The Moon is an astronomical body that orbits planet Earth, being
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). Following Jupiter's satellite Io, the
Moon is the
second-densest satellite in the
Solar System among those whose
densities are known.
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 Moon is in synchronous rotation with Earth, always showing the
same face, with its near side marked by dark volcanic maria that fill
the spaces between the bright ancient crustal highlands and the
prominent impact craters. As seen from the Earth, it is the
second-brightest regularly visible celestial object in Earth's sky,
after the Sun. 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.
The Moon's average orbital distance at the present time is
384,402 km (238,856 mi), or 1.28 light-seconds. This
is about thirty times the diameter of Earth, with its apparent size in
the sky almost the same as that of the
Sun (due to it being 400x
farther and larger), resulting in the
Moon covering the
precisely in total solar eclipse. This matching of apparent visual
size will not continue in the far future, because the Moon's distance
Earth is slowly increasing.
The Soviet Union's
Luna program was the first to reach the
unmanned spacecraft in 1959; the United States'
NASA Apollo program
achieved the only manned missions to date, beginning with the first
manned lunar orbiting mission by
Apollo 8 in 1968, and six manned
lunar 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
later history. Since the
Apollo 17 mission in 1972, the
Moon has been
visited only by unmanned spacecraft.
Within human culture, both the Moon's natural prominence in the
earthly sky, and its regular cycle of phases as seen from the Earth
have provided cultural references and influences for human societies
and cultures since time immemorial. Such cultural influences can be
found in language, lunar based calendar systems, art, and mythology.
1 Name and etymology
3 Physical characteristics
3.1 Internal structure
3.2 Surface geology
3.2.2 Impact craters
3.2.3 Lunar swirls
3.2.4 Presence of water
3.3 Gravitational field
3.4 Magnetic field
3.5.2 Past thicker atmosphere
4 Relationship to Earth
4.2 Relative size
4.3 Appearance from Earth
4.4 Tidal effects
5 Observation and exploration
5.1 Ancient and medieval studies
5.2 By spacecraft
5.2.1 20th century
184.108.40.206 Soviet missions
220.127.116.11 United States missions
5.2.2 21st century
5.2.3 Planned commercial missions
Astronomy from the Moon
7 Legal status
8 In culture
9 See also
11 Further reading
12 External links
12.1 Cartographic resources
12.2 Observation tools
Name and etymology
The Moon, tinted reddish, during a lunar eclipse
See also: list of lunar deities
The usual English proper name for Earth's natural satellite is "the
Moon", which in nonscientific texts is usually not
capitalized. 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. 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 our moon.
The modern English adjective pertaining to the
Moon is lunar, derived
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. 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. Both the Greek goddess
Selene and the Roman goddess
Diana were alternatively called Cynthia. The names Luna, Cynthia,
Selene are reflected in terminology for lunar orbits in words such
as apolune, pericynthion, and selenocentric. The name Diana comes from
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".
Near side of the Moon
Far side of the Moon
Lunar north pole
Lunar south pole
Origin of the Moon
Origin of the Moon and Giant-impact hypothesis
Several mechanisms have been proposed for the Moon's formation 4.51
billion years ago,[f] and some 60 million years after the origin of
the Solar System. These mechanisms included the fission of the
Moon from Earth's crust through centrifugal force (which would
require too great an initial spin of Earth), the gravitational
capture of a pre-formed Moon (which would require an unfeasibly
extended atmosphere of
Earth to dissipate the energy of the passing
Moon), 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). These hypotheses also cannot account for the
high angular momentum of the Earth–
The evolution of the
Moon and a tour of the Moon
The prevailing hypothesis is that the Earth–
Moon system formed as a
result of the impact of a Mars-sized body (named Theia) with the
Earth (giant impact), that blasted material into orbit about the
Earth that then accreted to form the present Earth-Moon
The far side of the
Moon has a crust that is 30 mi (48 km)
thicker than the near side of the Moon. This is thought to be due to
Moon having been amalgamated 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 popular.
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.
