# Moon

Last updated

Moon
Full moon seen from North America
Designations
Orbital characteristics
Epoch J2000
Perigee 362600 km
(356400370400 km)
Apogee 405400 km
(404000406700 km)
384399 km  (0.00257 AU) [1]
Eccentricity 0.0549 [1]
27.321661  d
(27d 7h 43min 11.5s [1] )
29.530589 d
(29d 12h 44min 2.9s)
Average orbital speed
1.022  km/s
Inclination 5.145° to the ecliptic [2] [lower-alpha 1]
Regressing by one revolution in 18.61 years
Progressing by one revolution in 8.85 years
Satellite of Earth [lower-alpha 2] [3]
Physical characteristics
1737.1 km  (0.2727 of Earth's) [1] [4] [5]
1738.1 km  (0.2725 of Earth's) [4]
1736.0 km  (0.2731 of Earth's) [4]
Flattening 0.0012 [4]
Circumference10921 km
3.793×107 km2  (0.074 of Earth's)
Volume 2.1958×1010 km3  (0.020 of Earth's) [4]
Mass 7.342×1022 kg  (0.012300 of Earth's) [1] [4] [6]
Mean density
3.344  g/cm3 [1] [4]
0.606 × Earth
1.62  m/s2   (0.1654  g ) [4]
0.3929±0.0009 [7]
2.38  km/s
Sidereal rotation period
27.321661 d
Equatorial rotation velocity
4.627 m/s
North pole
•  17h 47m 26s
• 266.86° [9]
North pole declination
65.64° [9]
Albedo 0.136 [10]
Surface temp. minmeanmax
Equator100 K 220 K390 K
85°N 150 K230 K [11]
29.3 to 34.1 arcminutes [4] [lower-alpha 4]
Atmosphere [12]
Surface pressure
Composition by volume

The Moon, also known as Luna, is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits (its primary). The Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known.

An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms object and body are often used interchangeably. However, an astronomical body or celestial body is a single, tightly bound, contiguous entity, while an astronomical or celestial object is a complex, less cohesively bound structure, which may consist of multiple bodies or even other objects with substructures.

In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. To a close approximation, planets and satellites follow elliptic orbits, with the central mass being orbited at a focal point of the ellipse, as described by Kepler's laws of planetary motion.

Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space, especially the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times.

## Contents

The Moon is thought to have formed about 4.51 billion years ago, not long after Earth. The most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia.

The age of the Earth is 4.54 ± 0.05 billion years (4.54 × 109 years ± 1%). This age may represent the age of the Earth's accretion, of core formation, or of the material from which the Earth formed. This dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.

The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, in the Hadean eon; about 20 to 100 million years after the Solar System coalesced. The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks, published in a 2016 report, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies.

Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. In English, Mars carries a name of the Roman god of war, and is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance that is distinctive among the astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

The Moon is in synchronous rotation with Earth, and thus always shows the same side to Earth, the near side. The near side is marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. After the Sun, the Moon is the second-brightest regularly visible celestial object in Earth's sky. Its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.

Tidal locking occurs when the long-term interaction between a pair of co-orbiting astronomical bodies drives the rotation rate of at least one of them into the state where there is no more net transfer of angular momentum between this body and its orbit around the second body ; this condition of "no net transfer" must be satisfied over the course of one orbit around the second body. This does not mean that the rotation and spin rates are always perfectly synchronized throughout an orbit, as there can be some back and forth transfer over the course of an orbit. This effect arises from the gravitational gradient between the co-orbiting bodies, acting over a sufficiently long period of time.

The near side of the Moon is the lunar hemisphere that is permanently turned towards Earth, whereas the opposite side is the far side. Only one side of the Moon is visible from Earth because the Moon rotates on its axis at the same rate that the Moon orbits the Earth – a situation known as synchronous rotation, or tidal locking.

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich composition, and hence appear dark to the naked eye. The maria cover about 16% of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus (lake), palus (marsh), and sinus (bay). The last three are smaller than maria, but have the same nature and characteristics.

The Moon's average orbital distance is 384,402 km (238,856 mi), [13] [14] or 1.28 light-seconds. This is about thirty times the diameter of Earth. The Moon's apparent size in the sky is almost the same as that of the Sun, since the star is about 400 times the lunar distance and diameter. Therefore, the Moon covers the Sun nearly precisely during a total solar eclipse. This matching of apparent visual size will not continue in the far future because the Moon's distance from Earth is gradually increasing.

Lunar distance, also called Earth–Moon distance, Earth–Moon characteristic distance, or distance to the Moon, is a unit of measure in astronomy. It is the average distance from the center of Earth to the center of the Moon. More technically, it is the mean semi-major axis of the geocentric lunar orbit. It may also refer to the time-averaged distance between the centers of the Earth and the Moon, or less commonly, the instantaneous Earth–Moon distance. The lunar distance is approximately a quarter of a million miles.

The angular diameter, angular size, apparent diameter, or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is the angular aperture. The angular diameter can alternatively be thought of as the angle through which an eye or camera must rotate to look from one side of an apparent circle to the opposite side. Angular radius equals half the angular diameter.

A solar eclipse occurs when an observer passes through the shadow cast by the Moon which fully or partially blocks ("occults") the Sun. This can only happen when the Sun, Moon and Earth are nearly aligned on a straight line in three dimensions (syzygy) during a new moon when the Moon is close to the ecliptic plane. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured.

The Moon was first reached in September 1959 by the Soviet Union's Luna 2, an unmanned spacecraft. The United States' NASA Apollo program achieved the only manned lunar missions to date, beginning with the first manned orbital mission by Apollo 8 in 1968, and six manned landings between 1969 and 1972, with the first being Apollo 11. These missions returned lunar rocks which have been used to develop a geological understanding of the Moon's origin, internal structure, and the Moon's later history. Since the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft.

The Soviet Union, officially the Union of Soviet Socialist Republics (USSR), was a socialist state in Eurasia that existed from 1922 to 1991. Nominally a union of multiple national Soviet republics, its government and economy were highly centralized. The country was a one-party state, governed by the Communist Party with Moscow as its capital in its largest republic, the Russian Soviet Federative Socialist Republic. Other major urban centres were Leningrad, Kiev, Minsk, Alma-Ata, and Novosibirsk. It spanned over 10,000 kilometres east to west across 11 time zones, and over 7,200 kilometres north to south. It had five climate zones: tundra, taiga, steppes, desert and mountains.

Luna 2 or Lunik 2 was the sixth of the Soviet Union's Luna programme spacecraft launched to the Moon. It was the first spacecraft to reach the surface of the Moon, and the first human-made object to make contact with another celestial body. On September 13, 1959, it hit the Moon's surface east of Mare Imbrium near the craters Aristides, Archimedes, and Autolycus.

The United States of America (USA), commonly known as the United States or America, is a country comprising 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is slightly smaller than the entire continent of Europe's 3.9 million square miles. With a population of over 327 million people, the U.S. is the third most populous country. The capital is Washington, D.C., and the largest city by population is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico. The State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean. The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The extremely diverse geography, climate, and wildlife of the United States make it one of the world's 17 megadiverse countries.

Both the Moon's natural prominence in the earthly sky and its regular cycle of phases as seen from Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in language, lunar calendar systems, art, and mythology.

A lunar calendar is a calendar based upon the monthly cycles of the Moon's phases, in contrast to solar calendars, whose annual cycles are based only directly upon the solar year. The most commonly used calendar, the Gregorian calendar, is a solar calendar system that originally evolved out of a lunar calendar system. A purely lunar calendar is also distinguished from a lunisolar calendar, whose lunar months are brought into alignment with the solar year through some process of intercalation. The details of when months begin varies from calendar to calendar, with some using new, full, or crescent moons and others employing detailed calculations.

The Moon has been the subject of many works of art and literature and the inspiration for countless others. It is a motif in the visual arts, the performing arts, poetry, prose and music.

## Name and etymology

The usual English proper name for Earth's natural satellite is "the Moon", which in nonscientific texts is usually not capitalized. [15] [16] [17] [18] [19] The noun moon is derived from Old English mōna, which (like all Germanic language cognates) stems from Proto-Germanic *mēnô, which comes from Proto-Indo-European *mḗh₁n̥s "moon", "month", which comes from the Proto-Indo-European root *meh₁- "to measure", the month being the ancient unit of time measured by the Moon. [20] [21] Occasionally, the name "Luna" is used. In literature, especially science fiction, "Luna" is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of Earth's moon. [22]

The modern English adjective pertaining to the Moon is lunar, derived from the Latin word for the Moon, luna. The adjective selenic (usually only used to refer to the chemical element selenium) is so rarely used to refer to the Moon that this meaning is not recorded in most major dictionaries. [23] [24] [25] It is derived from the Ancient Greek word for the Moon, σελήνη (selḗnē), from which is however also derived the prefix "seleno-", as in selenography , the study of the physical features of the Moon, as well as the element name selenium. [26] [27] Both the Greek goddess Selene and the Roman goddess Diana were alternatively called Cynthia. [28] The names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana comes from the Proto-Indo-European *diw-yo, "heavenly", which comes from the PIE root *dyeu- "to shine," which in many derivatives means "sky, heaven, and god" and is also the origin of Latin dies, "day".

The Moon

## Formation

The Moon formed 4.51 billion years ago, [lower-alpha 6] some 60 million years after the origin of the Solar System. Several forming mechanisms have been proposed, [29] including the fission of the Moon from Earth's crust through centrifugal force [30] (which would require too great an initial spin of Earth), [31] the gravitational capture of a pre-formed Moon [32] (which would require an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon), [31] and the co-formation of Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metals in the Moon). [31] These hypotheses also cannot account for the high angular momentum of the Earth–Moon system. [33]

The prevailing hypothesis is that the Earth–Moon system formed after an impact of a Mars-sized body (named Theia ) with the proto-Earth (giant impact). The impact blasted material into Earth's orbit and then the material accreted and formed the Moon. [34] [35]

The Moon's far side has a crust that is 30 mi (48 km) thicker than that of the near side. This is thought to be because the Moon fused from two different bodies.

This hypothesis, although not perfect, perhaps best explains the evidence. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most consensual theory.

Before the conference, there were partisans of the three "traditional" theories, plus a few people who were starting to take the giant impact seriously, and there was a huge apathetic middle who didn’t think the debate would ever be resolved. Afterward, there were essentially only two groups: the giant impact camp and the agnostics. [36]

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 also show that most of the Moon derived from the impactor, rather than the proto-Earth. [37] However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. [38] [39] [40] [41] 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, [42] although this is debated. [43]

The impact released a lot of energy and then the released material re-accreted into the Earth–Moon system. This would have melted the outer shell of Earth, and thus formed a magma ocean. [44] [45] Similarly, the newly formed Moon would also have been affected and had its own lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles). [44]

While the giant impact hypothesis might explain many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. [46]

Oceanus Procellarum ("Ocean of Storms")
Ancient rift valleys – rectangular structure (visible – topography – GRAIL gravity gradients)
Ancient rift valleys – context.
Ancient rift valleys – closeup (artist's concept).

In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks. [47] To their surprise, the rocks from the Apollo program had the same isotopic signature as rocks from Earth, however they differed from almost all other bodies in the Solar System. Indeed, this observation was unexpected, because most of the material that formed the Moon was thought to come from Theia and it was announced in 2007 that there was less than a 1% chance that Theia and Earth had identical isotopic signatures. [48] Other Apollo lunar samples had in 2012 the same titanium isotopes composition as Earth, [49] which conflicts with what is expected if the Moon formed far from Earth or is derived from Theia. These discrepancies may be explained by variations of the giant impact hypothesis.

