The Moon orbits Earth at an
average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter. Its gravitational influence is the main driver of Earth's
very slowly lengthens Earth's day. The
Moon's orbit around Earth has a
sidereal period of 27.3 days. During each
synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in
lunar phases that form the basis for the months of a
lunar calendar. The Moon is
tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (
the near side) to always face Earth, and the somewhat longer
lunar day is the same as the synodic period. However, 59% of the total lunar surface can be seen from Earth through cyclical shifts in perspective known as
Both the Moon's prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the
Soviet Union's uncrewed
Luna 2spacecraft in 1959; this was followed by the first successful soft landing by
Luna 9 in 1966. The only human lunar missions to date have been those of the
Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned
lunar rocks that have been used to develop a detailed
geological understanding of the
internal structure, and subsequent history. The Moon is the only
celestial body visited by humans.
Occasionally, the name Luna/ˈluːnə/ is used in scientific writing and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon.Cynthia/ˈsɪnθiə/ is another poetic name, though rare, for the Moon personified as a goddess, while Selene/səˈliːniː/ (literally "Moon") is the Greek goddess of the Moon.
The usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, lūna. The adjective selenian/səliːniən/, derived from the Greek word for the Moon, σελήνηselēnē, and used to describe the Moon as a world rather than as an object in the sky, is rare, while its cognate selenic was originally a rare synonym but now nearly always refers to the chemical element
selenium. The Greek word for the Moon does however provide us with the prefix seleno-, as in selenography, the study of the physical features of the Moon, as well as the element name selenium.
The Greek goddess of the wilderness and the hunt,
Artemis, equated with the Roman
Diana, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called
Cynthia, from her legendary birthplace on
Mount Cynthus. These names – Luna, Cynthia and Selene – are reflected in technical terms for
lunar orbits such as apolune, pericynthion and selenocentric.
astronomical symbol for the Moon is a crescent,
, for example in M☾ 'lunar mass' (also ML).
The prevailing theory is that the Earth–Moon system formed after a
giant impact of a
Mars-sized body (named Theia) with the
proto-Earth. The impact blasted material into orbit about the Earth and the material accreted and formed the Moon just beyond the Earth's
Roche limit of ~2.56
Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of the Moon derived from the impactor, rather than the proto-Earth. However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. Other bodies of the inner Solar System such as Mars and
Vesta have, according to meteorites from them, very different oxygen and tungsten
isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, although this is debated.
The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system. Similarly, the newly formed Moon would have had its own
lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).
While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.[example needed]Above a high resolution threshold for simulations, a study published in 2022 finds that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's
Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits.
Artist's impression of the Moon as it might have appeared in Earth's sky after the
Late Heavy Bombardment around 4 billion years ago. At that time the Moon orbited Earth much closer, appearing much larger.
After the Moon's formation the Moon settled in orbit around Earth much closer than today, making both bodies appear much larger in each's sky and causing on both more frequent and stronger
Since then, due to
tidal acceleration, the Moon's orbit around Earth has become significantly larger as well as longer,
tidally locking the so-called
lunar near side, always facing Earth with this same side.
The post formation cooled
lunar surface has been shaped by large and many small
impact events, retaining a broadly
cratered landscape of all ages, as well as by
volcanic activity, producing the prominent
lunar maria. Volcanically active until 1.2 billion years ago, most of the Moon's
mare basalts erupted during the
Imbrian period, 3.3–3.7 billion years ago, though some being as young as 1.2 billion years and some as old as 4.2 billion years. The causes for the eruption of mare basalts, particularly their uneven occurrence on mainly the near-side, like the
lunar highlands on the
far side, has been an unresolved issue due to differing explanations. One explanation suggests that large
meteorites were hitting the Moon in its early history leaving large craters which then were filled with
lava. Other explanations suggest processes of lunar volcanism.
The Moon is a very slightly
scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to adjust to its orbit.
Size comparison of the main moons of the Solar System with Earth to scale. Nineteen moons are large enough
to be round, several having
subsurface oceans and one, Titan, having a considerable atmosphere.