Giant impacts are thought to have been common in the early Solar
System. Computer simulations of a giant impact have produced results
that are consistent with the mass of the lunar core and the present
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. More recent simulations suggest a larger fraction of
Moon derived from the original
Earth mass. Studies
of meteorites originating from inner
Solar System bodies such as Mars
and Vesta show that they have very different oxygen and tungsten
isotopic compositions as compared to Earth, whereas
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
The great amount of energy released in the impact event and the
subsequent re-accretion of that material into the Earth-
would have melted the outer shell of Earth, forming a magma
ocean. Similarly, the newly formed
Moon would also have been
affected and had its own lunar magma ocean; estimates for its depth
range from about 500 km (300 miles) to its entire depth
(1,737 km (1,079 miles)).
While the giant impact hypothesis might explain many lines of
evidence, there are still some unresolved questions, most of which
involve the Moon's composition.
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. To their surprise, the team found that the rocks from the
Apollo program carried an isotopic signature that was identical with
rocks from Earth, and were different from almost all other bodies in
the Solar System. Because most of the material that went into orbit to
Moon was thought to come from Theia, this observation was
unexpected. In 2007, researchers from the California Institute of
Technology announced that there was less than a 1% chance that Theia
Earth had identical isotopic signatures. Published in 2012, an
analysis of titanium isotopes in Apollo lunar samples showed that the
Moon has the same composition as Earth, which conflicts with what
is expected if the
Moon formed far from
Earth's orbit or from Theia.
Variations on the giant impact hypothesis may explain this data.
Main article: Internal structure of the Moon
Structure of the Moon
Chemical composition of the lunar surface regolith (derived from
Composition (wt %)
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 km (150 mi) and a
fluid outer core primarily made of liquid iron with a radius of
roughly 300 km (190 mi). Around the core is a partially
molten boundary layer with a radius of about 500 km
(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 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. 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.
Consistent with this perspective, geochemical mapping made from orbit
suggests the crust of mostly anorthosite. 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. The crust is on average about
50 km (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 km (220 mi) or less, 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.
Geology of the Moon
Geology of the Moon and
Topography of the Moon
The topography of the
Moon has been measured with laser altimetry and
stereo image analysis. 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. At
13 km (8.1 mi) deep, its floor is the lowest point on the
surface of the Moon. 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. Other large impact basins, such as
Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess
regionally low elevations and elevated rims. The far side of the
lunar surface is on average about 1.9 km (1.2 mi) higher
than that of the near side.
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). Similar shrinkage
features exist on Mercury.
Main article: Lunar mare
Lunar nearside with major maria and craters labeled
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; 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. 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".
Evidence of young lunar volcanism
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 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. 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. Until recently, the
youngest eruptions, dated by crater counting, appeared to have been
only 1.2 billion years ago. In 2006, a study of Ina, a tiny
depression in Lacus Felicitatis, found jagged, relatively dust-free
features that, due to the lack of erosion by infalling debris,
appeared to be only 2 million years old.
Moonquakes and releases
of gas also indicate some continued lunar activity. 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 due to the
greater concentration of radioactive elements. Just
prior to this, evidence has been presented for 2–10 million years
younger basaltic volcanism inside Lowell crater, 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
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. 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
few tens of millions of years after their formation.
Further information: List of craters on the Moon
Lunar crater Daedalus on the Moon's far side
The other 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 alone. 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. 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
Apollo missions cluster between 3.8 and 4.1 billion
years old: this has been used to propose a
Late Heavy Bombardment
Late Heavy Bombardment of
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. 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. Beneath the finely
comminuted regolith layer is the megaregolith, a layer of highly
fractured bedrock many kilometres thick.
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.
Lunar swirls at Reiner Gamma
Main article: Lunar swirls
Lunar swirls are enigmatic features found across the Moon's surface,
which are characterized by a high albedo, appearing optically immature
(i.e. the optical characteristics of a relatively young regolith), and
often displaying a sinuous shape. Their curvilinear shape is often
accentuated by low albedo regions that wind between the bright swirls.
Presence of water
Main article: Lunar 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. 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
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
Earth aboard Apollo 15, showed small amounts of water in their
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. 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 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
Main article: Gravity of the Moon
GRAIL's gravity map of the Moon
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. 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.