## Physical characteristics

### Internal structure

Lunar surface chemical composition [50]
CompoundFormulaComposition
MariaHighlands
silica SiO245.4%45.5%
alumina Al2O314.9%24.0%
lime CaO11.8%15.9%
iron(II) oxide FeO14.1%5.9%
magnesia MgO9.2%7.5%
titanium dioxide TiO23.9%0.6%
sodium oxide Na2O0.6%0.6%
99.9%100.0%

The Moon is a differentiated body. It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi). [51] [52] This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago. [53]

Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallised, lower-density plagioclase minerals could form and float into a crust atop. [54] The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. [1]

Consistent with this perspective, geochemical mapping made from orbit suggests the crust of mostly anorthosite. [12] The Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth. [1] The crust is on average about 50 kilometres (31 mi) thick. [1]

The Moon is the second-densest satellite in the Solar System, after Io. [55] However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less, [1] around 20% of the radius of the Moon. Its composition is not well defined, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's time-variable rotation suggest that it is at least partly molten. [56]

### Surface geology

The topography of the Moon has been measured with laser altimetry and stereo image analysis. [57] Its most visible topographic feature is the giant far-side South Pole–Aitken basin, some 2,240 km (1,390 mi) in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. [58] [59] At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon. [58] [60] The highest elevations of the Moon's surface are located directly to the northeast, and it has been suggested might have been thickened by the oblique formation impact of the South Pole–Aitken basin. [61] Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims. [58] The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side. [1]

The discovery of fault scarp cliffs by the Lunar Reconnaissance Orbiter suggest that the Moon has shrunk within the past billion years, by about 90 metres (300 ft). [62] Similar shrinkage features exist on Mercury.

#### Volcanic features

The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called maria (Latin for "seas"; singular mare), as they were once believed to be filled with water; [63] they are now known to be vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. [64] The majority of these lavas erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side "maria". [65]

Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side, [66] compared with 2% of the far side. [67] This is thought to be due to a concentration of heat-producing elements under the crust on the near side, seen on geochemical maps obtained by Lunar Prospector's gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. [54] [68] [69] Most of the Moon's mare basalts erupted during the Imbrian period, 3.0–3.5 billion years ago, although some radiometrically dated samples are as old as 4.2 billion years. [70] Until recently, the youngest eruptions, dated by crater counting, appeared to have been only 1.2 billion years ago. [71] In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old. [72] Moonquakes and releases of gas also indicate some continued lunar activity. [72] In 2014 NASA announced "widespread evidence of young lunar volcanism" at 70 irregular mare patches identified by the Lunar Reconnaissance Orbiter, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements. [73] [74] [75] [76] Just prior to this, evidence has been presented for 2–10 million years younger basaltic volcanism inside Lowell crater, [77] [78] Orientale basin, located in the transition zone between the near and far sides of the Moon. An initially hotter mantle and/or local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities also on the far side in the Orientale basin. [79] [80]

The lighter-coloured regions of the Moon are called terrae, or more commonly highlands, because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean. [70] [71] In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events. [81]

The concentration of maria on the Near Side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after their formation. [82] [83]

#### Impact craters

The other major geologic process that has affected the Moon's surface is impact cratering, [84] with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon's near side alone. [85] The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale, structures characterized by multiple rings of uplifted material, between hundreds and thousands of kilometres in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon. [86] The lack of an atmosphere, weather and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface. [86] The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment of impacts. [87]

Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder. [88] The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–20 km (6.2–12.4 mi) in the highlands and 3–5 km (1.9–3.1 mi) in the maria. [89] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometres thick. [90]

Comparison of high-resolution images obtained by the Lunar Reconnaissance Orbiter has shown a contemporary crater-production rate significantly higher than previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimetres of regolith a hundred times more quickly than previous models suggested – on a timescale of 81,000 years. [91] [92]

#### Lunar swirls

Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and have often a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls.

#### Presence of water

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon. [93] [94] Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow. [95] The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive. [96]

In years since, signatures of water have been found to exist on the lunar surface. [97] In 1994, the bistatic radar experiment located on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters. [98] In 1998, the neutron spectrometer on the Lunar Prospector spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions. [99] Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior. [100]

The 2008 Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000  ppm. [101] Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018. [102] In 2009, LCROSS sent a 2,300 kg (5,100 lb) impactor into a permanently shadowed polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material. [103] [104] Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb). [105]

In May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220 was reported, [106] the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's upper mantle. Although of considerable selenological interest, this announcement affords little comfort to would-be lunar colonists – the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.

Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time "definitive evidence" for water-ice on the lunar surface. [107] [108] The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. [109] The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and cravices, allowing it to persist as ice on the surface since they are shielded from the sun. [107] [109]

### Gravitational field

The gravitational field of the Moon has been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. [110] [111] The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism. [112]

### Magnetic field

The Moon has an external magnetic field of about 1–100 nanoteslas, less than one-hundredth that of Earth. The Moon does not currently have a global dipolar magnetic field and only has crustal magnetization, probably acquired early in its history when a dynamo was still operating. [113] [114] Alternatively, some of the remnant magnetization may be from transient magnetic fields generated during large impacts through the expansion of an impact-generated plasma cloud in an ambient magnetic field. This is supported by the apparent location of the largest crustal magnetizations near the antipodes of the giant impact basins. [115]

### Atmosphere

The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 metric tons (9.8 long tons; 11 short tons). [118] The surface pressure of this small mass is around 3 × 10−15  atm (0.3  nPa); it varies with the lunar day. Its sources include outgassing and sputtering, a product of the bombardment of lunar soil by solar wind ions. [12] [119] Elements that have been detected include sodium and potassium, produced by sputtering (also found in the atmospheres of Mercury and Io); helium-4 and neon [120] from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. [121] [122] The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood. [121] Water vapour has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith. [123] These gases either return into the regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field. [121]

#### Dust

A permanent asymmetric moon dust cloud exists around the Moon, created by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface each 24 hours. The particles strike the Moon's surface ejecting moon dust above the Moon. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising to 100 kilometers above the surface. The dust measurements were made by LADEE's Lunar Dust EXperiment (LDEX), between 20 and 100 kilometers above the surface, during a six-month period. LDEX detected an average of one 0.3 micrometer moon dust particle each minute. Dust particle counts peaked during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon, pass through comet debris. The cloud is asymmetric, more dense near the boundary between the Moon's dayside and nightside. [124] [125]

#### Past thicker atmosphere

In October 2017, NASA scientists at the Marshall Space Flight Center and the Lunar and Planetary Institute in Houston announced their finding, based on studies of Moon magma samples retrieved by the Apollo missions, that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space. [126]

### Seasons

The Moon's axial tilt with respect to the ecliptic is only 1.5424°, [127] much less than the 23.44° of Earth. Because of this, the Moon's solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects. [128] From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of Peary Crater at the Moon's north pole may remain illuminated for the entire lunar day, creating peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters, [95] and these "craters of eternal darkness" are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F) [129] and just 26 K (−247 °C; −413 °F) close to the winter solstice in north polar Hermite Crater. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto. [128] Average temperatures of the Moon's surface are reported, but temperatures of different areas will vary greatly depending upon whether they are in sunlight or shadow. [130]

## Earth-Moon system

### Orbit

The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days [lower-alpha 7] (its sidereal period). However, because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days [lower-alpha 8] (its synodic period). [66] Unlike most satellites of other planets, the Moon orbits closer to the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbit gradually rotates once every 18.61 [131] years, which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws. [132]

### Relative size

The Moon is exceptionally large relative to Earth: Its diameter is more than a quarter and its mass is 1/81 of Earth's. [66] It is the largest moon in the Solar System relative to the size of its planet, [lower-alpha 9] though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass. [lower-alpha 10] [133] The Earth and the Moon's barycentre, their common centre of mass, is located 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath Earth's surface.

The Earth revolves around the Earth-Moon barycentre once a sidereal month, with 1/81 the speed of the Moon, or about 12.5 metres (41 ft) per second. This motion is superimposed on the much larger revolution of the Earth around the Sun at a speed of about 30 kilometres (19 mi) per second.

### Appearance from Earth

The Moon is in synchronous rotation as it orbits Earth; it rotates about its axis in about the same time it takes to orbit Earth. This results in it always keeping nearly the same face turned towards Earth. However, because of the effect of libration, about 59% of the Moon's surface can actually be seen from Earth. The side of the Moon that faces Earth is called the near side, and the opposite the far side. The far side is often inaccurately called the "dark side", but it is in fact illuminated as often as the near side: once every 29.5 Earth days. During new moon, the near side is dark. [134]

The Moon had once rotated at a faster rate, but early in its history, its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by Earth. [135] With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists, using data collected on the much earlier NASA Lunar Prospector mission, found two hydrogen-rich areas on opposite sides of the Moon, probably in the form of water ice. It is speculated that these patches were the poles of the Moon billions of years ago, before it was tidally locked to Earth. [136]

The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun. [66] [lower-alpha 11] This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon. [137] Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the centre, without limb darkening, because of the reflective properties of lunar soil, which retroreflects light more towards the Sun than in other directions. The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the moon illusion, first described in the 7th century BC. [138] The full Moon's angular diameter is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see § Eclipses).

The Moon's highest altitude at culmination varies by its phase and time of year. The full moon is highest in the sky during winter (for each hemisphere). The 18.61-year nodal cycle has an influence on lunar standstill. When the ascending node of the lunar orbit is in the vernal equinox, the lunar declination can reach up to plus or minus 28° each month. This means the Moon can pass overhead if viewed from latitudes up to 28° north or south (of the Equator), instead of only 18°. The orientation of the Moon's crescent also depends on the latitude of the viewing location; an observer in the tropics can see a smile-shaped crescent Moon. [139] The Moon is visible for two weeks every 27.3 days at the North and South Poles. Zooplankton in the Arctic use moonlight when the Sun is below the horizon for months on end. [140]

The distance between the Moon and Earth varies from around 356,400 km (221,500 mi) to 406,700 km (252,700 mi) at perigee (closest) and apogee (farthest), respectively. On 14 November 2016, it was closer to Earth when at full phase than it has been since 1948, 14% closer than its farthest position in apogee. [143] Reported as a "supermoon", this closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because its angular diameter is 14% greater and ${\displaystyle \scriptstyle 1.14^{2}\approx 1.30}$. [144] [145] [146] At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula: [147] [148]

${\displaystyle {\text{perceived reduction}}\%=100\times {\sqrt {{\text{actual reduction}}\% \over 100}}}$

When the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is about 0.877, or 1.00 / 1.14. This gives a maximum perceived increase of 14% between apogee and perigee moons of the same phase. [149]

There has been historical controversy over whether features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km (1.9 mi) diameter region of the lunar surface was modified by a gas release event about a million years ago. [150] [151]

The Moon's appearance, like the Sun's, can be affected by Earth's atmosphere. Common optical effects are the 22° halo ring, formed when the Moon's light is refracted through the ice crystals of high cirrostratus clouds, and smaller coronal rings when the Moon is seen through thin clouds. [152]

The monthly changes in the angle between the direction of sunlight and view from Earth, and the phases of the Moon that result, as viewed from the Northern Hemisphere. The Earth–Moon distance is not to scale.

The illuminated area of the visible sphere (degree of illumination) is given by ${\displaystyle (1-\cos e)/2=\sin ^{2}(e/2)}$, where ${\displaystyle e}$ is the elongation (i.e., the angle between Moon, the observer (on Earth) and the Sun).

