8 cm per year. (3,805 ± 0.004 cm/year.) Atomic clocks also show the opposite effect, namely that the day on Earth lengthens by about 15 microseconds each year, forcing coordinated universal time to be adjusted with leap seconds.
Were it to run its course, this tidal trail would continue until the rotation of the Earth and the orbital period of the Moon correspond, creating a mutual locking by tidal forces between the two celestial bodies. As a result, the Moon would be suspended in the sky above a meridian, as is the case, for example, between Pluto and its moon Charon. However, the Sun will become a red giant and engulf the Earth-Moon system long before this event.
In the same way, the lunar surface undergoes tides with an amplitude of about 10 cm every 27 days, with two components: a fixed one due to the Earth because in synchronous rotation, and a variable due to the Sun. The Earth-induced component comes from libration, the result of the Moon’s orbital eccentricity — if the Moon’s orbit were perfectly circular, there would only be solar tides. The cumulative effects of these tidal stresses produce lunar earthquakes. These phenomena remain much less common and less intense than earthquakes, although they can take place for up to an hour due to the absence of water to dampen seismic vibrations. The existence of these earthquakes is an unexpected discovery of seismographs placed on the Moon during the Apollo missions from 1969 to 1972.
Moreover, these tidal forces have a detectable impact on the climate in the context of atmospheric tides. During the different phases of the Moon, the tidal force attracts more or less the atmosphere and thus participates, up to a few percent, in the phenomena of overpressure and depression.
Finally, the presence of the Moon has an influence on the stabilization of the inclination of the Earth’s axis. Indeed, the obliquity of the Earth varies between 21 and 24 ° approximately with respect to the plane of the ecliptic while Mars, which does not have such a massive natural satellite, sees its obliquity vary from 20 to 60 ° over millions of years. Similarly, before the formation of the Moon, the Earth’s axis of rotation oscillated chaotically, which would have made it impossible for life to appear on its surface because of the climatic disturbances caused; this disappeared once the tidal gravitational lock between the Earth and its natural satellite was put in place.
Lunar influence
Lunar influence is the pseudo-scientific belief of a correlation between specific stages of the lunar cycle and physiological changes in living things on Earth, including humans.
The Moon has long been particularly associated with madness and irrationality, with words like lunatic being derived from the Moon’s Latin name, Luna. The philosophers Aristotle and Pliny the Elder argue that the full moon causes madness in sentient individuals, believing that the brain, which is mainly composed of water, must be affected by the moon and its power over the tides. In reality, the power of lunar gravity is too weak for this to be the case. In a contemporary way, the existence of a lunar influence claiming that admissions to psychiatric hospitals, road accidents, homicides or suicides would increase during full moons is sometimes defended, even if many studies invalidate this. Similarly, while the influence of the Moon on agriculture or forestry is sometimes assumed, no exploitable effect has ever been demonstrated.
On the other hand, selenotropism — that is, the orientation of an organism vis-à-vis the Moon — is demonstrated in some species of palolo worms such as the Eunice Fuscata of the tropical Pacific or in zooplankton in the Arctic during the polar night. In addition, the growth of certain animals such as the nautilus would be influenced by the Moon and the observation of their shells allows, with ancient fossil specimens, to independently confirm the lengthening of the lunar month on a geological scale due to the increase in the Earth-Moon distance. However, this hypothesis remains contested.
Observation
Visibility
The Moon has an exceptionally low geometric albedo of 0.12, which gives it a slightly higher reflectance than asphalt. However, with an apparent magnitude of -12.6 during the full moon, the Moon is the most visible star in the terrestrial sky, after the Sun and in front of Venus, thanks to its close proximity to the Earth. It is thus easily observable with the naked eye at night, or even in broad daylight. Binoculars can be used to distinguish seas and larger impact craters.
In addition, the satellite benefits from an improvement in brightness thanks to the opposition effect: the full moon is twelve times brighter than a quarter of the moon, even if the illuminated angular surface is only twice as high. In addition, the color constancy of the human visual system recalibrates the relationships between an object’s colors and its environment, which explains why the sunlit moon emerges when the surrounding sky is relatively dark. The edges of the full moon appear as bright as the center, without darkening center-edge, due to the reflective properties of the lunar soil, which reflects light back more to the Sun than in other directions.
The orientation of the Moon in the sky varies according to the latitude of the terrestrial observer. Indeed, since the Moon orbits near the ecliptic, someone looking at it from a positive latitude (north of the Earth’s equator) will see for example the prominent crater Tycho closer to the horizon while an observer from a negative latitude (south of the equator), will see it “upside down”. On the two photographs opposite, we observe the crater at the bottom of the image for a full moon seen in Belgium while it is at the top of the image for a full moon seen in Australia.
The altitude reached by the moon in the sky during its culmination varies according to its phase and the time of year. The full moon is highest in winter for each hemisphere.
