The
Moon is
Earth's only
natural satellite and the
fifth largest satellite in the
Solar System. The average
centre-to-centre distance from the Earth to the Moon is , about
thirty times the diameter of the Earth. The common centre of mass
of the system (the
barycentre) is located
at about —a quarter the Earth's
radius—beneath the surface of the Earth. The Moon
makes a complete
orbit around the Earth every
27.3 days (the
orbital period),
and the periodic variations in the geometry of the
Earth–Moon–
Sun system are responsible for the
phases of the Moon, which repeat every
29.5 days (the
synodic
period).
The Moon's diameter is , a little more than a quarter of that of
the Earth.
Thus, the Moon's surface area is less than a tenth of
the Earth (about a quarter of Earth's land area, approximately as
large as Russia, Canada, and the
United
States combined), and its volume is about 2 percent that
of Earth. The pull of
gravity at
its surface is about 17 percent of that at the Earth's
surface.
The Moon is the only
celestial
body on which human beings have made a manned
landing.
While the Soviet Union's Luna programme was
the first to reach the Moon with unmanned spacecraft, the NASA Apollo program achieved the only manned
missions to date, beginning with the first manned lunar mission by
Apollo 8 in 1968, and six manned lunar
landings between 1969 and 1972 – the first being Apollo 11 in 1969. Human exploration of the
Moon temporarily ceased with the conclusion of the Apollo program,
although a few robotic landers and orbiters have been sent to the
Moon since that time. The U.S. has committed to return to the Moon
by 2018. On November 13, 2009, NASA announced the discovery of
proof that water exists on the Moon, based on data obtained from
the
LCROSS lunar impact mission.
Name and etymology
The proper
English name for Earth's
natural satellite is, simply, the Moon (capitalized).
Moon
is a
Germanic word, related to
the Latin (month). It is ultimately a derivative of the
Proto-Indo-European root
me-, also represented
in
measure (
time), with reminders of
its importance in measuring time in words derived from it like
Monday,
month and
menstrual. The related adjective is
lunar, as well as an adjectival prefix
seleno-
and suffix
-selene (from
selēnē, , the
Ancient Greek word for the Moon). In English,
the word
moon exclusively meant "the Moon" until 1665,
when it was extended to refer to the recently discovered
natural satellites of other planets.
Subsequently, these objects were given distinct names in order to
avoid confusion. The Moon is occasionally referred to by its Latin
name , primarily in science fiction.
Lunar surface
Two sides of the Moon
The Moon is in
synchronous
rotation, which means it rotates about its axis in about the
same time it takes to
orbit the Earth. This
results in it keeping nearly the same face turned towards the Earth
at all times. The Moon used to rotate at a faster rate, but early
in its history, its rotation slowed and became
locked in this orientation as a result of
frictional effects associated with
tidal deformations caused by the Earth.
Small variations (
libration) in the angle
from which the Moon is seen allow about 59% of its surface to be
seen from the Earth (but only half at any instant).
The side of the Moon that faces Earth is called the
near side, and the opposite side the
far side. The far side is often
inaccurately called the "dark side," but in fact, it is illuminated
exactly as often as the near side: once per lunar day, during the
new Moon phase we observe on Earth when the near side is dark. The
far side of the Moon was first photographed by the Soviet probe
Luna 3 in 1959. One distinguishing feature of
the far side is its almost complete lack of
maria.
Maria
The dark and relatively featureless lunar plains which can clearly
be seen with the naked eye are called
maria (singular
mare), Latin for
seas, since they were believed by ancient
astronomers to be filled with water. These are
now known to be vast solidified pools of ancient
basaltic lava. The majority of these lavas erupted or
flowed into the depressions associated with
impact basins that formed by the collisions of
meteors and comets with the lunar surface.
(Oceanus
Procellarum is a major exception in that it does not correspond
to a known impact basin). Maria are found almost exclusively
on the near side of the Moon, with the far side having only a few
scattered patches covering about 2% of its surface, compared with
about 31% on the near side. The most likely explanation for this
difference is related to a higher concentration of heat-producing
elements on the near-side hemisphere, as has been demonstrated by
geochemical maps obtained from the
Lunar Prospector gamma-ray spectrometer.
Several provinces containing
shield
volcanoes and volcanic
dome are found
within the near side maria.
Terrae
The lighter-colored regions of the Moon are called
terrae,
or more commonly just
highlands, since they are higher
than most maria. Several prominent mountain ranges on the near side
are found along the periphery of the giant
impact basins, many of which have been filled
by mare basalt. These are hypothesized to be the surviving remnants
of the impact basin's outer rims. In contrast to the Earth, no
major lunar mountains are believed to have formed as a result of
tectonic events.