### Tidal effects

The gravitational attraction that masses have for one another decreases inversely with the square of the distance of those masses from each other. As a result, the slightly greater attraction that the Moon has for the side of Earth closest to the Moon, as compared to the part of the Earth opposite the Moon, results in tidal forces. Tidal forces affect both the Earth's crust and oceans.

The most obvious effect of tidal forces is to cause two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. This results in elevated sea levels called ocean tides. [153] As the Earth spins on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours. [153] Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon's time to orbit the Earth. The Sun has the same tidal effect on the Earth, but its forces of attraction are only 40% that of the Moon's; the Sun's and Moon's interplay is responsible for spring and neap tides. [153] If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects: the frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that grow shallower near land, the sloshing of water between different ocean basins. [154] As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory.

While gravitation causes acceleration and movement of the Earth's fluid oceans, gravitational coupling between the Moon and Earth's solid body is mostly elastic and plastic. The result is a further tidal effect of the Moon on the Earth that causes a bulge of the solid portion of the Earth nearest the Moon that acts as a torque in opposition to the Earth's rotation. This "drains" angular momentum and rotational kinetic energy from Earth's spin, slowing the Earth's rotation. [153] [155] That angular momentum, lost from the Earth, is transferred to the Moon in a process (confusingly known as tidal acceleration), which lifts the Moon into a higher orbit and results in its lower orbital speed about the Earth. Thus the distance between Earth and Moon is increasing, and the Earth's spin is slowing in reaction. [155] Measurements from laser reflectors left during the Apollo missions (lunar ranging experiments) have found that the Moon's distance increases by 38 mm (1.5 in) per year [156] (roughly the rate at which human fingernails grow). [157] Atomic clocks also show that Earth's day lengthens by about 15  microseconds every year, [158] slowly increasing the rate at which UTC is adjusted by leap seconds. Left to run its course, this tidal drag would continue until the spin of Earth and the orbital period of the Moon matched, creating mutual tidal locking between the two. As a result, the Moon would be suspended in the sky over one meridian, as is already currently the case with Pluto and its moon Charon. However, the Sun will become a red giant engulfing the Earth-Moon system long before this occurrence. [159] [160]

In a like manner, the lunar surface experiences tides of around 10 cm (4 in) amplitude over 27 days, with two components: a fixed one due to Earth, because they are in synchronous rotation, and a varying component from the Sun. [155] The Earth-induced component arises from libration, a result of the Moon's orbital eccentricity (if the Moon's orbit were perfectly circular, there would only be solar tides). [155] Libration also changes the angle from which the Moon is seen, allowing a total of about 59% of its surface to be seen from Earth over time. [66] The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of the absence of water to damp out the seismic vibrations. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972. [161]

### Eclipses

From Earth, the Moon and the Sun appear the same size, as seen in the 1999 solar eclipse (left), whereas from the STEREO-B spacecraft in an Earth-trailing orbit, the Moon appears much smaller than the Sun (right). [162]

Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses. [163] In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Because the distance between the Moon and Earth is very slowly increasing over time, [153] the angular diameter of the Moon is decreasing. Also, as it evolves toward becoming a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing. [lower-alpha 12] The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur. [164]

Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun, 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. [165] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years. [166]

Because the Moon is continuously blocking our view of a half-degree-wide circular area of the sky, [lower-alpha 13] [167] the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted. [168]

## History, observation and exploration

### Ancient and medieval studies

Understanding of the Moon's cycles was an early development of astronomy: by the 5th century BC, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, [169] and Indian astronomers had described the Moon's monthly elongation. [170] The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses. [171] Later, the physical form of the Moon and the cause of moonlight became understood. The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. [172] [173] Although the Chinese of the Han Dynasty believed the Moon to be energy equated to qi , their 'radiating influence' theory also recognized that the light of the Moon was merely a reflection of the Sun, and Jing Fang (78–37 BC) noted the sphericity of the Moon. [174] In the 2nd century AD, Lucian wrote the novel A True Story , in which the heroes travel to the Moon and meet its inhabitants. In 499 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. [175] The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions. [176] Shen Kuo (1031–1095) of the Song dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent. [177]

In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries. [178] However, in the 2nd century BC, Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun. [179] In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. These figures were greatly improved by Ptolemy (90–168 AD): his values of a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively. [180] Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. [181]

During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth". [182]

In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had mountains and craters. Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834–36 Mappa Selenographica of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book Der Mond, the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography. [183] Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions. [66] This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s, [184] leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology. [66]

### By spacecraft

#### 20th century

##### Soviet missions

The Cold War-inspired Space Race between the Soviet Union and the U.S. led to an acceleration of interest in exploration of the Moon. Once launchers had the necessary capabilities, these nations sent unmanned probes on both flyby and impact/lander missions. Spacecraft from the Soviet Union's Luna program were the first to accomplish a number of goals: following three unnamed, failed missions in 1958, [185] the first human-made object to escape Earth's gravity and pass near the Moon was Luna 1 ; the first human-made object to impact the lunar surface was Luna 2 , and the first photographs of the normally occluded far side of the Moon were made by Luna 3 , all in 1959.

The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first unmanned vehicle to orbit the Moon was Luna 10 , both in 1966. [66] Rock and soil samples were brought back to Earth by three Luna sample return missions ( Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976), which returned 0.3 kg total. [186] Two pioneering robotic rovers landed on the Moon in 1970 and 1973 as a part of Soviet Lunokhod programme.

Luna 24 was the last Soviet/Russian mission to the Moon.

##### United States missions

During the late 1950s at the height of the Cold War, the United States Army conducted a classified feasibility study that proposed the construction of a manned military outpost on the Moon called Project Horizon with the potential to conduct a wide range of missions from scientific research to nuclear Earth bombardment. The study included the possibility of conducting a lunar-based nuclear test. [187] [188] The Air Force, which at the time was in competition with the Army for a leading role in the space program, developed its own similar plan called Lunex. [189] [190] [187] However, both these proposals were ultimately passed over as the space program was largely transferred from the military to the civilian agency NASA. [190]

Following President John F. Kennedy's 1961 commitment to a manned moon landing before the end of the decade, the United States, under NASA leadership, launched a series of unmanned probes to develop an understanding of the lunar surface in preparation for manned missions: the Jet Propulsion Laboratory's Ranger program produced the first close-up pictures; the Lunar Orbiter program produced maps of the entire Moon; the Surveyor program landed its first spacecraft four months after Luna 9. The manned Apollo program was developed in parallel; after a series of unmanned and manned tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar flight, in 1968 Apollo 8 made the first manned mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race. [191]

Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969. [192] An estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time. [193] [194] The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) returned 380.05 kilograms (837.87 lb) of lunar rock and soil in 2,196 separate samples. [195] The American Moon landing and return was enabled by considerable technological advances in the early 1960s, in domains such as ablation chemistry, software engineering, and atmospheric re-entry technology, and by highly competent management of the enormous technical undertaking. [196] [197]

Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations, [198] [199] but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used. Ranging to the stations is routinely performed from Earth-based stations with an accuracy of a few centimetres, and data from this experiment are being used to place constraints on the size of the lunar core. [200]

##### 1980s–2000

After the first Moon race there were years of near quietude but starting in the 1990s, many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft. The spacecraft released a smaller probe, Hagoromo, in lunar orbit, but the transmitter failed, preventing further scientific use of the mission. [201] In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. [202] This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. [203]

India, Japan, China, the United States, and the European Space Agency each sent lunar orbiters, and especially ISRO's Chandrayaan-1 has contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. The post-Apollo era has also seen two rover missions: the final Soviet Lunokhod mission in 1973, and China's ongoing Chang'e 3 mission, which deployed its Yutu rover on 14 December 2013. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.

#### 21st century

The European spacecraft SMART-1 , the second ion-propelled spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface. [204]

The ambitious Chinese Lunar Exploration Program began with Chang'e 1 , which successfully orbited the Moon from 5 November 2007 until its controlled lunar impact on 1 March 2009. [205] It obtained a full image map of the Moon. Chang'e 2 , beginning in October 2010, reached the Moon more quickly, mapped the Moon at a higher resolution over an eight-month period, then left lunar orbit for an extended stay at the Earth–Sun L2 Lagrangian point, before finally performing a flyby of asteroid 4179 Toutatis on 13 December 2012, and then heading off into deep space. On 14 December 2013, Chang'e 3 landed a lunar lander onto the Moon's surface, which in turn deployed a lunar rover, named Yutu (Chinese: 玉兔; literally "Jade Rabbit"). This was the first lunar soft landing since Luna 24 in 1976, and the first lunar rover mission since Lunokhod 2 in 1973. China intends to launch another rover mission ( Chang'e 4 ) before 2020, followed by a sample return mission ( Chang'e 5 ) soon after. [206]

Between 4 October 2007 and 10 June 2009, the Japan Aerospace Exploration Agency's Kaguya (Selene) mission, a lunar orbiter fitted with a high-definition video camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit. [207] [208] India's first lunar mission, Chandrayaan I , orbited from 8 November 2008 until loss of contact on 27 August 2009, creating a high resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil. [209] The Indian Space Research Organisation planned to launch Chandrayaan II in 2013, which would have included a Russian robotic lunar rover. [210] [211] However, the failure of Russia's Fobos-Grunt mission has delayed this project, and is now scheduled to be launched no earlier than January 2019. [212]

Copernicus's central peaks as observed by the LRO, 2012
The Ina formation, 2009

The U.S. co-launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor and follow-up observation orbiter on 18 June 2009; LCROSS completed its mission by making a planned and widely observed impact in the crater Cabeus on 9 October 2009, [213] whereas LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery. In November 2011, the LRO passed over the large and bright Aristarchus crater. NASA released photos of the crater on 25 December 2011. [214]

Two NASA GRAIL spacecraft began orbiting the Moon around 1 January 2012, [215] on a mission to learn more about the Moon's internal structure. NASA's LADEE probe, designed to study the lunar exosphere, achieved orbit on 6 October 2013.

Upcoming lunar missions include Russia's Luna-Glob : an unmanned lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission. [216] [217] Privately funded lunar exploration has been promoted by the Google Lunar X Prize, announced 13 September 2007, which offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria. [218] Shackleton Energy Company is building a program to establish operations on the south pole of the Moon to harvest water and supply their Propellant Depots. [219] NASA began to plan to resume manned missions following the call by U.S. President George W. Bush on 14 January 2004 for a manned mission to the Moon by 2019 and the construction of a lunar base by 2024. [220] The Constellation program was funded and construction and testing begun on a manned spacecraft and launch vehicle, [221] and design studies for a lunar base. [222] However, that program has been cancelled in favor of a manned asteroid landing by 2025 and a manned Mars orbit by 2035. [223] India has also expressed its hope to send a manned mission to the Moon by 2020. [224] On 28 February 2018, SpaceX, Vodafone, Nokia and Audi announced a collaboration to install a 4G wireless communication network on the Moon, with the aim of streaming live footage on the surface to Earth. [225] #### Planned commercial missions In 2007, the X Prize Foundation together with Google launched the Google Lunar X Prize to encourage commercial endeavors to the Moon. A prize of$20 million was to be awarded to the first private venture to get to the Moon with a robotic lander by the end of March 2018, with additional prizes worth \$10 million for further milestones. [226] [227] As of August 2016, 16 teams were reportedly participating in the competition. [228] In January 2018 the foundation announced that the prize would go unclaimed as none of the finalist teams would be able to make a launch attempt by the deadline. [229]

In August 2016, the US government granted permission to US-based start-up Moon Express to land on the Moon. [230] This marked the first time that a private enterprise was given the right to do so. The decision is regarded as a precedent helping to define regulatory standards for deep-space commercial activity in the future, as thus far companies' operation had been restricted to being on or around Earth. [230]

On November 29, 2018 NASA announced that nine commercial companies would compete to win a contract to send small payloads to the Moon in what is known as Commercial Lunar Payload Services. According to NASA administrator Jim Bridenstine, "We are building a domestic American capability to get back and forth to the surface of the moon." [231]

## Astronomy from the Moon

For many years, the Moon has been recognized as an excellent site for telescopes. [233] It is relatively nearby; astronomical seeing is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth. [234] The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies and employed in the construction of mirrors up to 50 meters in diameter. [235] A lunar zenith telescope can be made cheaply with an ionic liquid. [236]

In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph. [237]

Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation claims ownership of any part of the Moon's surface. [238] Russia, China, and the U.S. are party to the 1967 Outer Space Treaty, [239] which defines the Moon and all outer space as the "province of all mankind". [238] This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. [240] The 1979 Moon Agreement was created to restrict the exploitation of the Moon's resources by any single nation, but as of November 2016, it has been signed and ratified by only 18 nations, none of which engages in self-launched human space exploration or has plans to do so. [241] Although several individuals have made claims to the Moon in whole or in part, none of these are considered credible. [242] [243] [244]

## In culture

### Mythology

A 5,000-year-old rock carving at Knowth, Ireland, may represent the Moon, which would be the earliest depiction discovered. [245] The contrast between the brighter highlands and the darker maria creates the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others. In many prehistoric and ancient cultures, the Moon was personified as a deity or other supernatural phenomenon, and astrological views of the Moon continue to be propagated today.