The Moon's diameter is about 3,500 km, more than a quarter of Earth's, with the face of the Moon comparable to the width of
Australia. The whole surface area of the Moon is about 38 million square kilometers, between the size of the
South America) and
The Moon's mass is 1/81 of Earth's, being the second densest among the planetary moons, and having the second highest
surface gravity, after
Io, at 0.1654
g and an escape velocity of 2.38 km/s (8600 km/h; 5300 mph).
Moon's internal structure: solid inner core (iron-metallic), molten outer core, hardened mantle and crust. The crust on the Moon's near side permanently facing Earth is thinner, featuring larger areas flooded by material of the once molten mantle forming today's
The Moon is a
differentiated body that was initially in
hydrostatic equilibrium but has since departed from this condition. It has a
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). 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.
Crystallization of this magma ocean would have created a
mafic mantle from the
precipitation and sinking of the minerals
orthopyroxene; after about three-quarters of the magma ocean had crystallized, lower-density
plagioclase minerals could form and float into a crust atop. The final liquids to crystallize would have been initially sandwiched between the crust and mantle, with a high abundance of
incompatible and heat-producing elements. Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly
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. The crust is on average about 50 kilometres (31 mi) thick.
The Moon is the second-densest satellite in the Solar System, after
Io. However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less, around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's time-variable rotation suggest that it is at least partly molten. The pressure at the lunar core is estimated to be 5 GPa (49,000 atm).
Magnetic and gravitational fields
John Young test driving the
Lunar Roving Vehicle. Lunar dust there falls much slower than objects on Earth in a vacuum.
Moon has an external magnetic field of less than 0.2
nanoteslas, or less than one hundred thousandth
that of Earth. The Moon does not currently have a global
dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating. However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized. Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the
antipodes of the giant impact basins.
On average the Moon's surface gravity is 1.62
ft/s2), about half of the surface gravity of
Mars and about a sixth of Earth's. The
Moon's gravitational field is not uniform. The details of the gravitational field have been measured through tracking the
Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are
mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. 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.
The Moon has an
atmosphere so tenuous as to be nearly
vacuum, with a total mass of less than 10 tonnes (9.8 long tons; 11 short tons). The surface pressure of this small mass is around 3 × 10−15atm (0.3
nPa); it varies with the lunar day. Its sources include
sputtering, a product of the bombardment of lunar soil by solar wind ions. Elements that have been detected include
potassium, produced by sputtering (also found in the atmospheres of
neon from the solar wind; and
polonium-210, outgassed after their creation by
radioactive decay within the crust and mantle. The absence of such neutral species (atoms or molecules) as
magnesium, which are present in the
regolith, is not understood. Water vapor 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. 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.
Studies of Moon magma samples retrieved by the
Apollo missions demonstrate 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.
Moon dust cloud exists around the Moon, generated by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. 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 up to 100 kilometers above the surface. Dust counts made by
LADEE's Lunar Dust EXperiment (LDEX) found particle counts peaked during the
Northern Taurid, and
Omicron Centauridmeteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being more dense near the boundary between the Moon's dayside and nightside.
Gene Cernan with lunar dust stuck on his suit. Lunar dust is highly abrasive and can cause damage to human lungs, nervous, and cardiovascular systems.
The surface is exposed to drastic temperature differences ranging from 140 °C to −171 °C depending on the
Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow, making topographical details play a decisive role on local
Parts of many craters, particularly the bottoms of many polar craters, are permanently shadowed, these "
craters of eternal darkness" have extremely low temperatures. The Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F) and just 26 K (−247 °C; −413 °F) close to the winter solstice in the north polar crater
Hermite. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of
Blanketed on top of the Moon's crust is a highly
comminuted (broken into ever smaller particles) and
impact gardened mostly gray 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. The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–15 m (33–49 ft) in the highlands and 4–5 m (13–16 ft) in the maria. Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick.
These extreme conditions for example are considered to make it unlikely for spacecraft to harbor bacterial spores at the Moon longer than just one lunar orbit.
The discovery of
fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. Similar shrinkage features exist on
Mercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon doesn't have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat.