From
images taken by the Clementine
mission in 1994, it appears that four mountainous regions on
the rim of the 73 km-wide Peary crater at the Moon's north pole remain illuminated for the
entire lunar day. These
peaks of eternal light are possible
because of the Moon's extremely small axial tilt to the
ecliptic plane.
No similar regions of
eternal light were found at the south pole, although the rim of
Shackleton
crater is illuminated for about 80% of the lunar
day. Other consequences of the Moon's small axial tilt are
regions that remain in permanent shadow at the bottoms of many
polar craters.
Impact craters
The surface of Earth's Moon is marked by
impact craters which form when asteroids and
comets collide with the lunar surface. There are about half a
million craters with diameters greater than 1 km on the Moon.
Since impact craters accumulate at a nearly constant rate, the
number of craters per unit area superposed on a geologic unit can
be used to estimate the age of the surface (see
crater counting). The lack of an atmosphere,
weather and recent geological processes ensures that many of these
craters have remained relatively well preserved in comparison to
those on Earth.
The largest crater on the Moon, which also has the distinction of
being one of the largest known craters in the Solar System, is the
South Pole-Aitken basin. It
is on the far side, between the South Pole and equator, and is some
2,240 km in diameter and 13 km in depth.
Prominent impact
basins on the near side include Imbrium, Serenitatis, Crisium, and
Nectaris.
Regolith
Blanketed atop the Moon's crust is a highly
comminuted (broken into ever smaller particles)
and "impact gardened" surface layer called
regolith. Since the regolith forms by impact
processes, the regolith of older surfaces is generally thicker than
for younger surfaces. In particular, it has been estimated that the
regolith varies in thickness from about 3–5 m in the maria,
and by about 10–20 m in the highlands. Beneath the finely
comminuted regolith layer is what is generally referred to as the
megaregolith. This layer is much thicker (on the order of
tens of kilometres) and comprises highly fractured bedrock.
Astronauts have reported that the
dust from the
surface felt like snow and smelled like spent
gunpowder. The dust is mostly made of
silicon dioxide glass (SiO
2),
most likely created from the meteors that have crashed into the
Moon's surface. It also contains
calcium and
magnesium.
Presence of water
The continuous bombardment of the Moon by
comets and
meteoroids has
most likely added small amounts of water to the lunar surface. If
so, sunlight would split much of this water into its constituent
elements of hydrogen and oxygen, both of which would ordinarily
escape into space over time, because of the Moon's weak gravity.
However,
because of the slightness of the axial tilt of the Moon's spin axis
to the ecliptic plane—only 1.5°—some deep craters near the poles
never receive direct light from the Sun and are thus in permanent
shadow (see Shackleton
crater). Water molecules that ended up in these
craters could be stable for long periods of time.
Clementine has mapped craters at the lunar south pole that are
shadowed in this way, and computer simulations suggest that up to
14,000 km² might be in permanent shadow. Results from the
Clementine mission bistatic radar
experiment are consistent with small, frozen pockets of water close
to the surface, and data from the
Lunar
Prospector neutron spectrometer indicate that anomalously high
concentrations of hydrogen are present in the upper metre of the
regolith near the polar regions. Estimate for the quantity of water
on the Moon is 32 ounces per one ton of top layer of Moon's
surface.
Water ice can be mined and then split into its constituent hydrogen
and oxygen atoms by means of nuclear generators or electric power
stations equipped with solar panels. The presence of usable
quantities of water on the Moon is an important factor in rendering
lunar habitation cost-effective,
since transporting water from Earth would be prohibitively
expensive.
However, recent observations made with the
Arecibo planetary radar suggest that some of the near-polar
Clementine radar data that were previously interpreted as being
indicative of water ice might instead be a result of rocks ejected
from young impact craters. The question of how much water
there is on the Moon has not been resolved.
In July 2008, small amounts of water were found in the interior of
volcanic pearls from the Moon (brought to Earth in 1971 by the
Apollo 15 astronauts).
On
September 24, 2009, the Indian Space Research
Organisation (ISRO) reported that their
first lunar mission, Chandrayaan-1
using NASA's Moon Mineralogy Mapper, found
evidence of large quantities of water on the Moon's surface, and
that water is still presently being formed. The instrument
observed an
absorption line in the
spectrum of sunlight reflected
from the Moon, indicating that light of a particular wavelength
(around 2.8
microns) is being absorbed more
readily than other nearby wavelengths. The position and shape of
the line indicate the absorption is due to water. A nearby line
also revealed the presence of the closely-related molecule
hydroxyl, which consists of an oxygen atom with a
single hydrogen atom. The exact abundance of water was not
determined, but the team believed it could be as high as 1,000
parts per million in the top layer
of Lunar soil.