The apparent size of the full moon averages about 0.52° of arc in the sky (or 31’2 of arc), which is about the same apparent size as the sun. However, it appears larger when close to the horizon due to a purely psychological effect, known as the lunar illusion, first described in the seventh century BC. Several explanations are proposed, such as the fact that the human brain perceives the sky as slightly flattened — which implies that an object on the horizon is considered larger — or that the relative size of the objects seen on the horizon makes the moon appear larger, as for the Ebbinghaus illusion.
The appearance of the Moon, like that of the Sun, can be affected by the Earth’s atmosphere. Common optical effects include a 22° halo ring, formed when light from the Moon is refracted through the ice crystals of high cirrostratus clouds, or smaller coronas when the Moon is seen through thin clouds.
Phases
Because of its synchronous rotation, the Moon always presents the same part of its surface to the Earth: the so-called “visible” face. However, half of the sphere illuminated by the sun’s rays—and thus both oriented toward both the Earth and the Sun—varies during the 29.53 days of its synodic period. This phenomenon gives rise to what are called lunar phases, which follow one another during a cycle called “lunation”. Over the course of the lunar cycle, the declination of the Moon varies: it increases during one half of the cycle and it decreases during the other half.
Since the Moon always has the same face towards the Earth and its orbit is slightly inclined, the lunar phases almost always present the same parts of the Moon from one cycle to another. There are mainly four characteristic points of the lunar appearance: the new moon when the Moon and the Sun are in conjunction with respect to the Earth, the first quarter when the Moon is in eastern quadrature, the full moon when the Moon and the Sun are in opposition with respect to the Earth and the last quarter when the Moon is in western quadrature. Between each of these characteristic points, we will speak successively of the first crescent, the waxing gibbous moon, the waning gibbous moon and finally the last crescent.
The illuminated part of the Moon being symmetrical with respect to the plane formed by the Sun, the Moon and the observer, the Moon presents at every moment the same phase to any terrestrial observer regardless of its latitude. However, the orientation of the terrestrial observer’s horizon varies with respect to this plane. Thus, for low latitudes – near the equator and in the tropics – the horizon is perpendicular to the plane and a crescent Moon will appear horizontal, like a “smile”. For higher latitudes, this neighborhood will appear more vertical, like a “C”. The Moon is visible for two weeks every 27.3 days at the North and South Poles.
Super moon
A supermoon is a full moon that coincides with a minimum distance from the satellite to Earth. It is not an astronomical term but rather a common expression used to designate certain astronomical phenomena.
On November 14, 2016, the Moon is at its closest full moon since 1948, 356,500 km from the center of the Earth. This full moon is then 30% brighter than when it is at its apogee, because its angular diameter is 14% larger and . It will not be closer until November 25, 2034.
. It will not be closer until November 25, 2034.Eclipses
Eclipses occur only when the Sun, Earth, and Moon are aligned, a phenomenon called “syzygy”.
Solar eclipses occur at the new moon, when the Moon lies between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when the Earth is between the Sun and the Moon. The existence of the former is a consequence of the fact that the apparent size of the Moon is about the same as that of the Sun, the two forming an angle of about 0.5° in the terrestrial sky. Indeed, if the Sun has a diameter 400 times larger than that of the Moon, it is also 400 times farther from the Earth than is the Moon.
The apparent size variations, due to non-circular orbits, are also almost identical, although occurring in different cycles. This makes it possible to sometimes have total solar eclipses — with the Moon appearing larger than the Sun — and annular eclipses — the Moon appearing smaller than the Sun. During a total eclipse, the Moon completely covers the Sun’s disk and the solar corona becomes visible to the naked eye.
As the distance between the Moon and Earth increases very slowly over time, the angular diameter of the Moon decreases in the Earth’s sky. In addition, as it evolves on its main sequence to become a red giant, the size of the Sun and its apparent diameter in the sky also increase. The combination of these two factors means that hundreds of millions of years ago, the Moon still completely covered the Sun during solar eclipses, and no annular eclipse was possible then. Similarly, within 600 million years, the Moon will no longer be able to completely cover the Sun and total solar eclipses will become impossible.
In addition, because the Moon’s orbit around the Earth is inclined about 5.145° to the plane of the ecliptic, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. The periodicity and recurrence of eclipses of the Sun by the Moon and the Moon by the Earth are described by the saros, whose period is about 18 years.
Because the Moon continually blocks the view of a circular area of the sky half a degree wide, a phenomenon called occultation occurs when a star or planet passes behind the Moon and is then hidden. Thus, a solar eclipse is a special case of the occultation of the Sun. Because the Moon is relatively close to Earth, occultations of individual stars are not visible everywhere on the planet, or at the same time. Due to the precession of the lunar orbit, different stars are occulted each year.