The names of the main
maria (blue) and some
crater (brown) features of the near side of the Moon
The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains called maria (singular mare;
Latin for "seas", as they were once believed to be filled with water) are vast solidified pools of ancient
basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. The majority of these lava deposits 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".
Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side compared with 2% of the far side. This is likely due to a
concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. Most of the Moon's
mare basalts erupted during the
Imbrian period, 3.3–3.7 billion years ago, though some being as young as 1.2 billion years and as old as 4.2 billion years.
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.Moonquakes and releases of gas indicate continued lunar activity. Evidence of recent lunar volcanism has been identified at 70
irregular mare patches, 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. Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell, inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities on the far side in the Orientale basin.
The lighter-colored 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
plagioclasecumulates of the lunar magma ocean. In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events.
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 the Moon's formation. Alternatively, it may be a consequence of asymmetrical
tidal heating when the Moon was much closer to the Earth.
A major geologic process that has affected the Moon's surface is
impact cratering, 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. The
lunar geologic timescale is based on the most prominent impact events, including
Orientale; structures characterized by multiple rings of uplifted material, between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional
stratigraphic horizon. 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. 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 period of increased impacts.
High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a contemporary crater-production rate significantly higher than was previously estimated. A secondary cratering process caused by
distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years. This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts.
Wide angle image of a lunar swirl, the 70 kilometer long
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 often have a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface
magnetic fields and many are located at the
antipodal point of major impacts. Well known swirls include the
Reiner Gamma feature and
Mare Ingenii. They are hypothesized to be areas that have been partially shielded from the
solar wind, resulting in slower
In 2008, NASA's Moon Mineralogy Mapper equipment on
India's Chandrayaan-1 discovered, for the first time, water-rich minerals (shown in blue around a small crater from which they were ejected).
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. Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow. 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.
In years since, signatures of water have been found to exist on the lunar surface. 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. 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. Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior.
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. Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018. 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. Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb).
In May 2011, 615–1410 ppm water in
melt inclusions in lunar sample 74220 was reported, 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 insight does not mean that water is easily available since 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. The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. 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 crevices, allowing it to persist as ice on the surface since they are shielded from the sun.
The Earth and the Moon form the Earth-Moon
satellite system with a shared center of mass, or
barycenter. This barycenter stays located at all times 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath the Earth's surface, making the Moon seemingly orbit the Earth.
orbital eccentricity is 0.055, indicating a slightly elliptical orbit.
Lunar distance, or the
semi-major axis of the geocentric lunar orbit, is approximately 400,000 km, which is a quarter of a million miles or 1.28 light-seconds, and a unit of measure in astronomy. This is not to be confused with the instantaneous Earth–Moon distance, or distance to the Moon, the momentanous distance from the center of Earth to the center of the Moon.
The Moon makes a complete orbit around Earth with respect to the fixed stars, its
sidereal period, about once every 27.3 days[h] However, because the Earth-Moon system moves at the same time in its orbit around the Sun, it takes slightly longer, 29.5 days,[i] to return at the same
lunar phase, completing a full cycle, as seen from Earth. This
synodic period or synodic month is commonly known as the
lunar month and is equal to the length of the
solar day on the Moon.
tidal locking, the Moon has a 1:1
spin–orbit resonance. This
orbit ratio makes the Moon's orbital periods around Earth equal to its corresponding
rotation periods. This is the reason for only one side of the Moon, its so-called
near side, being visible from Earth. That said, while the movement of the Moon is in resonance, it still is not without nuances such as
libration, resulting in slightly changing perspectives, making over time and location on Earth about 59% of the Moon's surface visible from Earth.
Unlike most satellites of other planets, the Moon's orbital plane is 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 18.61years, which affects other aspects of lunar motion. These follow-on effects are mathematically described by
Minimum, mean and maximum distances of the Moon from Earth with its angular diameter as seen from Earth's surface, to scale
Simplified diagram of the Moon's gravity tidal effect on the Earth
The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides of closest to each other, resulting in
Ocean tides are the most widely experienced result of this, but tidal forces considerably affect also other mechanics of Earth, as well as the Moon and their system.