Magnetic field of the Moon
The Moon has an external magnetic field of about 1–100 nanoteslas,
less than one-hundredth that of Earth. It does not currently have a
global dipolar magnetic field and only has crustal magnetization,
probably acquired early in lunar history when a dynamo was still
operating. Alternatively, some of the remnant magnetization
may be from transient magnetic fields generated during large impact
events through the expansion of an impact-generated plasma cloud in
the presence of an ambient magnetic field. This is supported by the
apparent location of the largest crustal magnetizations near the
antipodes of the giant impact basins.
Atmosphere of the Moon
Sketch by the
Apollo 17 astronauts. The lunar atmosphere was later
studied by LADEE.
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). 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. 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 from the solar wind; and argon-40, radon-222, 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 oxygen, nitrogen, carbon, hydrogen and
magnesium, which are present in the regolith, is not understood.
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.
These gases either return into the regolith due to 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
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.
Past thicker atmosphere
In October 2017,
NASA scientists at the Marshall Space Flight Center
Lunar and Planetary Institute
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.
The Moon's axial tilt with respect to the ecliptic is only
1.5424°, 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.
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, and these "craters of eternal darkness" are
Lunar Reconnaissance Orbiter
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
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. 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.
Relationship to Earth
Orbit of the
Moon and Lunar theory
Moon system (schematic)
DSCOVR satellite sees the
Moon passing in front of Earth
The Moon makes a complete orbit around
Earth with respect to the fixed
stars about once every 27.3 days[g] (its sidereal period).
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[h] (its synodic
period). Unlike most satellites of other planets, the
closer to the ecliptic plane than to the planet's equatorial plane.
The Moon's orbit is subtly perturbed by the
Earth in many
small, complex and interacting ways. For example, the plane of the
Moon's orbit gradually rotates once every 18.61 years, which
affects other aspects of lunar motion. These follow-on effects are
mathematically described by Cassini's laws.
The Moon is exceptionally large relative to Earth: a quarter its
diameter and 1/81 its mass. It is the largest moon in the Solar
System relative to the size of its planet,[i] though Charon is larger
relative to the dwarf planet Pluto, at 1/9 Pluto's mass.[j] 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.
Earth revolves around the Earth-
Moon barycentre once a sidereal
month, moving at 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
Moon setting in western sky over the High Desert in California
Moon above Makarska, Croatia
See also: Lunar observation, Lunar phase, Moonlight, and Earthlight
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, due to the effect of libration, about 59% of the
Moon's surface can actually be seen from Earth. The side of the Moon
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.
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. 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
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
Moon billions of years ago, before it was tidally locked to
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.[k] This is partly
due to the brightness enhancement of the opposition surge; the
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 centre, without limb darkening,
due to 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. 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 currently northernmost during winter.
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
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
The Moon is visible for two weeks every 27.3 days at the
North and South Poles.
Zooplankton in the
Arctic use moonlight when
Sun is below the horizon for months on end.
14 November 2016 supermoon was 356,511 kilometres (221,526 mi)
away from the center of Earth, the closest occurrence since 26
January 1948. It will not be closer until 25 November 2034.
The distance between the
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.
Reported as a "supermoon", this closest point coincides within an hour
of a full moon, and it was 30% more luminous than when at its greatest
distance due to its angular diameter being 14% greater, because
displaystyle scriptstyle 1.14^ 2 approx 1.30
. At lower levels, the human perception of reduced
brightness as a percentage is provided by the following
displaystyle text perceived reduction %=100times 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.
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.
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.
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
The illuminated area of the visible sphere (degree of illumination) is
displaystyle (1-cos e)/2=sin ^ 2 (e/2)
is the elongation (i.e., the angle between Moon, the observer (on
Earth) and the Sun).
Main articles: Tidal force, Tidal acceleration, Tide, and Theory of
The libration of the
Moon over a single lunar month. Also visible is
the slight variation in the Moon's visual size from Earth.
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. 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. 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. 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. As a result, the timing of the tides at most points
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. 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
Moon is increasing, and the Earth's spin 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 also
show that Earth's day lengthens by about 15 microseconds every
year, 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
Pluto and its moon Charon. However, the
Sun will become a
red giant engulfing the Earth-
Moon system long before this
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. 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).