Libration
The Moon always has the same hemisphere as the Earth, we call “librations” the phenomena of oscillation allowing an observer on the surface of the Earth to see more than 50% of the surface of the Moon. These phenomena can take four forms: librations in longitude, librations in latitude, parallactic librations and physical librations.
All these libration phenomena during successive lunations make it possible to observe about 59% of the lunar surface from the Earth’s surface. However, the additional areas thus offered for observation are very distorted by the perspective effect and it is difficult to distinguish the surface features of these regions from the ground.
Transient lunar phenomenon
There is a historical controversy that the characteristics of the lunar surface change over time. Today, many of these claims are considered a consequence of optical illusions, resulting from observation under different lighting conditions, poor visibility quality, or inadequate drawings. However, outgassing occurs occasionally and could be responsible for a very minor percentage of these observations, being part of the reported transient lunar phenomena. In 2006, it was suggested that a lunar surface 3 km in diameter would have been significantly modified by a clearance event about a million years ago.
Phenomena called “transients” of a few tenths of a millisecond can occur. Usually of magnitude 5 to 10 (but up to 3), they are only visible with a telescope or telescope associated with a camera and on the unlit part of the Moon. The lunar flash comes from the fall of bodies (mainly from swarms of comets) of 5 to 15 cm hitting the Moon at speeds of 20 to 30 km/s, which melts the rock on the surface at the point of impact and projects droplets of liquid rocks. The bright flash is produced by the energy released during this impact. Over the past five centuries, hundreds of these phenomena have been observed by many different observers.
History of observations
Before the invention of the telescope
One of the earliest possible representations of the Moon is a rock sculpture named Orthostat 47, dated to the third millennium BCE and discovered in Knowth, Ireland. The first written record of the observation of a solar eclipse dates from 1223 BC, found on a clay tablet in the ancient city of Ugarit. An inscription on a bone dating from 2136 BC is also suspected of being a trace of the observation of an eclipse.
The understanding of lunar cycles is an early development of astronomy: as early as the eighth century BC, Babylonian astronomers kept systematic records of solar eclipses and as early as the fifth century BC, they noted the saros, the 18-year period governing lunar eclipses. The Chinese astronomer Shi Shen gives in the fourth century BC instructions for predicting solar and lunar eclipses. Archimedes designed in the third century BC a planetarium capable of calculating the motions of the Moon and other objects in the Solar System.
The physical form of the Moon and the cause of moonlight are also understood early in the history of astronomy. The Greek philosopher Anaxagoras estimates in the fifth century BC that the Sun and the Moon are both spherical rocks and that the latter reflects the light of the former. Moreover, Democritus assumes that the marks observed on the Moon are the consequence of the existence of mountains and valleys.
Although the Han Dynasty Chinese associated the Moon with an energy assimilated to ch’i, their theory of “radiant influence” also recognizes that the light of the Moon is simply the reflection of the Sun, and Jing Fang notes the sphericity of the Moon in the first century BC.
However, Aristotle theorizes conversely in From the Sky that the Moon marks the boundary between the spheres of the mutable elements (earth, water, air and fire) and the imperishable stars of the ether. The supralunar world is perfect, and therefore the Moon is a smooth and unalterable sphere. Aristotle’s disciple, Clearchus of Soles, explains the lunar spots by the fact that the Moon is a polished mirror that reflects the Earth’s landscape.
This theory, however, is invalidated by the observation that the surface of the Moon remains unchanged as it moves in front of the Earth, leading other scientists to imagine that the spots are condensed vapors from a cloud or emanate from the Earth. This Aristotelian conception of a smooth Moon remained in part until the end of the Middle Ages, even left traces as far as nineteenth-century Persia and twentieth-century European folklore.
In the second century BC, Seleucus of Seleucia rightly argues that the tides are due to the attraction of the Moon and that their height depends on the position of the Moon relative to the Sun. Previously, Aristarchus of Samos had calculated in the third century BC. In On dimensions and distances the size of the Moon and its distance, obtaining a value of about twenty times the radius of the Earth for distance. These values were greatly improved by Hipparchus in the second century BC in On the magnitudes and distances of the Sun and the Moon. This text is lost but its results reported by Ptolemy in the second century, estimating the lunar distance at 59 times the radius of the Earth and its diameter at 0.292 times that of the planet. These estimates are already very close to reality, which is about 60 and 0.273 respectively.
Also in the second century, Plutarch wrote in his Moral Works that “the Moon is a celestial earth” and that dark areas are depressions filled with water. They are thus called maria (Latin word meaning “seas” in the plural), while the light-colored highlands are called terrae (“land”). These names, although incorrect, remain in the current nomenclature.