The lunar solid crust experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth, because they are in
synchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun. The Earth-induced variable component arises from changing distance and
libration, a result of the Moon's orbital eccentricity and inclination (if the Moon's orbit were perfectly circular and un-inclined, there would only be solar tides). According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining
Earth's magnetic field.
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 scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moonquakes was an unexpected discovery from
seismometers placed on the Moon by
Apolloastronauts from 1969 through 1972.
The most commonly known effect of tidal forces are elevated sea levels called ocean tides. While the Moon exerts most of the tidal forces, the Sun also exerts tidal forces and therefore contributes to the tides as much as 40% of the Moon's tidal force; producing in interplay the
spring and neap tides.
The tides are two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. As the Earth rotates 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. 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.
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 sloshing of water between different ocean basins
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.
Delays in the tidal peaks of both ocean and solid-body tides cause
torque in opposition to the Earth's rotation. This "drains"
angular momentum and rotational
kinetic energy from Earth's rotation, slowing the Earth's rotation. That angular momentum, lost from the Earth, is transferred to the Moon in a process known as
tidal acceleration, which lifts the Moon into a higher orbit while lowering orbital speed around the Earth.
Thus the distance between Earth and Moon is increasing, and the Earth's rotation is slowing in reaction. 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 (roughly the rate at which human fingernails grow).Atomic clocks show that Earth's day lengthens by about 17
microseconds every year, slowly increasing the rate at which
UTC is adjusted by
This tidal drag makes the rotation of the Earth and the orbital period of the Moon very slowly match. This matching first results in
tidally locking the lighter body of the orbital system, as is already the case with the Moon. Theoretically, in 50 billion years, the Earth's rotation will have slowed to the point of matching the Moon's orbital period, causing the Earth to always present the same side to the Moon. However, the Sun will become a
red giant, engulfing the Earth-Moon system, long before then.
Libration, the slight variation in the Moon's
apparent size and viewing angle over a single lunar month as viewed from Earth's north
The Moon's highest
culmination varies by its
lunar phase, or more correctly its orbital position, and time of the year, or more correctly the position of the Earth's axis. The full moon is highest in the sky during winter and lowest during summer (for each hemisphere respectively), with its altitude changing towards dark moon to the opposite.
The apparent orientation of the Moon depends on its position in the sky and the hemisphere of the Earth from which it is being viewed. In the
northern hemisphere it is seen upside down compared to the view in the
southern hemisphere. Sometimes the "horns" of a crescent moon appear to be pointing more upwards than sideways. This phenomenon is called a
wet moon and occurs more frequently in 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, making the Moon's apparent size fluctuate. On average the Moon's
angular diameter is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see
§ Eclipses). Additionally when close to the horizon a purely psychological effect, known as the
Moon illusion, makes the Moon appear larger.
Despite the Moon's tidal locking, the effect of
libration makes about 59% of the Moon's surface visible from Earth over the course of one month.
Comparison between the Moon on the left, rotating tidally locked (correct), and with the Moon on the right, without rotation (incorrect)
tidally locked synchronous rotation of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. 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
dark moon to
new moon, the near side is dark.
The Moon originally 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. 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 1998-99 NASA Lunar Prospector mission, found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth.
Half of the Moon's surface is always illuminated by the Sun (except during a
lunar eclipse). Earth also reflects light onto the Moon, observable at times as
Earthlight when it is again reflected back to Earth from areas of the
near side of the Moon that are not illuminated by the Sun.
With the different positions of the Moon, different areas of it are illuminated by the Sun. This illumination of different lunar areas, as viewed from Earth, produces the different
lunar phases during the
synodic month. A phase is equal to the area of the visible lunar sphere that is illuminated by the Sun. This area or degree of illumination is given by , where is the
elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).
On 14 November 2016, the Moon was at full phase closer to Earth than it had been since 1948. It was 14% closer and larger than its farthest position in apogee. This closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because of its increased apparent diameter, which made it a particularly notable example of a "
At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula:
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.