Libration also changes the angle from which
Moon is seen, allowing a total of about 59% of its surface to be
Earth over time. The cumulative effects of stress built
up by these tidal forces produces moonquakes.
Moonquakes are much less
common and weaker than are earthquakes, although moon quakes can last
for up to an hour—a significantly longer time 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
Main articles: Solar eclipse, Lunar eclipse, and
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
Eclipses can only occur when the Sun, Earth, and
Moon are all in a
straight line (termed "syzygy"). Solar eclipses occur at new moon,
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
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
Moon appearing larger than the Sun) and annular (with the Moon
appearing smaller than the Sun) solar eclipses. In a total
Moon completely covers the disc of the
Sun and the solar
corona becomes visible to the naked eye. Because the distance between
Earth is very slowly increasing over time, 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.[l] The combination of these two
changes means that hundreds of millions of years ago, the
always completely cover the
Sun on solar eclipses, and no annular
eclipses were possible. Likewise, hundreds of millions of years in 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
Earth, is described by the saros, which has a period of approximately
Moon is continuously blocking our view of a
half-degree-wide circular area of the sky,[m] the related
phenomenon of occultation occurs when a bright star or planet passes
Moon and is occulted: hidden from view. In this way, a
solar eclipse is an occultation of the Sun. Because the
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
Observation and exploration
Ancient and medieval studies
Main articles: Exploration of the Moon: Early history, Selenography,
and Lunar theory
Map of the
Johannes Hevelius from his
the first map to include the libration zones
A study of the
Moon in Robert Hooke's Micrographia, 1665
Understanding of the Moon's cycles was an early development of
astronomy: by the 5th century BC, Babylonian astronomers had recorded
Saros cycle of lunar eclipses, and Indian astronomers
had described the Moon's monthly elongation. The Chinese
Shi Shen (fl. 4th century BC) gave instructions for
predicting solar and lunar eclipses. Later, the physical form of
Moon and the cause of moonlight became understood. The ancient
Anaxagoras (d. 428 BC) reasoned that the
Moon were both giant spherical rocks, and that the latter reflected
the light of the former. 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
merely a reflection of the Sun, and
Jing Fang (78–37 BC) noted
the sphericity of the Moon. In the 2nd century AD,
the novel A True Story, in which the heroes travel to the
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
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.
Galileo's sketches of the
Moon from Sidereus Nuncius
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.
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.
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.
Archimedes (287–212 BC) designed a planetarium that could calculate
the motions of the
Moon and other objects in the Solar System.
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".
Galileo Galilei drew one of the first telescopic drawings of
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
Francesco Maria Grimaldi
Francesco Maria Grimaldi led to the system of naming of
lunar features in use today. The more exact 1834–36 Mappa
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 astrogeology.
See also: Robotic exploration of the Moon, List of proposed missions
to the Moon, Colonization of the Moon, and List of artificial objects
on the Moon
Luna program and
Luna 2, the first human-made object to reach the surface of the Moon
(left) and Soviet
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
Luna program were the first to accomplish a number of
goals: following three unnamed, failed missions in 1958, 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. Rock and soil samples were brought back to
three Luna sample return missions (
Luna 16 in 1970,
Luna 20 in 1972,
Luna 24 in 1976), which returned 0.3 kg total. Two
pioneering robotic rovers landed on the
Moon in 1970 and 1973 as a
part of Soviet
Luna 24 was the last Soviet/Russian mission to the Moon.
United States missions
Apollo program and
Earthrise (Apollo 8, 1968)
Moon rock (Apollo 17, 1972)
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.
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. However, both these proposals were
ultimately passed over as the space program was largely transferred
from the military to the civilian agency NASA.
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
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
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.
Neil Armstrong working at the lunar module
"That's one small step ..."
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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
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) returned 380.05 kilograms
(837.87 lb) of lunar rock and soil in 2,196 separate
samples. 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.
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 due to budgetary
considerations, 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.
An artificially coloured mosaic constructed from a series of 53 images
taken through three spectral filters by Galileo' s imaging system
as the spacecraft flew over the northern regions of the
Moon on 7
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. In 1994, the U.S. sent the joint Defense
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. 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.