In the fifth century, the Indian astronomer Aryabhata mentions in his Aryabhatiya that the cause of the brightness of the Moon is reflected sunlight. Al-Marwazi, a Persian astronomer, estimates the diameter of the Moon at about 3,000 km and its distance from Earth at about 346,000 km in the ninth century. The eleventh-century astronomer and physicist Alhazen elaborates by arguing that sunlight is not reflected by the Moon like a mirror, but that light is emitted from every part of the Moon’s sunny surface in all directions. Shen Kuo, of the Song dynasty, then created an allegory equating the growth and decline of the Moon with a round ball of silver which, when sprayed with white powder and seen from the side, would appear as a crescent.
After the invention of the telescope
Precise selenography does not begin until the fifteenth century, the first published drawings being those of William Gilbert in 1603, from observations with the naked eye. In 1610, Galileo published in Sidereus Nuncius one of the first drawings of the Moon made with an instrument – his telescope – and noted that the star is not smooth but has mountains and craters. Thomas Harriot made similar drawings with a telescope a few months earlier but did not publish them.
The cartography of the Moon follows in the seventeenth century with the first attempts, including that of Claude Mellan around 1634, then the first map published by the Dutch cartographer Michael Florent van Langren in 1645 from telescopic observations. It is the first to distinctly mark Maria, craters and mountains and adopts the first Catholic nomenclature after kings and saints. Two years later, Johannes Hevelius published Selenographia, the first treatise and atlas entirely devoted to the Moon. This included a new, more detailed map of the lunar surface and included a new nomenclature that would remain popular for a time in Protestant countries. However, it is the nomenclature proposed by Giovanni Battista Riccioli and his assistant Francesco Maria Grimaldi in 1651 in the Almagestum Novum — naming the craters after astronomers and famous people — that remains in posterity.
A large four-leaf map of the Moon named Mappa Selenographica, drawn up by Guillaume Beer and Johann Heinrich von Mädler between 1834 and 1836 and published in Der Mond in 1837, provides the first trigonometrically accurate study of lunar features. It includes the indication of the altitude of more than a thousand mountains with precisions similar to those of the first attempts at terrestrial geography. Moreover, the authors conclude that the Moon has neither a significant body of water nor a significant atmosphere.
All measurements were made through direct observations until John William Draper created astrophotography in March 1840 with a daguerreotype of the Moon. The quality of photographs of the Moon then progressed rapidly until lunar photography was recognized in the late nineteenth century as a sub-discipline of astronomy.
The lunar craters, first indicated by Galileo, were considered to be of volcanic origin until Richard A. Proctor according to which they would actually be impact craters created by collisions of asteroids or comets. This view gained the support in 1892 of the geologist Grove Karl Gilbert who found these results through experimentation. Comparative studies of these craters from 1920 to the 1940s led to the development of the lunar geological time scale, which became in the 1950s a new and growing branch of planetary geology. However, observation from Earth remains limited to the near side and it is in particular through space exploration that knowledge of the natural satellite increases, the first image of the far side of the Moon being obtained for example in 1959 thanks to the Soviet space probe Luna 3.
Exploration of the Moon
The Space Race (1959–1976)
Between the beginning of the Soviet Luna program in 1959 and until the 1970s with the last manned missions of the American Apollo program and the last Luna mission in 1976, the Cold War-inspired space race between the Soviet Union and the United States led to an acceleration of interest in exploring the Moon. As soon as their launchers manage to place spacecraft into orbit, the two countries begin sending probes to the natural satellite.
Luna Program
The Soviet Union began its lunar space program with a series of three failed unnamed missions in 1958.
However, the fourth was a success and the first flyby of the Moon was carried out by the Soviet probe Luna 1 on January 3, 1959, which is also the first spacecraft in history to be placed in heliocentric orbit. It was quickly followed by the first man-made object to reach the Moon — and generally to touch a celestial body other than Earth — the Luna 2 probe, which crashed there in September 1959. The first pictures of the far side of the moon are then sent on October 7, 1959by the Luna 3 probe.
The first mapping of the lunar surface is produced thanks to the photographs taken by Zond 3 on July 18, 1965, images covering 19,000,000 km2 and contributing to the development of selenography.
Russian engineers then progressed during the 1960s from craft only capable of flying over or crashing on the Moon to landers. Luna 9 is thus the first probe to land on the Moon rather than crash on February 3, 1966, returning photographs of the lunar surface. The first probe put into orbit around the Moon is Luna 10, on April 3, 1966.
On November 17, 1970, the Lunokhod 1 rover, carried by Luna 17, is the first robotic vehicle to explore its surface. Three years later, the Lunokhod 2 rover, carried by Luna 21, was the first craft to cover the distance of a marathon (42.1 km) on another celestial body.
Finally, the USSR developed three sample return missions to the Moon that brought back 0.3 kg of lunar rocks to Earth: Luna 16 in 1970, Luna 20 in 1972 and Luna 24 in 1976. The latter was the last Soviet mission to the Moon.