Albedo and color
The changing apparent color of the Moon, filtered by Earth's atmosphere
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.[j] 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. 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 center, without
limb darkening, because of the
reflective properties of
lunar soil, which
retroreflects light more towards the Sun than in other directions. The Moon's color depends on the light the Moon reflects, which in turn depends on the Moon's surface and its features, having for example large darker regions. In general the lunar surface reflects a brown-tinged gray light.
Viewed from Earth the air filters the reflected light, at times giving it a red color depending on the angle of the Moon in the sky and thickness of the atmosphere, or a blue tinge depending on the particles in the air, as in cases of volcanic particles. The terms
blood moon and
blue moon do not necessarily refer to circumstances of red or blue
moonlight, but are rather particular cultural references such as particular
full moons of a year.
There has been historical controversy over whether observed 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.
Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "
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. 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, the angular diameter of the Moon is decreasing. As it evolves toward becoming a
red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing.[k] 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.
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. 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.
Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky,[l] 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.
It is believed by some that 20–30,000 year old
tally sticks, were used to observe the phases of the Moon, keeping time using the waxing and waning of
the Moon's phases.
One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving Orthostat 47 at
Although the Chinese of the
Han Dynasty believed the Moon to be energy equated to qi, their 'radiating influence' theory 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.: 413–414 Ptolemy (90–168 AD) greatly improved on the numbers of Aristarchus, calculating a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters, close to the correct values of about 60 and 0.273 respectively. 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. 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.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.: 415–416
Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth".
Galileo's sketches of the Moon from the ground-breaking Sidereus Nuncius (1610), publishing among other findings the first descriptions of the Moons topography
Galileo Galilei used an early telescope to make drawings of the Moon for his book Sidereus Nuncius, and deduced that it was not smooth but had mountains and craters.
Thomas Harriot had made, but not published such drawings a few months earlier.
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 1834–1836 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. Lunar craters, first noted by Galileo, were thought to be
volcanic until the 1870s proposal of
Richard Proctor that they were formed by collisions. This view gained support in 1892 from the experimentation of geologist
Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s, leading to the development of
lunar stratigraphy, which by the 1950s was becoming a new and growing branch of
First view of the far side of the Moon, taken by
Luna 3, 7 October 1959
After the first spaceflight of
Sputnik 1 in 1957 during
International Geophysical Year the spacecrafts of the Soviet Union's
Luna program were the first to accomplish a number of goals. Following three unnamed failed missions in 1958, the first human-made object Luna 1 escaped Earth's gravity and passed near the Moon in 1959. Later that year the first human-made object Luna 2 reached the Moon's surface by
intentionally impacting. By the end of the year Luna 3 reached as the first human-made object the normally occluded
far side of the Moon, taking the first photographs of it.
The first spacecraft to perform a successful lunar
soft landing was Luna 9 and the first vehicle to orbit the Moon was Luna 10, both in 1966.
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 uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the
Jet Propulsion Laboratory's
Ranger program, the
Lunar Orbiter program and the
Surveyor program. The crewed
Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential
Soviet lunar human landing, in 1968
Apollo 8 made the first human mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race.
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. 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. The Apollo missions 11 to 17 (except
Apollo 13, which aborted its planned lunar landing) removed 380.05 kilograms (837.87 lb) of lunar rock and soil in 2,196
Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived
instrument stations, including heat flow probes,
magnetometers, were installed at the
17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations, but as the stations'
lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used.Apollo 17 in 1972 remains the last crewed mission to the Moon.
Explorer 49 in 1973 was the last dedicated U.S. probe to the Moon until the 1990s.
The until 1979 negotiated
Moon treaty, with its ratification in 1984 by its few signatories was about the only major activity regarding the Moon until 1990.
Renewed exploration (1990-present)
Map of all soft landing sites on the near side of the Moon
In 1990 Hiten-Hagoromo, the first dedicated lunar mission since 1976, reached the Moon. Sent by
Japan, it became the first mission that was not a Soviet Union or U.S. mission to the Moon.