On November 13, 2009, NASA announced the results of the
Lunar Crater Observation and Sensing Satellite,
saying that "not just water, but lots of water" had been found by
the mission near the southern pole.
Physical characteristics
Internal structure
Schematic illustration of the internal
structure of the Moon
The Moon is a
differentiated body, being
composed of a geochemically distinct
crust,
mantle, and
core. This structure is hypothesized to have
resulted from the
fractional
crystallization of a
magma
ocean shortly after its formation, at about 4.4 billion years
ago. The energy required to melt the outer portion of the Moon is
commonly attributed to a
giant
impact event that is postulated to have formed the Earth-Moon
system, and the subsequent reaccretion of material in Earth orbit.
Crystallization of this magma ocean would have given rise to a
mafic mantle and a
plagioclase-rich crust (see
Origin and
geologic evolution below).
Geochemical mapping from orbit implies that the crust of the Moon
is largely
anorthositic in composition,
consistent with the magma ocean hypothesis. In terms of elements,
the crust is composed primarily of
oxygen
(41% to 46% by mass),
silicon (21%),
magnesium (6%),
iron
(13%),
calcium (8%), and
aluminium (7%). Based on geophysical techniques,
its thickness is estimated to be on average about 50 km.
Partial melting within the mantle of the Moon gave rise to the
eruption of mare basalts on the lunar surface. Analyses of these
basalts indicate that the mantle is composed predominantly of the
minerals
olivine,
orthopyroxene and
clinopyroxene, and that the lunar mantle is
more iron rich than that of the Earth. Some lunar basalts contain
high abundances of
titanium (present in the
mineral
ilmenite), suggesting that the
mantle is highly heterogeneous in composition. Moonquakes have been
found to occur deep within the mantle of the Moon about a thousand
kilometres below the surface. These occur with monthly
periodicities and are related to tidal stresses caused by the
eccentric orbit of the Moon about the Earth.
The Moon has a mean density of 3 346.4 kg/m³, making it
the second densest moon in the Solar System after
Io. Nevertheless, several lines of evidence imply
that the core of the Moon is small, with a radius of about
350 km or less. This corresponds to only about 20% the size of
the Moon, in contrast to about 50% as is the case for most other
terrestrial bodies. The composition of the lunar core is not well
constrained, but most believe that it is composed of metallic iron
alloyed with a small amount of
sulfur and
nickel. Analyses of the Moon's time-variable
rotation indicate that the core is at least partly molten.
Topography
Topography of the Moon, referenced to
the lunar geoid
The
topography of the Moon has been
measured by the methods of laser altimetry and stereo image
analysis, most recently from data obtained during the
Clementine mission. The most visible
topographic feature is the giant far side
South Pole-Aitken basin, which
possesses the lowest elevations of the Moon. The highest elevations
are found just to the north-east of this basin, and it has been
suggested that this area might represent thick
ejecta deposits that were emplaced during an oblique
South Pole-Aitken basin impact event.
Other large impact
basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and
elevated rims. Another distinguishing feature of the Moon's
shape is that the elevations are on average about 1.9 km
higher on the far side than the near side.
Gravity field
The gravitational field of the Moon has been determined through
tracking of radio signals emitted by orbiting spacecraft. The
principle used depends on the
Doppler
effect, whereby the spacecraft acceleration in the
line-of-sight direction can be determined by means of small shifts
in frequency of the radio signal, and the distance from the
spacecraft to a station on Earth. However, because of the Moon's
synchronous rotation it is not
possible to track spacecraft much over the
limb of the Moon, and the farside gravity
field is thus only poorly characterised.
Radial gravitational anomaly at the surface of the Moon
The major characteristic of the Moon's gravitational field is the
presence of
mascons, which are large positive
gravitational anomalies associated with some of the giant
impact basins. These anomalies greatly
influence the orbit of spacecraft about the Moon, and an accurate
gravitational model is necessary in the planning of both manned and
unmanned missions. The mascons are in part due to the presence of
dense mare basaltic lava flows that fill some of the impact basins.
However, lava flows by themselves can not explain the entirety of
the gravitational signature, and uplift of the crust-mantle
interface is required as well. Based on
Lunar Prospector gravitational models, it
has been suggested that some mascons exist that do not show
evidence for mare basaltic volcanism.
The huge expanse of
mare basaltic volcanism associated with Oceanus
Procellarum does not possess a positive gravitational
anomaly.