Apollo Program
The American space program was first entrusted to the military before being largely transferred to the civilian agency NASA.
Following the 1961 commitment of President John F. Kennedy then his famous speech where he delivered We choose to go to the Moon in 1962, various space programs are launched with the promise that an American will walk on the Moon before the end of the decade. Among them, the Ranger program produced the first close-up photos of the satellite, the Lunar Orbiter program mapped the entire Moon, and the Surveyor program resulted in the landing of Surveyor 1 on June 2, 1966, four months after Luna 9. The use of the term “landing” is however preferred, especially by the CNRS and the Academy of Sciences, even in the case of the Moon.
The Apollo program was developed in parallel, stimulated by a potential Soviet manned lunar program. After a series of unmanned and crewed tests in Earth orbit, the first human mission to lunar orbit was carried out in December 1968 by Apollo 8. The members of its crew (Frank Borman, James Lovell, and William Anders) are thus the first humans to see directly the far side of the Moon.
The Apollo 11 landing on July 21, 1969, is considered the culmination of the space race between the United States and the USSR during the Cold War. At 02:56 UTC, the first human to set foot on the Moon was Neil Armstrong, commander of the mission, followed by Buzz Aldrin. About 500 million people followed the event in Mondovision, the largest television audience for a live broadcast at the time.
In 2020, the last humans to walk on the lunar soil are Harrison Schmitt and Eugene Cernan, during the Apollo 17 mission in December 1972. The Apollo 11 to 17 missions (except Apollo 13, which canceled its landing during the mission) collected 380 kg of moon rock and soil from 2,196 samples. Sets of scientific instruments were installed on the lunar surface during the Apollo program, including the Apollo Lunar Surface Experiments Package. This includes long-life instruments, including heat flow probes, seismometers and magnetometers. Direct transmission of data to Earth ended at the end of 1977 for budgetary reasons.
Lunar reflectors are also deposited during these missions to measure the Earth-Moon distance with an accuracy of a few centimeters thanks to a laser beam. Passive instruments, they are still in use. The Soviet probes of the Lunokhod program also deposit.
In total, in the twentieth century and up to the present day, 24 astronauts have orbited the Moon and 12 of them have walked on it, all during the Apollo program.
Since the 1970s
The Moon began in 1974 to be abandoned by space powers in favor of other celestial bodies of the Solar System, including the outer Solar System for NASA with the Pioneer and Voyager programs, and the construction of space stations.
In the 1990s, the Moon became the main destination for probes of new space nations developing programs to explore the Solar System, mainly Japan, China and India. Thus, in 1990, Japan was the third country to place an orbiter in lunar orbit, Hagoromo dropped by the Hiten probe.
Interest in the Moon revived following two small NASA missions, Clementine and Lunar Prospector respectively launched in 1994 and 1998, which allowed the realization of the first quasi-global topographic map of the Moon as well as the discovery of an excess of hydrogen at the lunar poles, probably due to the presence of water ice in craters of eternal darkness.
In the 2000s, many missions to the Moon were carried out by different space agencies. The European Space Agency launches SMART-1 in September 2003 in order to carry out a study of the chemical elements of the lunar surface up to its impact in September 2006. Japan Aerospace Exploration Agency launches SELENE (or KAGUYA) orbiter in October 2007, which obtains lunar geophysics data and takes the first high-definition film beyond Earth orbit with an end of the mission in June 2009. The Indian Space Research Organization puts its first probe into lunar orbit, Chandrayaan-1, from November 2008 until it lost contact in August 2009, confirming the presence of water on the Moon. Chandrayaan-2 is launched in July 2019 but his lander Vikram failed to land.
China’s ambitious lunar exploration program (CLEP) begins with Chang’e 1, which orbits the Moon in November 2007 until its controlled lunar impact in March 2009, returns a complete map of the Moon. His Chang ‘e 2 linings reached the Moon in October 2010then became the first spacecraft to travel from lunar orbit to L2 in August 2011, before finally going to perform a flyby of asteroid 4179 Toutatis in December 2012.
The Chang’e 3 lander lands in December 2013 in the Sea of Rains and then deploys a lunar rover named Yutu. This is the first landing on the Moon since Luna 24 in 1976 and the first lunar rover since Lunokhod 2 in 1973. Her Chang ‘e 4 linings became the first mission to land on the far side of the Moon in Von Kármán crater in January 2019 and deploys the Yutu 2 rover. The Chang’e 5 sample return mission brings back to December 2020 the first lunar samples since Luna 24 in 1976 and completed the first automatic docking outside Earth’s orbit.
In the 2010s, NASA again implemented missions to the Moon. The Lunar Reconnaissance Orbiter is launched in June 2009 with the LCROSS impactor. If the latter completes its mission with a planned impact in the crater Cabeus in October 2009, the LRO is still active by regularly providing accurate lunar altimetry — allowing for topographic mapping — and high-resolution images. Two more orbiters were launched by NASA in January 2012 then in October 2013: GRAIL to study the internal structure of the Moon and LADEE to study the lunar exosphere, with mission endings in December 2012 and April 2014 respectively.