In 1994, the U.S. dedicated a mission to fly a spacecraft (Clementine) to the Moon again for the first time since 1973. This mission obtained the first near-global topographic map of the Moon, and the first global
multispectral images of the lunar surface. In 1998, this was followed 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.
The next years saw a row of first missions to the Moon by a new group of states actively exploring the Moon.
Between 2004 and 2006 the first spacecraft by the
European Space Agency (ESA) (SMART-1) reached the Moon, recording the first detailed survey of chemical elements on the lunar surface.
Chinese Lunar Exploration Program began with Chang'e 1 between 2007 and 2009, obtaining a full image map of the Moon.
India reached the Moon in 2008 for the first time with its Chandrayaan-1, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of
water molecules in lunar soil.
The U.S. launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor on 18 June 2009. LCROSS completed its mission by making a planned and widely observed impact in the crater
Cabeus on 9 October 2009, whereas LRO is currently in operation, obtaining precise lunar
altimetry and high-resolution imagery.
With the signing of the U.S.-led
Artemis Accords in 2020, the
Artemis program aims to return the astronauts to the Moon in the 2020s. The Accords have been joined by a growing number of countries. The introduction of the Artemis Accords has fueled a renewed discussion about the international framework and cooperation of lunar activity, building on the
Moon Treaty and the ESA-led
Moon Village concept. The U.S.
developed plans for returning to the Moon beginning in 2004, which resulted in several programs. The Artemis program has advanced the farthest, and includes plans to send the first woman to the Moon as well as build an international lunar space station called
Upcoming lunar missions include the
Artemis program missions and Russia's first lunar mission, Luna-Glob: an uncrewed lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission.
While the Moon has the lowest
planetary protection target-categorization, its degradation as a pristine body and scientific place has been discussed. If there is
astronomy performed from the Moon, it will need to be free from any physical and
radio pollution. While the Moon has no significant atmosphere, traffic and impacts on the Moon causes clouds of dust that can spread far and possibly contaminate the original state of the Moon and its special scientific content. Scholar
Alice Gorman asserts that, although the Moon is inhospitable, it is not dead, and that sustainable human activity would require treating the Moon's ecology as a co-participant.
Space debris beyond Earth around the Moon has been considered as a future challenge with increasing numbers of missions to the Moon, particularly as a danger for such missions. As such lunar waste management has been raised as an issue which future lunar missions, particularly on the surface, need to tackle.
Longterm missions continuing to be active are some orbiters such as the 2009-launched
Lunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launched
Chang'e 3 with its Lunar Ultraviolet Telescope still operational.
Five retroreflectors have been installed on the Moon since the 1970s and since used for accurate measurements of the physical
laser ranging to the Moon.
Since 2020, countries have joined the U.S. in their
Artemis Accords, which are challenging the treaty. The U.S. has furthermore emphasized in a presidential
executive order ("Encouraging International Support for the Recovery and Use of Space Resources.") that "the United States does not view outer space as a 'global commons'" and calls the Moon Agreement "a failed attempt at constraining free enterprise."
With Australia signing and ratifying both the Moon Treaty in 1986 as well as the Artemis Accords in 2020, there has been a discussion if they can be harmonized. In this light an Implementation Agreement for the Moon Treaty has been advocated for, as a way to compensate for the shortcomings of the Moon Treaty and to harmonize it with other laws, allowing it to be more widely accepted.
In the face of such increasing commercial and national interest, particularly prospecting territories, U.S. lawmakers have introduced in late 2020 specific regulation for the conservation of historic landing sites and interest groups have argued for making such sites
World Heritage Sites and zones of scientific value protected zones, all of which add to the legal availability and territorialization of the Moon.
In 2021, the Declaration of the Rights of the Moon was created by a group of "lawyers, space archaeologists and concerned citizens", drawing on precedents in the
Rights of Nature movement and the concept of legal personality for non-human entities in space.
In light of future development on the Moon some international and multi-
space agency organizations have been created:
Since pre-historic times people have taken note of
the Moon's phases, its waxing and waning, and used it to keep record of time.