India, Japan, China, the United States, and the European Space Agency
each sent lunar orbiters, and especially ISRO's
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
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.
Artistic representation of a future
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.
Chinese Lunar Exploration Program
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. 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
Sun L2 Lagrangian point, before finally performing a flyby of
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
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.
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. 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. The
Indian Space Research Organisation
Indian Space Research Organisation planned to launch
Chandrayaan II in
2013, which would have included a Russian robotic lunar
rover. However, the failure of Russia's
has delayed this project.
Copernicus's central peaks as observed by the LRO, 2012
The Ina formation, 2009
The U.S. co-launched the
Lunar Reconnaissance Orbiter
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, 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.
NASA GRAIL spacecraft began orbiting the
Moon around 1 January
2012, on a mission to learn more about the Moon's internal
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. 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. Shackleton
Energy Company is building a program to establish operations on the
south pole of the
Moon to harvest water and supply their Propellant
NASA began to plan to resume manned missions following the call by
George W. Bush
George W. Bush on 14 January 2004 for a manned mission
Moon by 2019 and the construction of a lunar base by 2024.
Constellation program was funded and construction and testing
begun on a manned spacecraft and launch vehicle, and design
studies for a lunar base. However, that program has been
cancelled in favor of a manned asteroid landing by 2025 and a manned
Mars orbit by 2035.
India has also expressed its hope to send a
manned mission to the
Moon by 2020.
On February 28, 2018, SpaceX, Vodafone,
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
Planned commercial missions
In 2007, the X Prize Foundation together with
Google launched the
Google Lunar X Prize
Google Lunar X Prize to encourage commercial endeavors to the Moon. A
prize of $20 million will 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.
As of August 2016, 16 teams are participating in the competition.
In August 2016, the US government granted permission to US-based
Moon Express to land on the Moon. 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
Astronomy from the Moon
A false-color image of
Earth in ultraviolet light taken from the
surface of the
Moon on the
Apollo 16 mission. The day-side reflects a
lot of UV light from the Sun, but the night-side shows faint bands of
UV emission from the aurora caused by charged particles.
For many years, the
Moon has been recognized as an excellent site for
telescopes. 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. 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. A lunar zenith telescope can be made cheaply with an
In April 1972, the
Apollo 16 mission recorded various astronomical
photos and spectra in ultraviolet with the Far Ultraviolet
Main article: Space law
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. Russia and the U.S. are party to the 1967
Outer Space Treaty, which defines the
Moon and all outer space as
the "province of all mankind". This treaty also restricts the use
Moon to peaceful purposes, explicitly banning military
installations and weapons of mass destruction. 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.
Although several individuals have made claims to the
Moon in whole or
in part, none of these are considered credible.
Luna, the Moon, from a 1550 edition of Guido Bonatti's Liber
Moon in fiction
Further information: Lunar deity, Selene, Luna (goddess), Man in the
Moon, and Crescent
Moon with faces (1493 woodcut)
A 5,000-year-old rock carving at Knowth, Ireland, may represent the
Moon, which would be the earliest depiction discovered. 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
Moon was personified as a deity or other supernatural
phenomenon, and astrological views of the
Moon continue to be
Proto-Indo-European religion, the moon was personified as the male
god *Meh1not. The ancient Sumerians believed that the moon was
the god Nanna, who was the father of Inanna, the goddess of
the planet Venus, and Utu, the god of the sun.
Nanna was later known as Sîn, and was particularly
associated with magic and sorcery. In Greco-Roman mythology, the
Sun and the
Moon are represented as male and female, respectively
(Helios/Sol and Selene/Luna); this is a development unique to the
eastern Mediterranean and traces of an earlier male moon god in
the Greek tradition are preserved in the figure of Menelaus.
In Mesopotamian iconography, the crescent was the primary symbol of
Nanna-Sîn. In ancient Greek art, the
represented wearing a crescent on her headgear in an arrangement
reminiscent of horns. The star and crescent arrangement also
goes back to the Bronze Age, representing either the
Sun and Moon, or
Moon and planet Venus, in combination. It came to represent the
Artemis or Hecate, and via the patronage of
Hecate came to be
used as a symbol of Byzantium.