Other satellites, such as the Deep Space Climate Observatory located at point L1 of the Earth-Sun system, periodically provide images of the Moon.
Human presence on the Moon
Return to the Moon
The colonization of the Moon is the project of installing one or more permanent manned bases on the Moon, although this is not yet rationally possible. A human presence at least temporary on a planetary body other than the Earth is already a recurring theme of science fiction, but would have a practical interest here because the Moon would then constitute a preparation for more distant journeys.
NASA begins planning for the resumption of human missions following the call of US President George W. Bush in January 2004 with the space policy program Vision for Space Exploration. A human mission to the Moon before 2020 is then planned. The Constellation program was therefore funded and tests began on a crewed spacecraft called Orion as well as for a lunar base. The program was finally canceled in 2010 by President Barack Obama due to the budget.
However, at the instigation of US President Donald Trump, the return of Man to the Moon is put forward in April 2019, through the Artemis program. NASA’s manned space program, it plans to land a crew by 2024. It should lead to a sustainable exploration of the satellite by the organization of regular missions whose culmination would be the installation of a permanent station on the Moon.
The program would also develop the equipment and procedures needed for hypothetical manned missions to Mars. The Space Launch System (SLS) heavy launcher and the Orion spacecraft, whose development has already begun, will be used. In addition, a future space station, the Lunar Gateway, placed in orbit around the Moon, will serve as a relay between the Earth and the surface of the Moon. The landing sites selected for the various missions are located at the south pole of the Moon, because the water ice reserves present in the craters of eternal darkness are of strategic interest from the perspective of long-duration missions.
Legal status
Although the landers of the Luna program scattered pennants in the colors of the USSR on the Moon and American and Chinese flags were symbolically planted at the landing sites of their probes, no nation claims ownership of part of the surface of the Moon. Russia, China, India and the United States are signatories to the Outer Space Treaty — which entered into force on October 10, 1967— which defines the Moon and all outer space as belonging to the whole of humanity. This treaty also restricts the use of the Moon for peaceful purposes, explicitly prohibiting military installations and weapons of mass destruction, including nuclear weapons.
In 1979, the Moon Treaty was created to restrict the exploitation of the Moon’s natural resources by a single nation. However, it is considered a failure because no nation with human spaceflight programs or projects signs it. Although several natural persons have claimed the Moon in whole or in part, none of these claims is considered credible.
In August 2016, the US government allows the American start-up Moon Express to land on the Moon. This is the first time that a private company has been granted this right to do so. The decision is seen as a precedent helping to set regulatory standards for deep-space business activities in the future, as until now, companies’ activities have been limited to Earth or its surroundings.
In 2020, U.S. President Donald Trump signed an executive order entitled ” Encouraging International Support for the Recovery and Use of Space Resources“. The order emphasizes that the United States does not consider space a common good and reiterates the criticisms made of the Moon Treaty.
Since a Chinese space program official stated in 2013 that the Moon contains enough helium-3 to meet humanity’s energy needs for 10,000 years through nuclear fusion, the extraction of natural resources on the Moon could raise geopolitical problems.
Astronomy from the Moon
The Moon is recognized as an excellent site for telescopes. Indeed, it is relatively close and the quality of visibility is excellent in the absence of light pollution and atmosphere. Also, some craters near the poles are permanently in the dark and cold, so they are particularly suitable for infrared telescopes. In addition, radio telescopes on the far side would be protected from radio emissions from Earth.
Lunar soil can be mixed with carbon nanotubes and polyepoxides for use in the construction of mirrors with a diameter of up to 50 meters. A lunar zenithal telescope could be manufactured cheaply with an ionic liquid.
These properties are already being put to good use in April 1972, during the Apollo 16 mission, where various photos and astronomical spectra were taken from the lunar surface.
Human impact
In addition to traces of human activity on the Moon from experiments carried out on site, such as the Apollo Lunar Surface Experiments Package, permanent installations such as works of art are found on the lunar soil, such as the Moon Museum, the Apollo 11 Goodwill Messages, the lunar plates or the Fallen Astronaut. There are also some artifacts, such as the famous flags of the United States planted during each Apollo mission. Personal belongings left by the astronauts are also still present, such as the golf balls left by Alan Shepard during the Apollo 14 mission or a Bible deposited by David Scott during Apollo 15.
In total, space exploration has left nearly 180 tons of Earth-based material on the Moon. The heaviest objects include the third stages of several Saturn V rockets used during manned missions. Apart from the Chinese Yutu-2 rover, the only objects still used for scientific experiments are lunar reflectors to accurately measure the Earth-Moon distance.