In Proto-Indo-European religion, the moon was personified as the male god *Meh1not. [246] The ancient Sumerians believed that the Moon was the god Nanna, [247] [248] who was the father of Inanna, the goddess of the planet Venus, [247] [248] and Utu, the god of the sun. [247] [248] Nanna was later known as Sîn, [248] [247] and was particularly associated with magic and sorcery. [247] In Greco-Roman mythology, the Sun and the Moon are represented as male and female, respectively (Helios/Sol and Selene/Luna); [246] this is a development unique to the eastern Mediterranean [246] and traces of an earlier male moon god in the Greek tradition are preserved in the figure of Menelaus. [246]

In Mesopotamian iconography, the crescent was the primary symbol of Nanna-Sîn. [248] In ancient Greek art, the Moon goddess Selene was represented wearing a crescent on her headgear in an arrangement reminiscent of horns. [249] [250] The star and crescent arrangement also goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and planet Venus, in combination. It came to represent the goddess Artemis or Hecate, and via the patronage of Hecate came to be used as a symbol of Byzantium.

An iconographic tradition of representing Sun and Moon with faces developed in the late medieval period.

The splitting of the moon (Arabic : انشقاق القمر) is a miracle attributed to Muhammad. [251]

### Calendar

The Moon's regular phases make it a very convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. [252] [253] [254] The ~30-day month is an approximation of the lunar cycle. The English noun month and its cognates in other Germanic languages stem from Proto-Germanic *mǣnṓth-, which is connected to the above-mentioned Proto-Germanic *mǣnōn, indicating the usage of a lunar calendar among the Germanic peoples (Germanic calendar) prior to the adoption of a solar calendar. [255] The PIE root of moon, *méh1nōt, derives from the PIE verbal root *meh1-, "to measure", "indicat[ing] a functional conception of the Moon, i.e. marker of the month" (cf. the English words measure and menstrual), [256] [257] [258] and echoing the Moon's importance to many ancient cultures in measuring time (see Latin mensis and Ancient Greek μείς (meis) or μήν (mēn), meaning "month"). [259] [260] [261] [262] Most historical calendars are lunisolar. The 7th-century Islamic calendar is an exceptional example of a purely lunar calendar. Months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon. [263]

### Lunacy

The Moon has long been associated with insanity and irrationality; the words lunacy and lunatic (popular shortening loony) are derived from the Latin name for the Moon, Luna. Philosophers Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person. [264] Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims. [264] [265] [266] [267] [268]

## Notes

1. Between 18.29° and 28.58° to Earth's equator. [1]
2. There are a number of near-Earth asteroids, including 3753 Cruithne, that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term (Morais et al, 2002). These are quasi-satellites  – they are not moons as they do not orbit Earth. For more information, see Other moons of Earth.
3. The maximum value is given based on scaling of the brightness from the value of −12.74 given for an equator to Moon-centre distance of 378 000 km in the NASA factsheet reference to the minimum Earth–Moon distance given there, after the latter is corrected for Earth's equatorial radius of 6 378 km, giving 350 600 km. The minimum value (for a distant new moon) is based on a similar scaling using the maximum Earth–Moon distance of 407 000 km (given in the factsheet) and by calculating the brightness of the earthshine onto such a new moon. The brightness of the earthshine is [ Earth albedo × Radius of Moon's orbit)2 ] relative to the direct solar illumination that occurs for a full moon. (Earth albedo = 0.367; Earth radius = (polar radius × equatorial radius)½ = 6 367 km.)
4. The range of angular size values given are based on simple scaling of the following values given in the fact sheet reference: at an Earth-equator to Moon-centre distance of 378 000 km, the angular size is 1896  arcseconds. The same fact sheet gives extreme Earth–Moon distances of 407 000 km and 357 000 km. For the maximum angular size, the minimum distance has to be corrected for Earth's equatorial radius of 6 378 km, giving 350 600 km.
5. Lucey et al. (2006) give 107 particles cm−3 by day and 105 particles cm−3 by night. Along with equatorial surface temperatures of 390  K by day and 100 K by night, the ideal gas law yields the pressures given in the infobox (rounded to the nearest order of magnitude): 10−7  Pa by day and 10−10 Pa by night.
6. This age is calculated from isotope dating of lunar zircons.
7. More accurately, the Moon's mean sidereal period (fixed star to fixed star) is 27.321661 days (27 d 07 h 43 min 11.5 s), and its mean tropical orbital period (from equinox to equinox) is 27.321582 days (27 d 07 h 43 min 04.7 s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
8. More accurately, the Moon's mean synodic period (between mean solar conjunctions) is 29.530589 days (29 d 12 h 44 min 02.9 s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
9. There is no strong correlation between the sizes of planets and the sizes of their satellites. Larger planets tend to have more satellites, both large and small, than smaller planets.
10. With 27% the diameter and 60% the density of Earth, the Moon has 1.23% of the mass of Earth. The moon Charon is larger relative to its primary Pluto, but Pluto is now considered to be a dwarf planet.
11. The Sun's apparent magnitude is −26.7, while the full moon's apparent magnitude is −12.7.
12. See graph in Sun#Life phases. At present, the diameter of the Sun is increasing at a rate of about five percent per billion years. This is very similar to the rate at which the apparent angular diameter of the Moon is decreasing as it recedes from Earth.
13. On average, the Moon covers an area of 0.21078 square degrees on the night sky.

## Related Research Articles

Callisto is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System after Ganymede and Saturn's largest moon Titan, and the largest object in the Solar System not to be properly differentiated. Callisto was discovered in 1610 by Galileo Galilei. At 4821 km in diameter, Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1883000 km. It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated. Callisto's rotation is tidally locked to its orbit around Jupiter, so that the same hemisphere always faces inward; Jupiter appears to stand nearly still in Callisto's sky. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt.

Triton is the largest natural satellite of the planet Neptune, and the first Neptunian moon to be discovered. The discovery was made on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. At 2,710 kilometres (1,680 mi) in diameter, it is the seventh-largest moon in the Solar System, the only satellite of Neptune massive enough to be in hydrostatic equilibrium and the second-largest planetary moon in relation to its primary, after Earth's Moon. Because of its retrograde orbit and composition similar to Pluto's, Triton is thought to have been a dwarf planet captured from the Kuiper belt.

Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall.

Ganymede, a satellite of Jupiter, is the largest and most massive of the Solar System's moons. The ninth largest object in the Solar System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km (3,273 mi) and is 8% larger than the planet Mercury, although only 45% as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. Outward from Jupiter, it is the seventh satellite and the third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

Mare Imbrium is a vast lava plain within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System. The Imbrium Basin formed from the collision of a proto-planet during the Late Heavy Bombardment. Basaltic lava later flooded the giant crater to form the flat volcanic plain seen today. The basin's age has been estimated using uranium–lead dating methods to 3938 ± 4 million years ago, the diameter of the impactor has been estimated to be 250 ± 25 km. The Moon's maria have fewer features than other areas of the Moon because molten lava pooled in the craters and formed a relatively smooth surface. Mare Imbrium is not as flat as it was originally because later events have altered its surface.

In astronomy and astrophysics, a mass concentration is a region of a planet or moon's crust that contains a large positive gravitational anomaly. In general, the word "mascon" can be used as a noun to refer to an excess distribution of mass on or beneath the surface of an astronomical body, such as is found around Hawaii on Earth. However, this term is most often used to describe a geologic structure that has a positive gravitational anomaly associated with a feature that might otherwise have been expected to have a negative anomaly, such as the "mascon basins" on the Moon.

Colonization of the Moon is the proposed establishment of a permanent human community or robotic industries on the Moon.

The far side of the Moon is the hemisphere of the Moon that always faces away from Earth. The far side's terrain is rugged with a multitude of impact craters and relatively few flat lunar maria. It has one of the largest craters in the Solar System, the South Pole–Aitken basin. Both sides of the Moon experience two weeks of sunlight followed by two weeks of night; the far side is sometimes called the "dark side of the Moon", meaning unseen rather than lacking light.

Shackleton is an impact crater that lies at the south pole of the Moon. The peaks along the crater's rim are exposed to almost continual sunlight, while the interior is perpetually in shadow. The low-temperature interior of this crater functions as a cold trap that may capture and freeze volatiles shed during comet impacts on the Moon. Measurements by the Lunar Prospector spacecraft showed higher than normal amounts of hydrogen within the crater, which may indicate the presence of water ice. The crater is named after Antarctic explorer Ernest Shackleton.

Chandrayaan-1 was India's first lunar probe. It was launched by the Indian Space Research Organisation in October 2008, and operated until August 2009. The mission included a lunar orbiter and an impactor. India launched the spacecraft using a PSLV-XL rocket, serial number C11, on 22 October 2008 at 00:52 UTC from Satish Dhawan Space Centre, at Sriharikota about 80 km (50 mi) north of Chennai. Prime Minister Atal Bihari Vajpayee announced the project on course in his Independence Day speech on 15 August 2003. The mission was a major boost to India's space program, as India researched and developed its own technology in order to explore the Moon. The vehicle was inserted into lunar orbit on 8 November 2008.

A Moon landing is the arrival of a spacecraft on the surface of the Moon. This includes both manned and unmanned (robotic) missions. The first human-made object to reach the surface of the Moon was the Soviet Union's Luna 2 mission, on 13 September 1959.

Lunar water is water that is present on the Moon. Liquid water cannot persist at the Moon's surface, and water vapor is decomposed by sunlight, with hydrogen quickly lost to outer space. However, scientists have conjectured since the 1960s that water ice could survive in cold, permanently shadowed craters at the Moon's poles. Water molecules are also detected in the thin layer of gases above the lunar surface.

Lunar craters are impact craters on Earth's Moon. The Moon's surface has many craters, almost all of which were formed by impacts.

The geology of the Moon is quite different from that of Earth. The Moon lacks a significant atmosphere, which eliminates erosion due to weather; it does not have any form of plate tectonics, it has a lower gravity, and because of its small size, it cooled more rapidly. The complex geomorphology of the lunar surface has been formed by a combination of processes, especially impact cratering and volcanism. The Moon is a differentiated body, with a crust, mantle, and core.