An iconographic tradition of representing
Moon with faces
developed in the late medieval period.
The splitting of the moon (Arabic: انشقاق القمر) is a
miracle attributed to Muhammad.
Further information: Lunar calendar, Lunisolar calendar, Metonic
cycle, Blue moon, and Movable feast
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. 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
Proto-Germanic *mǣnōn, indicating the usage of a
lunar calendar among the
Germanic peoples (Germanic calendar) prior to
the adoption of a solar calendar. 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),
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"). 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.
"Moonrise", 1884, picture by
Stanisław Masłowski (National Museum,
Kraków, Gallery of Sukiennice Museum)
Further information: Lunar effect
The Moon has long been associated with insanity and irrationality; the
words lunacy and lunatic (popular shortening loony) are derived from
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. 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
Solar System portal
List of lunar features
Former classification of planets
Other moons of Earth
List of natural satellites
Tourism on the Moon
Timeline of the far future
^ Between 18.29° and 28.58° to Earth's equator.
^ 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.
^ 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
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 × (
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
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): 10−7 Pa by day and 10−10 Pa by night.
^ This age is calculated from isotope dating of lunar zircons.
^ 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
^ 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.
^ With 27% the diameter and 60% the density of Earth, the
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.
^ The Sun's apparent magnitude is −26.7, while the full moon's
apparent magnitude is −12.7.
^ See graph in Sun#Life phases. At present, the diameter of the
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
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Find more about
Moon at's sister projects
Definitions from Wiktionary
Media from Wikimedia Commons
News from Wikinews
Quotations from Wikiquote
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Textbooks from Wikibooks
Travel guide from Wikivoyage
Learning resources from Wikiversity
NASA images and videos about the Moon
Album of images and high-resolution overflight videos by Seán Doran,
based on LRO data
The Moon on
Google Maps, a 3-D rendition of the moon akin to Google
"Consolidated Lunar Atlas". Lunar and Planetary Institute. Retrieved
26 February 2012.
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next new crescent moon is visible for any location.
Lunar shelter (building a lunar base with 3D printing)
Perigee & apogee
Total penumbral lunar eclipse
Solar eclipses on the Moon
Peak of eternal light
Crater of eternal darkness
South Pole–Aitken basin
Lunar basalt 70017
Transient lunar phenomenon
Lunar magma ocean
Late Heavy Bombardment
Lunar laser ranging
Lunar sample displays
Super & micro
Man in the Moon
Craters named after people
Memorials on the Moon
Moon in fiction
Moon in mythology
Hjúki and Bil
Moon is made of green cheese"
Lilith (hypothetical second moon)
Earth sciences portal
Solar System portal
Age of the Earth
Evolutionary history of life
Formation and evolution of the Solar System
Future of Earth
Geologic time scale
Geological history of Earth
History of Earth
Timeline of evolution
Extremes on Earth
Geology of solar terrestrial planets
Structure of the Earth
Human impact on the environment
NASA World Wind
Earth in culture
Earth in science fiction
Etymology of the word "Earth"
History of the world
List of countries
Flag of Earth
Claimed moons of Earth
Earth's location in the Universe
Earth sciences portal
Solar System portal
Natural satellites of the Solar System
≥ 100 km)
largest / 2634 km / 0.413 Earths
smallest / 106 km / 0.017 Earths
NOTE: Italicized moons are not close to being in hydrostatic
equilibrium; [bracketed] moons may or may not be close to being in
The Solar System
S/2015 (136472) 1
Solar System objects
By discovery date
Gravitationally rounded objects
Possible dwarf planets
first discovered: Ceres
Planets beyond Neptune
List of crewed spacecraft
List of probes
Outline of the Solar System
Solar System → Local Interstellar Cloud → Local
Bubble → Gould Belt → Orion Arm → Milky
Milky Way subgroup → Local Group → Virgo
Supercluster → Laniakea Supercluster → Observable
universe → Universe
Each arrow (→) may be read as "within" or "part of".
BNF: cb119358377 (data)