In November 2018, NASA announced that nine commercial companies would compete for a contract to send small payloads to the Moon as part of Commercial Lunar Payload Services, new scientific instruments for the lunar soil.
In culture
Beliefs and mythologies
The contrast between the light plateaus and the darker seas on the surface of the Moon creates patterns for the human observer through a psychological phenomenon called pareidolia. These are noted and interpreted by many cultures, including the motifs of man in the Moon or the lunar rabbit. In Chinese mythology, the latter is notably the companion of the moon goddess Chang’e — who gives her name to the probes of the Chinese lunar exploration program — and in Aztec mythology, it serves as food for Quetzalcoatl.
In Proto-Indo-European religion, the Moon is personified as the male god *Meh1 non. The ancient Sumerians associate the Moon with the god Nanna, father of Ishtar, the goddess of the planet Venus and Utu, the god of the Sun. Nanna was later known as Sin.
In Greco-Roman mythology, the Sun and the Moon are represented respectively by a man and a woman (Helios and Selene for the Greeks and Sol and Luna for the Romans). This is a development unique to the eastern Mediterranean and traces of an earlier male lunar god in Greek tradition are preserved in the figure of Menelaus.
In Mesopotamian iconography, the crescent is the main symbol of Nanna-Sîn. In ancient Greek art, the moon goddess Selene is depicted wearing a crescent headgear evoking horns. The arrangement of star and crescent also dates back to the Bronze Age, representing the association of either the Sun and the Moon, or the Moon and the planet Venus. This arrangement is used to represent the goddesses Artemis (Diana in Roman mythology) and Hecate. Via Hecate’s patronage, it was then used as a symbol of Byzantium, and was later taken over by the Ottoman Empire. In Hindu mythology, the Moon is a male entity and is called Chandra.
The Moon also plays a prominent role in Muslim religious culture. Not only is it the basis for the construction of the Muslim lunar calendar, it is also evoked in the various religious biographies of Muhammad as part of the miracle of the division of the moon (Arabic: انشقاق القمر).
Legends about therianthropy—the transformation of a human being into another animal—are traditionally associated with the Moon. The most famous is that of the lycanthrope, or werewolf, drawing its strength from the Moon and able to change from its human form to its bestial form during full moon nights. Phenomena such as total solar eclipses created until the seventeenth-century myths and legends associated with the disappearance of the sun, although their explanation was already known by scholars.
Calendar
The regular phases of the Moon make it a very convenient element for measuring time; The periods of its rise and fall are consequently the basis of many of the oldest calendars. Archaeologists estimate that counting sticks, serrated bones dating from 20 to 30,000 years ago, would mark the phases of the Moon.
Indeed, the study of the phases of the moon is easy and a cycle of seasons – corresponding to one year – is carried out in about twelve lunations (354 days). Historically, lunar calendars were therefore used by early civilizations, such as in Mesopotamia and ancient Egypt. However, if they are adapted to nomadic peoples, they are problematic for peoples practicing agriculture because of the gradual shift they present with the seasons, forcing regular adjustments. Moreover, the modern definition of the month of about 30 days follows this tradition and is an approximation of the lunar cycle.
In order to take into account this discrepancy, many subsequent calendars are lunisolar with, among others, the Gallic calendars of Coligny, Hebrew or Traditional Chinese. They aim to match the cycle of the seasons with that of the lunar months, the Greek astronomer Meton having noticed in particular in the fifth century BC that 19 solar years correspond to 235 lunar months, in order to put them back in phase. They remain complex and subsequent civilizations will quickly prefer solar calendars.
The most famous purely lunar calendar is the Hijri calendar, dating from the seventh century. The months are then traditionally determined by visual observation of the hilal, the first crescent moon above the horizon.
The English name month (“month”) and its relatives in other Germanic languages come from the Proto-Germanic *mǣnṓth-, indicating the use of a lunar calendar among the Germans before the adoption of a solar calendar. This derives from the common Indo-European verbal root *meh 1 – “to measure”, allowing to go back to a functional conception of the Moon as a marker of the month and therefore of time.
This echoes the importance of the Moon in many ancient cultures for the measurement of time such as Latin mensis and Ancient Greek μείς (meis) or μήν (mēn) meaning “month”). In French, this root is found in particular in the words mois and menstruation (a term derived from the Latin menstrues which means “monthly”). In Chinese and Japanese, the character used to note the month in a date is that of the Moon (月), that of the day being that of the Sun (日).
Name and etymology
The feminine noun moon comes from the Latin lūna, attested since Ennius. It is then attested in French from the eleventh century: its first known occurrence is found in the Chanson de Roland, dated to about 1080.
Another term, *louksnā (“the luminous”), is a formation derived from *loukís, lūx (light) in Latin (also related to the Greek leukos “white”) describes the moon as a luminous star for the nocturnal clarity it brings. Authors such as Varron and Cicero, already derived luna from the intransitive verb lucere, meaning “to light, to shine, to enlighten”.