A sample-return mission is a spacecraft mission with the goal of collecting and returning samples from an extraterrestrial location to Earth for analysis. Sample-return missions may bring back merely atoms and molecules or a deposit of complex compounds such as loose material ("soil") and rocks. These samples may be obtained in a number of ways, such as soil and rock excavation or a collector array used for capturing particles of solar wind or cometary debris.

The physical exploration of the Moon began when Luna 2, a space probe launched by the Soviet Union, made an impact on the surface of the Moon on September 14, 1959. Prior to that the only available means of exploration had been observation from Earth. The invention of the optical telescope brought about the first leap in the quality of lunar observations. Galileo Galilei is generally credited as the first person to use a telescope for astronomical purposes; having made his own telescope in 1609, the mountains and craters on the lunar surface were among his first observations using it.

The Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. Data collected by LRO has been described as essential for planning NASA's future human and robotic missions to the Moon. Its detailed mapping program is identifying safe landing sites, locating potential resources on the Moon, characterizing the radiation environment, and demonstrating new technologies.

The lunar south pole is of special interest to scientists because of the occurrence of water ice in permanently shadowed areas around it. Of the lunar poles, the south pole is of greater interest because the area that remains in shadow is much larger than that at the north pole. The lunar south pole craters are unique in that sunlight does not reach the bottom. Such craters are cold traps that contain a fossil record of the early Solar System.

Peter H. Schultz is Professor of Geological Sciences at Brown University specializing in the study of planetary geology, impact cratering on the Earth and other objects in the Solar System, and volcanic modifications of planetary surfaces. He was co-investigator to the NASA Science Mission Directorate spacecraft Deep Impact and the Lunar Crater Observation and Sensing Satellite (LCROSS). He was awarded the Barringer Medal of the Meteoritical Society in 2004 for his theoretical and experimental studies of impact craters.

Lunar swirls are enigmatic features found across the Moon's surface, which are characterized by having a high albedo, appearing optically immature, and (often) having a sinuous shape. Their curvilinear shape is often accentuated by low albedo regions that wind between the bright swirls. They appear to overlay the lunar surface, superposed on top of craters and ejecta deposits, but impart no observable topography. Swirls have been identified on the lunar maria and highlands - they are not associated with a specific lithologic composition. Swirls on the maria are characterized by strong albedo contrasts and complex, sinuous morphology, whereas those on highland terrain appear less prominent and exhibit simpler shapes, such as single loops or diffuse bright spots.