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. The counting of the days between the Moon's phases gave eventually rise to generalized
time periods of the full lunar cycle as
months, and possibly of its phases as
The words for the month in a range of different languages carry this relation between the period of the month and the Moon etymologically. The English month as well as moon, and its cognates in other Indo-European languages (e.g. the
Ancient Greekμείς (meis) or μήν (mēn), meaning "month") stem from the
Proto-Indo-European (PIE) root of moon, *méh1nōt, derived 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). To give another example from a different
language family, the
Chinese language uses the same word (月) for moon as well as for month, which furthermore can be found in the symbols for the word week (星期).
This lunar timekeeping gave rise to the historically dominant, but varied,
lunisolar calendars. The 7th-century
Islamic calendar is an example of a purely
lunar calendar, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon.
For the representation of the Moon, especially its
lunar phases, the
crescent symbol (🌙) has been particularly used by many cultures. In
writing systems such as Chinese the crescent has developed into the symbol 月, the word for Moon, and in ancient Egyptian it was the symbol
𓇹, which is spelled like the ancient Egyptian lunar deity Iah, meaning Moon.
The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans. The Moon has long been associated with insanity and irrationality; the words lunacy and lunatic are derived from the Latin name for the Moon, Luna. Philosophers
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. 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.
^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
Earth radius / 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.)
^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.
^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): 10−7Pa by day and 10−10 Pa by night.
abWith 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 Earth and the Moon are different since Pluto is considered a
dwarf planet and not a planet, unlike Earth.
^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.
^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).
^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).
^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.
^On average, the Moon covers an area of 0.21078 square degrees on the night sky.
abBugby, D. C.; Farmer, J. T.; O’Connor, B. F.; Wirzburger, M. J.; C. J. Stouffer, E. D. Abel (January 2010). Two‐Phase Thermal Switching System for a Small, Extended Duration Lunar Surface Science Platform. AIP Conference Proceedings. Vol. 1208. pp. 76–83.
^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.
^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.
^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.
^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.
Walker, John (May 1997).
"Inconstant Moon". Earth and Moon Viewer. Fourth paragraph of "How Bright the Moonlight":
Archived from the original on 14 December 2013. Retrieved 23 January 2014. 14% [...] due to the logarithmic response of the human eye.
^Gibbs, Philip (May 1997).
"Why is the sky blue?". math.ucr.edu.
Archived from the original on 2 November 2015. Retrieved 4 November 2015. ... may cause the moon to have a blue tinge since the red light has been scattered out.
^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.
^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.
^Pino, Paolo; Salmeri, Antonino; Hugo, Adam; Hume, Shayna (27 August 2021). "Waste Management for Lunar Resources Activities: Toward a Circular Lunar Economy". New Space. Mary Ann Liebert Inc. 10 (3): 274–283.
^"It seems possible, though not certain, that after the conquest Mehmed took over the crescent and star as an emblem of sovereignty from the Byzantines. The half-moon alone on a blood red flag, allegedly conferred on the Janissaries by Emir Orhan, was much older, as is demonstrated by numerous references to it dating from before 1453. But since these flags lack the star, which along with the half-moon is to be found on Sassanid and Byzantine municipal coins, it may be regarded as an innovation of Mehmed. It seems certain that in the interior of Asia tribes of Turkish nomads had been using the half-moon alone as an emblem for some time past, but it is equally certain that crescent and star together are attested only for a much later period. There is good reason to believe that old Turkish and Byzantine traditions were combined in the emblem of Ottoman and, much later, present-day Republican Turkish sovereignty." Franz Babinger (William C. Hickman Ed., Ralph Manheim Trans.), Mehmed the Conqueror and His Time, Princeton University Press, 1992, p 108
^Kelly, Ivan; Rotton, James; Culver, Roger (1986), "The Moon Was Full and Nothing Happened: A Review of Studies on the Moon and Human Behavior", Skeptical Inquirer, 10 (2): 129–143. Reprinted in The Hundredth Monkey – and other paradigms of the paranormal, edited by Kendrick Frazier, Prometheus Books. Revised and updated in The Outer Edge: Classic Investigations of the Paranormal, edited by
Barry Karr, and Tom Genoni, 1996,