The names of the goddesses associated with the satellite, Luna, Selene and Cynthia (poetic name of Artemis, her mythical birthplace being Mount Cynthus) are also found in astronomical terms related to the Moon such as apolune, pericynthion and selenocentric orbit.
Personalized by the goddess Luna in Roman mythology, the Moon also gives its name to Monday (from lunis dies, in Latin, for “day of the Moon”).
Source of Inspiration
In vexillology, the full moon appears on coats of arms and flags such as the flag of Laos, Mongolia or Palau. Also, the symbol of the crescent and especially the association of the star and crescent having become the emblems of the Ottoman Empire after having been those of Byzantium, these motifs appear on multiple flags of Muslim countries including, among others, those of Turkey, Tunisia, Algeria or Pakistan. The crescent is also used independently of Islam, notably on the flag of Singapore.
In music, the Moon is a source of inspiration for many creations. Classical music compositions refer directly to it, such as Ludwig van Beethoven’s Moonlight Sonata (1802) — although this name was given after the composer’s death — or Claude Debussy’s Clair de Lune movement (1905). Then follow the ballads Blue Moon (1934) by Richard Rodgers and Lorenz Hart who will know success with various performers and Fly Me to the Moon which will be especially popularized by Frank Sinatra (1964).
The satellite was the theme of many rock songs, including Creedence Clearwater Revival’s Bad Moon Rising (1969), The Police’s Walking on the Moon (1979) and R.E.M. or the album The Dark Side of the Moon (1973) by Pink Floyd. In French, the most famous song is J’ai demande à la Lune (2002) by Indochine, with in another register the nursery rhyme Au clair de la Lune.
Moonlight is also celebrated by many poets and writers, including Paul Verlaine, author of Clair de Lune (1869), himself inspired by the work of Claude Debussy, and Guy de Maupassant, who draws two short stories (1882).
Finally, the representation of the moon in the terrestrial sky is common in painting, especially among the Romantics, because its disappearance can evoke the passage from life to death or an unhappy destiny.
Science fiction
In the second century, Lucian of Samosata wrote the satirical and imaginary travelogue True Stories, in which the heroes go to the Moon and meet its inhabitants the Selenites, named after Selene. This story is regularly cited as a precursor or even as the first work of science fiction in history.
During the Renaissance, other “proto-science fiction” writings emerged, including Le Songe ou l’Astronomie lunaire (1608) by Johannes Kepler or Histoire comique des États et Empires de la Lune (circa 1650) by Cyrano de Bergerac, again telling the journeys of men to the Moon, the latter even evoking a kind of rocket.
In the nineteenth century, Edgar Allan Poe published a journalistic hoax of a man going to the Moon in a balloon, Unparalleled Adventure of a certain Hans Pfaall (1835). However, the most famous science fiction novelist of the century was Jules Verne, author of From the Earth to the Moon (1865) and Around the Moon (1869). The other founding father of the genre, H. G. Wells published The First Men in the Moon in 1901.
From the twentieth century, the subject began to reach considerable popularity and many authors referred to it, among others in A Woman in the Moon (1928) by Thea von Harbou, Ashy Light (1955) by Arthur C. Clarke, Menace in the Sky (1960) by Algis Budrys and Revolt on the Moon (1966) by Robert A. Heinlein.
In comics, Hergé marked the genre with Objectif Lune (1953) then On a marche sur la Lune (1954). In American comics, the moon is often a place of fighting (this is where Jean Grey dies and thus concludes one of the most significant stories of the X-Men) or serves as a base for characters (in the Marvel universe, Uatu observes the Earth there).
Moreover, the Moon is a major theme in cinema, and this from its beginnings. Thus, the first science fiction film in history, Le Voyage dans la Lune (1902) by Georges Mélies is centered on the star and already addresses the subject of a team of explorers visiting it and meeting its mythical inhabitants, the same Selenites as those evoked by Lucien de Samosate. Thea von Harbou’s novel was also adapted into a silent film by Fritz Lang in The Woman on the Moon (1929).
After the Second World War, as geopolitical reality developed an interest in the star, the number of films increased; so comes Destination… Moon! (1950) by Irving Pichel and the adaptations From the Earth to the Moon (1958) by Byron Haskin, then The First Men in the Moon (1964) by Nathan Jura.
Space exploration considerably developed the genre of films related to the Moon, often based on real events, such as Apollo 13 (1995) by Ron Howard or First Man: The First Man on the Moon (2018) by Damien Chazelle, directly inspired by NASA missions. Pure science fiction films are also made, centered in Duncan Jones’ Moon (2009) or as a setting in Stanley Kubrick’s 2001: A Space Odyssey (1968).
References (sources)
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