## References

### Citations

1. Wieczorek, Mark A.; et al. (2006). "The constitution and structure of the lunar interior". Reviews in Mineralogy and Geochemistry . 60 (1): 221–364. Bibcode:2006RvMG...60..221W. doi:10.2138/rmg.2006.60.3.
2. Lang, Kenneth R. (2011), The Cambridge Guide to the Solar System Archived 1 January 2016 at the Wayback Machine , 2nd ed., Cambridge University Press.
3. Morais, M.H.M.; Morbidelli, A. (2002). "The Population of Near-Earth Asteroids in Coorbital Motion with the Earth". Icarus . 160 (1): 1–9. Bibcode:2002Icar..160....1M. doi:10.1006/icar.2002.6937.
4. Williams, Dr. David R. (2 February 2006). "Moon Fact Sheet". NASA/National Space Science Data Center. Archived from the original on 23 March 2010. Retrieved 31 December 2008.
5. Smith, David E.; Zuber, Maria T.; Neumann, Gregory A.; Lemoine, Frank G. (1 January 1997). "Topography of the Moon from the Clementine lidar". Journal of Geophysical Research . 102 (E1): 1601. Bibcode:1997JGR...102.1591S. doi:10.1029/96JE02940.
6. Terry 2013, p. 226.
7. Williams, James G.; Newhall, XX; Dickey, Jean O. (1996). "Lunar moments, tides, orientation, and coordinate frames". Planetary and Space Science . 44 (10): 1077–1080. Bibcode:1996P&SS...44.1077W. doi:10.1016/0032-0633(95)00154-9.
8. Makemson, Maud W. (1971); "Determination of Selenographic Positions", in The Moon, volume 2, issue 3, pp. 293-308, doi:10.1007/BF00561882, BibCode: 1971Moon....2..293M
9. Archinal, Brent A.; A'Hearn, Michael F.; Bowell, Edward G.; Conrad, Albert R.; Consolmagno, Guy J.; et al. (2010). "Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009" (PDF). Celestial Mechanics and Dynamical Astronomy. 109 (2): 101–135. Bibcode:2011CeMDA.109..101A. doi:10.1007/s10569-010-9320-4. Archived from the original (PDF) on 4 March 2016. Retrieved 24 September 2018.
10. Matthews, Grant (2008). "Celestial body irradiance determination from an underfilled satellite radiometer: application to albedo and thermal emission measurements of the Moon using CERES". Applied Optics . 47 (27): 4981–4993. Bibcode:2008ApOpt..47.4981M. doi:10.1364/AO.47.004981. PMID   18806861.
11. A.R. Vasavada; D.A. Paige & S.E. Wood (1999). "Near-Surface Temperatures on Mercury and the Moon and the Stability of Polar Ice Deposits". Icarus . 141 (2): 179–193. Bibcode:1999Icar..141..179V. doi:10.1006/icar.1999.6175.
12. Lucey, Paul; Korotev, Randy L.; et al. (2006). "Understanding the lunar surface and space-Moon interactions". Reviews in Mineralogy and Geochemistry . 60 (1): 83–219. Bibcode:2006RvMG...60...83L. doi:10.2138/rmg.2006.60.2.
13. "How far away is the moon?". Space Place. NASA. Archived from the original on 6 October 2016.
14. Scott, Elaine (2016). Our Moon: New Discoveries About Earth's Closest Companion. Houghton Mifflin Harcourt. p. 7. ISBN   978-0-544-75058-6.
15. Meaning of “moon” in the English Dictionary Cambridge Learner's Dictionary
16. "Naming Astronomical Objects: Spelling of Names". International Astronomical Union. Archived from the original on 16 December 2008. Retrieved 29 March 2010.
17. "Gazetteer of Planetary Nomenclature: Planetary Nomenclature FAQ". USGS Astrogeology Research Program. Archived from the original on 27 May 2010. Retrieved 29 March 2010.
18. Barnhart, Robert K. (1995). The Barnhart Concise Dictionary of Etymology. Harper Collins. p. 487. ISBN   978-0-06-270084-1.
19. Oxford English Dictionary , 2nd ed. "luna", Oxford University Press (Oxford), 2009.
20. "Oxford English Dictionary: lunar, a. and n." Oxford English Dictionary: Second Edition 1989. Oxford University Press . Retrieved 23 March 2010.
21. Imke Pannen (2010). When the Bad Bleeds: Mantic Elements in English Renaissance Revenge Tragedy. V&R unipress GmbH. pp. 96–. ISBN   978-3-89971-640-5. Archived from the original on 4 September 2016.
22. Barboni, M.; Boehnke, P.; Keller, C.B.; Kohl, I.E.; Schoene, B.; Young, E.D.; McKeegan, K.D. (2017). "Early formation of the Moon 4.51 billion years ago". Science Advances . 3 (1): e1602365. Bibcode:2017SciA....3E2365B. doi:10.1126/sciadv.1602365. PMC  . PMID   28097222.
23. Binder, A.B. (1974). "On the origin of the Moon by rotational fission". The Moon . 11 (2): 53–76. Bibcode:1974Moon...11...53B. doi:10.1007/BF01877794.
24. Stroud, Rick (2009). The Book of the Moon. Walken and Company. pp. 24–27. ISBN   978-0-8027-1734-4.
25. Mitler, H.E. (1975). "Formation of an iron-poor moon by partial capture, or: Yet another exotic theory of lunar origin". Icarus . 24 (2): 256–268. Bibcode:1975Icar...24..256M. doi:10.1016/0019-1035(75)90102-5.
26. Stevenson, D.J. (1987). "Origin of the moon–The collision hypothesis". Annual Review of Earth and Planetary Sciences . 15 (1): 271–315. Bibcode:1987AREPS..15..271S. doi:10.1146/annurev.ea.15.050187.001415.
27. Taylor, G. Jeffrey (31 December 1998). "Origin of the Earth and Moon". Planetary Science Research Discoveries. Hawai'i Institute of Geophysics and Planetology. Archived from the original on 10 June 2010. Retrieved 7 April 2010.
28. "Asteroids Bear Scars of Moon's Violent Formation". 16 April 2015. Archived from the original on 8 October 2016.
29. Dana Mackenzie (21 July 2003). The Big Splat, or How Our Moon Came to Be. John Wiley & Sons. pp. 166–168. ISBN   978-0-471-48073-0. Archived from the original on 1 January 2016.
30. Canup, R.; Asphaug, E. (2001). "Origin of the Moon in a giant impact near the end of Earth's formation". Nature . 412 (6848): 708–712. Bibcode:2001Natur.412..708C. doi:10.1038/35089010. PMID   11507633.
31. "Earth-Asteroid Collision Formed Moon Later Than Thought". National Geographic. 28 October 2010. Archived from the original on 18 April 2009. Retrieved 7 May 2012.
32. Kleine, Thorsten (2008). "2008 Pellas-Ryder Award for Mathieu Touboul" (PDF). Meteoritics and Planetary Science. 43 (S7): A11. Bibcode:2008M&PS...43...11K. doi:10.1111/j.1945-5100.2008.tb00709.x.
33. Touboul, M.; Kleine, T.; Bourdon, B.; Palme, H.; Wieler, R. (2007). "Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals". Nature . 450 (7173): 1206–1209. Bibcode:2007Natur.450.1206T. doi:10.1038/nature06428. PMID   18097403.
34. "Flying Oceans of Magma Help Demystify the Moon's Creation". National Geographic. 8 April 2015. Archived from the original on 9 April 2015.
35. Pahlevan, Kaveh; Stevenson, David J. (2007). "Equilibration in the aftermath of the lunar-forming giant impact". Earth and Planetary Science Letters . 262 (3–4): 438–449. arXiv:. Bibcode:2007E&PSL.262..438P. doi:10.1016/j.epsl.2007.07.055.
36. Nield, Ted (2009). "Moonwalk (summary of meeting at Meteoritical Society's 72nd Annual Meeting, Nancy, France)". Geoscientist . Vol. 19. p. 8. Archived from the original on 27 September 2012.
37. Warren, P.H. (1985). "The magma ocean concept and lunar evolution". Annual Review of Earth and Planetary Sciences . 13 (1): 201–240. Bibcode:1985AREPS..13..201W. doi:10.1146/annurev.ea.13.050185.001221.
38. Tonks, W. Brian; Melosh, H. Jay (1993). "Magma ocean formation due to giant impacts". Journal of Geophysical Research . 98 (E3): 5319–5333. Bibcode:1993JGR....98.5319T. doi:10.1029/92JE02726.
39. Daniel Clery (11 October 2013). "Impact Theory Gets Whacked". Science . 342 (6155): 183–185. Bibcode:2013Sci...342..183C. doi:10.1126/science.342.6155.183. PMID   24115419.
40. Wiechert, U.; et al. (October 2001). "Oxygen Isotopes and the Moon-Forming Giant Impact". Science . 294 (12): 345–348. Bibcode:2001Sci...294..345W. doi:10.1126/science.1063037. PMID   11598294. Archived from the original on 20 April 2009. Retrieved 5 July 2009.
41. Pahlevan, Kaveh; Stevenson, David (October 2007). "Equilibration in the Aftermath of the Lunar-forming Giant Impact". Earth and Planetary Science Letters . 262 (3–4): 438–449. arXiv:. Bibcode:2007E&PSL.262..438P. doi:10.1016/j.epsl.2007.07.055.
42. "Titanium Paternity Test Says Earth is the Moon's Only Parent (University of Chicago)". Astrobio.net. 5 April 2012. Retrieved 3 October 2013.
43. Taylor, Stuart R. (1975). Lunar Science: a Post-Apollo View. Oxford: Pergamon Press. p. 64. ISBN   978-0-08-018274-2.
44. Brown, D.; Anderson, J. (6 January 2011). "NASA Research Team Reveals Moon Has Earth-Like Core". NASA. NASA. Archived from the original on 15 March 2012.
45. Weber, R.C.; Lin, P.-Y.; Garnero, E.J.; Williams, Q.; Lognonne, P. (21 January 2011). "Seismic Detection of the Lunar Core" (PDF). Science. 331 (6015): 309–312. Bibcode:2011Sci...331..309W. doi:10.1126/science.1199375. PMID   21212323. Archived from the original (PDF) on 15 October 2015. Retrieved 10 April 2017.
46. Nemchin, A.; Timms, N.; Pidgeon, R.; Geisler, T.; Reddy, S.; Meyer, C. (2009). "Timing of crystallization of the lunar magma ocean constrained by the oldest zircon". Nature Geoscience . 2 (2): 133–136. Bibcode:2009NatGe...2..133N. doi:10.1038/ngeo417. hdl:20.500.11937/44375.
47. Shearer, Charles K.; et al. (2006). "Thermal and magmatic evolution of the Moon". Reviews in Mineralogy and Geochemistry . 60 (1): 365–518. Bibcode:2006RvMG...60..365S. doi:10.2138/rmg.2006.60.4.
48. Schubert, J. (2004). "Interior composition, structure, and dynamics of the Galilean satellites.". In F. Bagenal; et al. (eds.). Jupiter: The Planet, Satellites, and Magnetosphere. Cambridge University Press. pp. 281–306. ISBN   978-0-521-81808-7.
49. Williams, J.G.; Turyshev, S.G.; Boggs, D.H.; Ratcliff, J.T. (2006). "Lunar laser ranging science: Gravitational physics and lunar interior and geodesy". Advances in Space Research . 37 (1): 67–71. arXiv:. Bibcode:2006AdSpR..37...67W. doi:10.1016/j.asr.2005.05.013.
50. Spudis, Paul D.; Cook, A.; Robinson, M.; Bussey, B.; Fessler, B. (January 1998). "Topography of the South Polar Region from Clementine Stereo Imaging". Workshop on New Views of the Moon: Integrated Remotely Sensed, Geophysical, and Sample Datasets: 69. Bibcode:1998nvmi.conf...69S.
51. Spudis, Paul D.; Reisse, Robert A.; Gillis, Jeffrey J. (1994). "Ancient Multiring Basins on the Moon Revealed by Clementine Laser Altimetry". Science . 266 (5192): 1848–1851. Bibcode:1994Sci...266.1848S. doi:10.1126/science.266.5192.1848. PMID   17737079.
52. Pieters, C.M.; Tompkins, S.; Head, J.W.; Hess, P.C. (1997). "Mineralogy of the Mafic Anomaly in the South Pole‐Aitken Basin: Implications for excavation of the lunar mantle". Geophysical Research Letters . 24 (15): 1903–1906. Bibcode:1997GeoRL..24.1903P. doi:10.1029/97GL01718. hdl:2060/19980018038.
53. Taylor, G.J. (17 July 1998). "The Biggest Hole in the Solar System". Planetary Science Research Discoveries: 20. Bibcode:1998psrd.reptE..20T. Archived from the original on 20 August 2007. Retrieved 12 April 2007.
54. Schultz, P.H. (March 1997). "Forming the south-pole Aitken basin – The extreme games". Conference Paper, 28th Annual Lunar and Planetary Science Conference. 28: 1259. Bibcode:1997LPI....28.1259S.
55. "NASA's LRO Reveals 'Incredible Shrinking Moon'". NASA. 19 August 2010. Archived from the original on 21 August 2010.
56. Wlasuk, Peter (2000). Observing the Moon. Springer. p. 19. ISBN   978-1-85233-193-1.
57. Norman, M. (21 April 2004). "The Oldest Moon Rocks". Planetary Science Research Discoveries. Hawai'i Institute of Geophysics and Planetology. Archived from the original on 18 April 2007. Retrieved 12 April 2007.
58. Head, L.W.J.W. (2003). "Lunar Gruithuisen and Mairan domes: Rheology and mode of emplacement". Journal of Geophysical Research . 108 (E2): 5012. Bibcode:2003JGRE..108.5012W. doi:10.1029/2002JE001909. Archived from the original on 12 March 2007. Retrieved 12 April 2007.
59. Spudis, P.D. (2004). "Moon". World Book Online Reference Center, NASA. Archived from the original on 3 July 2013. Retrieved 12 April 2007.
60. Gillis, J.J.; Spudis, P.D. (1996). "The Composition and Geologic Setting of Lunar Far Side Maria". Lunar and Planetary Science . 27: 413. Bibcode:1996LPI....27..413G.
61. Lawrence, D.J., et al. (11 August 1998). "Global Elemental Maps of the Moon: The Lunar Prospector Gamma-Ray Spectrometer". Science . 281 (5382): 1484–1489. Bibcode:1998Sci...281.1484L. doi:10.1126/science.281.5382.1484. PMID   9727970. Archived from the original on 16 May 2009. Retrieved 29 August 2009.
62. Taylor, G.J. (31 August 2000). "A New Moon for the Twenty-First Century". Planetary Science Research Discoveries: 41. Bibcode:2000psrd.reptE..41T. Archived from the original on 15 March 2012. Retrieved 12 April 2007.
63. Papike, J.; Ryder, G.; Shearer, C. (1998). "Lunar Samples". Reviews in Mineralogy and Geochemistry . 36: 5.1–5.234.
64. Hiesinger, H.; Head, J.W.; Wolf, U.; Jaumanm, R.; Neukum, G. (2003). "Ages and stratigraphy of mare basalts in Oceanus Procellarum, Mare Numbium, Mare Cognitum, and Mare Insularum". Journal of Geophysical Research . 108 (E7): 1029. Bibcode:2003JGRE..108.5065H. doi:10.1029/2002JE001985.
65. Phil Berardelli (9 November 2006). "Long Live the Moon!". Science. Archived from the original on 18 October 2014.
66. Jason Major (14 October 2014). "Volcanoes Erupted 'Recently' on the Moon". Discovery News. Archived from the original on 16 October 2014.
67. "NASA Mission Finds Widespread Evidence of Young Lunar Volcanism". NASA. 12 October 2014. Archived from the original on 3 January 2015.
68. Eric Hand (12 October 2014). "Recent volcanic eruptions on the moon". Science. Archived from the original on 14 October 2014.
69. Braden, S.E.; Stopar, J.D.; Robinson, M.S.; Lawrence, S.J.; van der Bogert, C.H.; Hiesinger, H.doi=10.1038/ngeo2252 (2014). "Evidence for basaltic volcanism on the Moon within the past 100 million years". Nature Geoscience . 7 (11): 787–791. Bibcode:2014NatGe...7..787B. doi:10.1038/ngeo2252.
70. Srivastava, N.; Gupta, R.P. (2013). "Young viscous flows in the Lowell crater of Orientale basin, Moon: Impact melts or volcanic eruptions?". Planetary and Space Science . 87: 37–45. Bibcode:2013P&SS...87...37S. doi:10.1016/j.pss.2013.09.001.
71. Gupta, R.P.; Srivastava, N.; Tiwari, R.K. (2014). "Evidences of relatively new volcanic flows on the Moon". Current Science . 107 (3): 454–460.
72. Whitten, J.; et al. (2011). "Lunar mare deposits associated with the Orientale impact basin: New insights into mineralogy, history, mode of emplacement, and relation to Orientale Basin evolution from Moon Mineralogy Mapper (M3) data from Chandrayaan-1". Journal of Geophysical Research . 116: E00G09. Bibcode:2011JGRE..116.0G09W. doi:10.1029/2010JE003736.
73. Cho, Y.; et al. (2012). "Young mare volcanism in the Orientale region contemporary with the Procellarum KREEP Terrane (PKT) volcanism peak period 2 b.y. ago". Geophysical Research Letters . 39 (11): L11203. Bibcode:2012GeoRL..3911203C. doi:10.1029/2012GL051838.
74. Munsell, K. (4 December 2006). "Majestic Mountains". Solar System Exploration. NASA. Archived from the original on 17 September 2008. Retrieved 12 April 2007.
75. Richard Lovett (2011). "Early Earth may have had two moons : Nature News". Nature. doi:10.1038/news.2011.456. Archived from the original on 3 November 2012. Retrieved 1 November 2012.
76. "Was our two-faced moon in a small collision?". Theconversation.edu.au. Archived from the original on 30 January 2013. Retrieved 1 November 2012.
77. Melosh, H. J. (1989). Impact cratering: A geologic process. Oxford University Press. ISBN   978-0-19-504284-9.
78. "Moon Facts". SMART-1. European Space Agency. 2010. Retrieved 12 May 2010.
79. Wilhelms, Don (1987). "Relative Ages" (PDF). Geologic History of the Moon. U.S. Geological Survey. Archived (PDF) from the original on 11 June 2010.
80. Hartmann, William K.; Quantin, Cathy; Mangold, Nicolas (2007). "Possible long-term decline in impact rates: 2. Lunar impact-melt data regarding impact history". Icarus . 186 (1): 11–23. Bibcode:2007Icar..186...11H. doi:10.1016/j.icarus.2006.09.009.
81. "The Smell of Moondust". NASA. 30 January 2006. Archived from the original on 8 March 2010. Retrieved 15 March 2010.
82. Heiken, G. (1991). Vaniman, D.; French, B. (eds.). Lunar Sourcebook, a user's guide to the Moon. New York: Cambridge University Press. p. 736. ISBN   978-0-521-33444-0.
83. Rasmussen, K.L.; Warren, P.H. (1985). "Megaregolith thickness, heat flow, and the bulk composition of the Moon". Nature . 313 (5998): 121–124. Bibcode:1985Natur.313..121R. doi:10.1038/313121a0.
84. Boyle, Rebecca. "The moon has hundreds more craters than we thought". Archived from the original on 13 October 2016.
85. Speyerer, Emerson J.; Povilaitis, Reinhold Z.; Robinson, Mark S.; Thomas, Peter C.; Wagner, Robert V. (13 October 2016). "Quantifying crater production and regolith overturn on the Moon with temporal imaging". Nature . 538 (7624): 215–218. Bibcode:2016Natur.538..215S. doi:10.1038/nature19829. PMID   27734864 via www.nature.com.
86. Margot, J.L.; Campbell, D.B.; Jurgens, R.F.; Slade, M.A. (4 June 1999). "Topography of the Lunar Poles from Radar Interferometry: A Survey of Cold Trap Locations" (PDF). Science . 284 (5420): 1658–1660. Bibcode:1999Sci...284.1658M. CiteSeerX  . doi:10.1126/science.284.5420.1658. PMID   10356393.
87. Ward, William R. (1 August 1975). "Past Orientation of the Lunar Spin Axis". Science . 189 (4200): 377–379. Bibcode:1975Sci...189..377W. doi:10.1126/science.189.4200.377. PMID   17840827.
88. Martel, L.M.V. (4 June 2003). "The Moon's Dark, Icy Poles". Planetary Science Research Discoveries: 73. Bibcode:2003psrd.reptE..73M. Archived from the original on 15 March 2012. Retrieved 12 April 2007.
89. Seedhouse, Erik (2009). Lunar Outpost: The Challenges of Establishing a Human Settlement on the Moon. Springer-Praxis Books in Space Exploration. Germany: Springer Praxis. p. 136. ISBN   978-0-387-09746-6.
90. Coulter, Dauna (18 March 2010). "The Multiplying Mystery of Moonwater". NASA. Archived from the original on 16 May 2016. Retrieved 28 March 2010.
91. Spudis, P. (6 November 2006). "Ice on the Moon". The Space Review. Archived from the original on 22 February 2007. Retrieved 12 April 2007.
92. Feldman, W.C.; S. Maurice; A.B. Binder; B.L. Barraclough; R.C. Elphic; D.J. Lawrence (1998). "Fluxes of Fast and Epithermal Neutrons from Lunar Prospector: Evidence for Water Ice at the Lunar Poles". Science . 281 (5382): 1496–1500. Bibcode:1998Sci...281.1496F. doi:10.1126/science.281.5382.1496. PMID   9727973.
93. Saal, Alberto E.; Hauri, Erik H.; Cascio, Mauro L.; van Orman, James A.; Rutherford, Malcolm C.; Cooper, Reid F. (2008). "Volatile content of lunar volcanic glasses and the presence of water in the Moon's interior". Nature . 454 (7201): 192–195. Bibcode:2008Natur.454..192S. doi:10.1038/nature07047. PMID   18615079.
94. Pieters, C.M.; Goswami, J.N.; Clark, R.N.; Annadurai, M.; Boardman, J.; Buratti, B.; Combe, J.-P.; Dyar, M.D.; Green, R.; Head, J.W.; Hibbitts, C.; Hicks, M.; Isaacson, P.; Klima, R.; Kramer, G.; Kumar, S.; Livo, E.; Lundeen, S.; Malaret, E.; McCord, T.; Mustard, J.; Nettles, J.; Petro, N.; Runyon, C.; Staid, M.; Sunshine, J.; Taylor, L.A.; Tompkins, S.; Varanasi, P. (2009). "Character and Spatial Distribution of OH/H2O on the Surface of the Moon Seen by M3 on Chandrayaan-1". Science . 326 (5952): 568–572. Bibcode:2009Sci...326..568P. doi:10.1126/science.1178658. PMID   19779151.
95. Li, Shuai; Lucey, Paul G.; Milliken, Ralph E.; Hayne, Paul O.; Fisher, Elizabeth; Williams, Jean-Pierre; Hurley, Dana M.; Elphic, Richard C. (August 2018). "Direct evidence of surface exposed water ice in the lunar polar regions". Proceedings of the National Academy of Sciences. 115 (36): 8907–8912. doi:10.1073/pnas.1802345115. PMC  . PMID   30126996.
96. Lakdawalla, Emily (13 November 2009). "LCROSS Lunar Impactor Mission: "Yes, We Found Water!"". The Planetary Society. Archived from the original on 22 January 2010. Retrieved 13 April 2010.
97. Colaprete, A.; Ennico, K.; Wooden, D.; Shirley, M.; Heldmann, J.; Marshall, W.; Sollitt, L.; Asphaug, E.; Korycansky, D.; Schultz, P.; Hermalyn, B.; Galal, K.; Bart, G.D.; Goldstein, D.; Summy, D. (1–5 March 2010). "Water and More: An Overview of LCROSS Impact Results". 41st Lunar and Planetary Science Conference. 41 (1533): 2335. Bibcode:2010LPI....41.2335C.
98. Colaprete, Anthony; Schultz, Peter; Heldmann, Jennifer; Wooden, Diane; Shirley, Mark; Ennico, Kimberly; Hermalyn, Brendan; Marshall, William; Ricco, Antonio; Elphic, Richard C.; Goldstein, David; Summy, Dustin; Bart, Gwendolyn D.; Asphaug, Erik; Korycansky, Don; Landis, David; Sollitt, Luke (22 October 2010). "Detection of Water in the LCROSS Ejecta Plume". Science . 330 (6003): 463–468. Bibcode:2010Sci...330..463C. doi:10.1126/science.1186986. PMID   20966242.
99. Hauri, Erik; Thomas Weinreich; Albert E. Saal; Malcolm C. Rutherford; James A. Van Orman (26 May 2011). "High Pre-Eruptive Water Contents Preserved in Lunar Melt Inclusions". Science Express . 10 (1126): 213–215. Bibcode:2011Sci...333..213H. doi:10.1126/science.1204626. PMID   21617039.
100. Rincon, Paul (21 August 2018). "Water ice 'detected on Moon's surface'". BBC News. Retrieved 21 August 2018.
101. David, Leonard. "Beyond the Shadow of a Doubt, Water Ice Exists on the Moon". Scientific American. Retrieved 21 August 2018.
102. "Water Ice Confirmed on the Surface of the Moon for the 1st Time!". Space.com. Retrieved 21 August 2018.
103. Muller, P.; Sjogren, W. (1968). "Mascons: lunar mass concentrations". Science . 161 (3842): 680–684. Bibcode:1968Sci...161..680M. doi:10.1126/science.161.3842.680. PMID   17801458.
104. Richard A. Kerr (12 April 2013). "The Mystery of Our Moon's Gravitational Bumps Solved?". Science . 340 (6129): 138–139. doi:10.1126/science.340.6129.138-a. PMID   23580504.
105. Konopliv, A.; Asmar, S.; Carranza, E.; Sjogren, W.; Yuan, D. (2001). "Recent gravity models as a result of the Lunar Prospector mission" (PDF). Icarus . 50 (1): 1–18. Bibcode:2001Icar..150....1K. CiteSeerX  . doi:10.1006/icar.2000.6573. Archived from the original (PDF) on 13 November 2004.
106. Garrick-Bethell, Ian; Weiss, iBenjamin P.; Shuster, David L.; Buz, Jennifer (2009). "Early Lunar Magnetism". Science . 323 (5912): 356–359. Bibcode:2009Sci...323..356G. doi:10.1126/science.1166804. PMID   19150839.
107. "Magnetometer / Electron Reflectometer Results". Lunar Prospector (NASA). 2001. Archived from the original on 27 May 2010. Retrieved 17 March 2010.
108. Hood, L.L.; Huang, Z. (1991). "Formation of magnetic anomalies antipodal to lunar impact basins: Two-dimensional model calculations". Journal of Geophysical Research . 96 (B6): 9837–9846. Bibcode:1991JGR....96.9837H. doi:10.1029/91JB00308.
109. "Moon Storms". NASA. 27 September 2013. Archived from the original on 12 September 2013. Retrieved 3 October 2013.
110. Culler, Jessica (16 June 2015). "LADEE - Lunar Atmosphere Dust and Environment Explorer". Archived from the original on 8 April 2015.
111. Globus, Ruth (1977). "Chapter 5, Appendix J: Impact Upon Lunar Atmosphere". In Richard D. Johnson & Charles Holbrow (ed.). Space Settlements: A Design Study. NASA. Archived from the original on 31 May 2010. Retrieved 17 March 2010.
112. Crotts, Arlin P.S. (2008). "Lunar Outgassing, Transient Phenomena and The Return to The Moon, I: Existing Data" (PDF). The Astrophysical Journal . 687 (1): 692–705. arXiv:. Bibcode:2008ApJ...687..692C. doi:10.1086/591634. Archived (PDF) from the original on 20 February 2009.
113. Steigerwald, William (17 August 2015). "NASA's LADEE Spacecraft Finds Neon in Lunar Atmosphere". NASA. Retrieved 18 August 2015.
114. Stern, S.A. (1999). "The Lunar atmosphere: History, status, current problems, and context". Reviews of Geophysics . 37 (4): 453–491. Bibcode:1999RvGeo..37..453S. CiteSeerX  . doi:10.1029/1999RG900005.
115. Lawson, S.; Feldman, W.; Lawrence, D.; Moore, K.; Elphic, R.; Belian, R. (2005). "Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer". Journal of Geophysical Research . 110 (E9): 1029. Bibcode:2005JGRE..11009009L. doi:10.1029/2005JE002433.
116. R. Sridharan; S.M. Ahmed; Tirtha Pratim Dasa; P. Sreelathaa; P. Pradeepkumara; Neha Naika; Gogulapati Supriya (2010). "'Direct' evidence for water (H2O) in the sunlit lunar ambience from CHACE on MIP of Chandrayaan I". Planetary and Space Science . 58 (6): 947–950. Bibcode:2010P&SS...58..947S. doi:10.1016/j.pss.2010.02.013.
117. Drake, Nadia; 17, National Geographic PUBLISHED June (17 June 2015). "Lopsided Cloud of Dust Discovered Around the Moon". National Geographic News. Archived from the original on 19 June 2015. Retrieved 20 June 2015.
118. Horányi, M.; Szalay, J.R.; Kempf, S.; Schmidt, J.; Grün, E.; Srama, R.; Sternovsky, Z. (18 June 2015). "A permanent, asymmetric dust cloud around the Moon". Nature . 522 (7556): 324–326. Bibcode:2015Natur.522..324H. doi:10.1038/nature14479. PMID   26085272.
119. Hamilton, Calvin J.; Hamilton, Rosanna L., The Moon, Views of the Solar System Archived 4 February 2016 at the Wayback Machine , 1995–2011.
120. Amos, Jonathan (16 December 2009). "'Coldest place' found on the Moon". BBC News. Retrieved 20 March 2010.
121. "Diviner News". UCLA. 17 September 2009. Archived from the original on 7 March 2010. Retrieved 17 March 2010.
122. Rocheleau, Jake (21 May 2012). "Temperature on the Moon – Surface Temperature of the Moon – PlanetFacts.org". Archived from the original on 27 May 2015.
123. Haigh, I. D.; Eliot, M.; Pattiaratchi, C. (2011). "Global influences of the 18.61 year nodal cycle and 8.85 year cycle of lunar perigee on high tidal levels". J. Geophys. Res. 116 (C6): C06025. Bibcode:2011JGRC..116.6025H. doi:10.1029/2010JC006645.CS1 maint: Uses authors parameter (link)
124. V V Belet︠s︡kiĭ (2001). Essays on the Motion of Celestial Bodies. Birkhäuser. p. 183. ISBN   978-3-7643-5866-2.
125. "Space Topics: Pluto and Charon". The Planetary Society. Archived from the original on 15 March 2012. Retrieved 6 April 2010.
126. Phil Plait. "Dark Side of the Moon". Bad Astronomy: Misconceptions. Archived from the original on 12 April 2010. Retrieved 15 February 2010.
127. Alexander, M.E. (1973). "The Weak Friction Approximation and Tidal Evolution in Close Binary Systems". Astrophysics and Space Science . 23 (2): 459–508. Bibcode:1973Ap&SS..23..459A. doi:10.1007/BF00645172.
128. "Moon used to spin 'on different axis'". BBC News. BBC. 23 March 2016. Archived from the original on 23 March 2016. Retrieved 23 March 2016.
129. Luciuk, Mike. "How Bright is the Moon?". Amateur Astronomers. Archived from the original on 12 March 2010. Retrieved 16 March 2010.
130. Hershenson, Maurice (1989). The Moon illusion. Routledge. p. 5. ISBN   978-0-8058-0121-7.
131. Spekkens, K. (18 October 2002). "Is the Moon seen as a crescent (and not a "boat") all over the world?". Curious About Astronomy. Archived from the original on 16 October 2015. Retrieved 28 September 2015.
132. "Moonlight helps plankton escape predators during Arctic winters". New Scientist . 16 January 2016. Archived from the original on 30 January 2016.
133. ""Super Moon" exceptional. Brightest moon in the sky of Normandy, Monday, November 14 - The Siver Times". 12 November 2016. Archived from the original on 14 November 2016.
134. "Moongazers Delight – Biggest Supermoon in Decades Looms Large Sunday Night". 10 November 2016. Archived from the original on 14 November 2016.
135. "Supermoon November 2016". Space.com. 13 November 2016. Archived from the original on 14 November 2016. Retrieved 14 November 2016.
136. Tony Phillips (16 March 2011). "Super Full Moon". NASA. Archived from the original on 7 May 2012. Retrieved 19 March 2011.
137. Richard K. De Atley (18 March 2011). "Full moon tonight is as close as it gets". The Press-Enterprise. Archived from the original on 22 March 2011. Retrieved 19 March 2011.
138. "'Super moon' to reach closest point for almost 20 years". The Guardian. 19 March 2011. Archived from the original on 25 December 2013. Retrieved 19 March 2011.
139. Georgia State University, Dept. of Physics (Astronomy). "Perceived Brightness". Brightnes and Night/Day Sensitivity. Georgia State University. Archived from the original on 21 February 2014. Retrieved 25 January 2014.
140. Lutron. "Measured light vs. perceived light" (PDF). From IES Lighting Handbook 2000, 27-4. Lutron. Archived (PDF) from the original on 5 February 2013. Retrieved 25 January 2014.
141. Walker, John (May 1997). "Inconstant Moon". Earth and Moon Viewer. Fourth paragraph of "How Bright the Moonlight": Fourmilab. Archived from the original on 14 December 2013. Retrieved 23 January 2014. 14% [...] due to the logarithmic response of the human eye.
142. Taylor, G.J. (8 November 2006). "Recent Gas Escape from the Moon". Planetary Science Research Discoveries: 110. Bibcode:2006psrd.reptE.110T. Archived from the original on 4 March 2007. Retrieved 4 April 2007.
143. Schultz, P.H.; Staid, M.I.; Pieters, C.M. (2006). "Lunar activity from recent gas release". Nature . 444 (7116): 184–186. Bibcode:2006Natur.444..184S. doi:10.1038/nature05303. PMID   17093445.
144. "22 Degree Halo: a ring of light 22 degrees from the sun or moon". Department of Atmospheric Sciences, University of Illinois at Urbana–Champaign . Retrieved 13 April 2010.
145. Lambeck, K. (1977). "Tidal Dissipation in the Oceans: Astronomical, Geophysical and Oceanographic Consequences". Philosophical Transactions of the Royal Society A . 287 (1347): 545–594. Bibcode:1977RSPTA.287..545L. doi:10.1098/rsta.1977.0159.
146. Le Provost, C.; Bennett, A.F.; Cartwright, D.E. (1995). "Ocean Tides for and from TOPEX/POSEIDON". Science . 267 (5198): 639–642. Bibcode:1995Sci...267..639L. doi:10.1126/science.267.5198.639. PMID   17745840.
147. Touma, Jihad; Wisdom, Jack (1994). "Evolution of the Earth-Moon system". The Astronomical Journal . 108 (5): 1943–1961. Bibcode:1994AJ....108.1943T. doi:10.1086/117209.
148. Chapront, J.; Chapront-Touzé, M.; Francou, G. (2002). "A new determination of lunar orbital parameters, precession constant and tidal acceleration from LLR measurements". Astronomy and Astrophysics . 387 (2): 700–709. Bibcode:2002A&A...387..700C. doi:10.1051/0004-6361:20020420.
149. "Why the Moon is getting further away from Earth". BBC News. 1 February 2011. Archived from the original on 25 September 2015. Retrieved 18 September 2015.
150. Ray, R. (15 May 2001). "Ocean Tides and the Earth's Rotation". IERS Special Bureau for Tides. Archived from the original on 27 March 2010. Retrieved 17 March 2010.
151. Murray, C.D.; Dermott, Stanley F. (1999). Solar System Dynamics. Cambridge University Press. p. 184. ISBN   978-0-521-57295-8.
152. Dickinson, Terence (1993). From the Big Bang to Planet X. Camden East, Ontario: Camden House. pp. 79–81. ISBN   978-0-921820-71-0.
153. Latham, Gary; Ewing, Maurice; Dorman, James; Lammlein, David; Press, Frank; Toksőz, Naft; Sutton, George; Duennebier, Fred; Nakamura, Yosio (1972). "Moonquakes and lunar tectonism". Earth, Moon, and Planets . 4 (3–4): 373–382. Bibcode:1972Moon....4..373L. doi:10.1007/BF00562004.
154. Phillips, Tony (12 March 2007). "Stereo Eclipse". Science@NASA. Archived from the original on 10 June 2008. Retrieved 17 March 2010.
155. Espenak, F. (2000). "Solar Eclipses for Beginners". MrEclip]]. Retrieved 17 March 2010.
156. Walker, John (10 July 2004). "Moon near Perigee, Earth near Aphelion". Fourmilab. Archived from the original on 8 December 2013. Retrieved 25 December 2013.
157. Thieman, J.; Keating, S. (2 May 2006). "Eclipse 99, Frequently Asked Questions". NASA. Archived from the original on 11 February 2007. Retrieved 12 April 2007.
158. Espenak, F. "Saros Cycle". NASA. Archived from the original on 24 May 2012. Retrieved 17 March 2010.
159. Guthrie, D.V. (1947). "The Square Degree as a Unit of Celestial Area". Popular Astronomy . Vol. 55. pp. 200–203. Bibcode:1947PA.....55..200G.
160. "Total Lunar Occultations". Royal Astronomical Society of New Zealand. Archived from the original on 23 February 2010. Retrieved 17 March 2010.
161. Aaboe, A.; Britton, J.P.; Henderson, J.A.; Neugebauer, Otto; Sachs, A.J. (1991). "Saros Cycle Dates and Related Babylonian Astronomical Texts". Transactions of the American Philosophical Society . 81 (6): 1. doi:10.2307/1006543. JSTOR   1006543. One comprises what we have called "Saros Cycle Texts", which give the months of eclipse possibilities arranged in consistent cycles of 223 months (or 18 years).
162. Sarma, K.V. (2008). "Astronomy in India". In Helaine Selin (ed.). Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures. Encyclopaedia of the History of Science (2 ed.). Springer. pp. 317–321. Bibcode:2008ehst.book.....S. ISBN   978-1-4020-4559-2.
163. Needham 1986, p. 411.
164. O'Connor, J.J.; Robertson, E.F. (February 1999). "Anaxagoras of Clazomenae". University of St Andrews. Archived from the original on 15 March 2012. Retrieved 12 April 2007.
165. Needham 1986, p. 227.
166. Needham 1986, p. 413–414.
167. Robertson, E.F. (November 2000). "Aryabhata the Elder". Scotland: School of Mathematics and Statistics, University of St Andrews. Archived from the original on 11 July 2015. Retrieved 15 April 2010.
168. A.I. Sabra (2008). "Ibn Al-Haytham, Abū ʿAlī Al-Ḥasan Ibn Al-Ḥasan". Dictionary of Scientific Biography. Detroit: Charles Scribner's Sons. pp. 189–210, at 195.
169. Needham 1986, p. 415–416.
170. Lewis, C.S. (1964). The Discarded Image. Cambridge: Cambridge University Press. p. 108. ISBN   978-0-521-47735-2.
171. van der Waerden, Bartel Leendert (1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy". Annals of the New York Academy of Sciences . 500 (1): 1–569. Bibcode:1987NYASA.500....1A. doi:10.1111/j.1749-6632.1987.tb37193.x. PMID   3296915.
172. Evans, James (1998). The History and Practice of Ancient Astronomy. Oxford & New York: Oxford University Press. pp. 71, 386. ISBN   978-0-19-509539-5.
173. "Discovering How Greeks Computed in 100 B.C." The New York Times. 31 July 2008. Archived from the original on 4 December 2013. Retrieved 9 March 2014.
174. Van Helden, A. (1995). "The Moon". Galileo Project. Archived from the original on 23 June 2004. Retrieved 12 April 2007.
175. Consolmagno, Guy J. (1996). "Astronomy, Science Fiction and Popular Culture: 1277 to 2001 (And beyond)". Leonardo . 29 (2): 127–132. doi:10.2307/1576348. JSTOR   1576348.
176. Hall, R. Cargill (1977). "Appendix A: Lunar Theory Before 1964". NASA History Series. Lunar Impact: A History of Project Ranger. Washington, DC: Scientific and Technical Information Office, NASA. Archived from the original on 10 April 2010. Retrieved 13 April 2010.
177. Zak, Anatoly (2009). "Russia's unmanned missions toward the Moon". Archived from the original on 14 April 2010. Retrieved 20 April 2010.
178. "Rocks and Soils from the Moon". NASA. Archived from the original on 27 May 2010. Retrieved 6 April 2010.
179. "Soldiers, Spies and the Moon: Secret U.S. and Soviet Plans from the 1950s and 1960s". The National Security Archive. National Security Archive. Archived from the original on 19 December 2016. Retrieved 1 May 2017.
180. Brumfield, Ben (25 July 2014). "U.S. reveals secret plans for '60s moon base". CNN. Archived from the original on 27 July 2014. Retrieved 26 July 2014.
181. Teitel, Amy (11 November 2013). "LUNEX: Another way to the Moon". Popular Science. Archived from the original on 16 October 2015.
182. Logsdon, John (2010). John F. Kennedy and the Race to the Moon. Palgrave Macmillan. ISBN   978-0-230-11010-6.
183. Coren, M. (26 July 2004). "'Giant leap' opens world of possibility". CNN. Archived from the original on 16 March 2012. Retrieved 16 March 2010.
184. "Record of Lunar Events, 24 July 1969". Apollo 11 30th anniversary. NASA. Archived from the original on 8 April 2010. Retrieved 13 April 2010.
185. "Manned Space Chronology: Apollo_11". Spaceline.org. Archived from the original on 14 February 2008. Retrieved 6 February 2008.
186. "Apollo Anniversary: Moon Landing "Inspired World"". National Geographic. Archived from the original on 9 February 2008. Retrieved 6 February 2008.
187. Orloff, Richard W. (September 2004) [First published 2000]. "Extravehicular Activity". Apollo by the Numbers: A Statistical Reference. NASA History Division, Office of Policy and Plans. The NASA History Series. Washington, DC: NASA. ISBN   978-0-16-050631-4. LCCN   00061677. NASA SP-2000-4029. Archived from the original on 6 June 2013. Retrieved 1 August 2013.
188. Launius, Roger D. (July 1999). "The Legacy of Project Apollo". NASA History Office]]. Archived from the original on 8 April 2010. Retrieved 13 April 2010.
189. SP-287 What Made Apollo a Success? A series of eight articles reprinted by permission from the March 1970 issue of Astronautics & Aeronautics, a publication of the American Institute of Aeronautics and Astronautics. Washington, DC: Scientific and Technical Information Office, National Aeronautics and Space Administration. 1971.
190. "NASA news release 77-47 page 242" (PDF) (Press release). 1 September 1977. Archived (PDF) from the original on 26 June 2011. Retrieved 16 March 2010.
191. Appleton, James; Radley, Charles; Deans, John; Harvey, Simon; Burt, Paul; Haxell, Michael; Adams, Roy; Spooner N.; Brieske, Wayne (1977). "NASA Turns A Deaf Ear To The Moon". OASI Newsletters Archive. Archived from the original on 10 December 2007. Retrieved 29 August 2007.
192. Dickey, J.; et al. (1994). "Lunar laser ranging: a continuing legacy of the Apollo program".