Magnetic field
The Moon has an external
magnetic
field of the order of one to a hundred
nanotesla—less than one hundredth that of the
Earth, which is
30–60 microtesla. Other major differences are that the Moon
does not currently have a
dipolar magnetic
field (as would be generated by a
geodynamo in its core), and the magnetizations
that are present are almost entirely crustal in origin. One
hypothesis holds that the crustal magnetizations were acquired
early in lunar history when a geodynamo was still operating. The
small size of the lunar core, however, is a potential obstacle to
this theory. Alternatively, it is possible that on an airless body
such as the Moon, transient magnetic fields could be generated
during large impact events. In support of this, it has been noted
that the largest crustal magnetizations appear to be located near
the
antipodes of the giant impact basins.
It has been proposed that such a phenomenon could result from the
free expansion of an impact generated plasma cloud around the Moon
in the presence of an ambient magnetic field.
Atmosphere
The Moon has an atmosphere so thin as to be almost negligible, with
a total atmospheric mass of less than 10
4 kg. The
effective surface pressure of this small mass is around 3
atm."Moon." Encyclopædia Britannica. 2008.
Encyclopædia Britannica Online. 10 Sep. 2008
/www.britannica.com/EBchecked/topic/391266/Moon>. This pressure
varies, of course, with the diurnal moon cycle. One source of its
atmosphere is
outgassing—the release of
gases such as
radon that originate by
radioactive decay processes within the
crust and mantle. Another important source is generated through the
process of
sputtering, which involves the
bombardment of micrometeorites, solar wind ions, electrons, and
sunlight. Gases that are released by sputtering can either
reimplant into the
regolith as a result of
the Moon's gravity, or can be lost to space either by solar
radiation pressure or by being swept away by the solar wind
magnetic field if they are ionised. The elements
sodium (Na) and
potassium
(K) have been detected using earth-based spectroscopic methods,
whereas the element
radon–222
(
222Rn) and
polonium-210
(
210Po) have been inferred from data obtained from the
Lunar Prospector alpha particle spectrometer.
Argon–40 (
40Ar),
helium-4 (
4He),
oxygen (O
2) and/or
methane (CH
4),
nitrogen (N
2) and/or
carbon monoxide (CO), and
carbon dioxide (CO
2) were detected
by in-situ detectors placed by the Apollo astronauts.
Surface temperature
During the lunar day, the surface temperature averages 107 °C,
and during the lunar night, it averages −153 °C.
Origin and geologic evolution
Formation
Several mechanisms have been suggested for the Moon's formation.
The formation of the Moon is hypothesized to have occurred
4.527 ± 0.010 billion years ago, about 30–50 million years
after the origin of the Solar System.
- Fission hypothesis
- Early
speculation proposed that the Moon broke off from the Earth's crust
because of centrifugal forces,
leaving a basin presumed to be the Pacific Ocean behind as a scar. This idea, however, would
require too great an initial spin of the Earth and also would have
resulted in the Moon's orbit following Earth's equatorial plane rather than its current
path.
- Capture hypothesis
- Other speculation has centered on the Moon being formed
elsewhere and subsequently being captured by Earth's gravity.
However, the conditions conjectured necessary for such a mechanism
to work, such as an extended
atmosphere of the Earth in order to dissipate the energy of the passing Moon, are
improbable.
- Co-formation hypothesis
- The co-formation hypothesis proposes that the Earth and the
Moon formed together at the same time and place from the primordial
accretion disk. The Moon would have
formed from material surrounding the proto-Earth, similar to the
formation of the planets around the Sun. Some suggest that this
hypothesis fails to adequately explain the depletion of metallic
iron in the Moon.
- A major deficiency in all these hypotheses is that they cannot
readily account for the high angular
momentum of the Earth–Moon system.
- Giant impact hypothesis
- The prevailing hypothesis today is that the Earth–Moon system
formed as a result of a giant
impact. A Mars-sized body (labelled
"Theia") is
hypothesized to have hit the proto-Earth, blasting sufficient
material into orbit around the proto-Earth to form the Moon through
accretion. As accretion is the process by which all planetary
bodies are therorized to have formed, giant impacts are thought to
have affected most if not all planets. Computer simulations
modelling a giant impact are consistent with measurements of the
angular momentum of the Earth–Moon
system, as well as the small size of the lunar core. Unresolved
questions regarding this theory concern the determination of the
relative sizes of the proto-Earth and Theia and of how much
material from these two bodies formed the Moon. Recent oxygen
isotope composition analysis of the Moon shows its oxygen isotope
composition is more similar to the Earth's than this hypothesis
would suggest.
Lunar magma ocean
As a result of the large amount of energy converted during both the
giant impact event and the subsequent reaccretion of material in
Earth orbit, it is commonly hypothesized that a large portion of
the Moon was once initially molten. The molten outer portion of the
Moon at this time is referred to as a
magma ocean, and estimates for its depth
range from about 500 km to the entire radius of the
Moon.
As the magma ocean cooled, it fractionally crystallised and
differentiated, giving
rise to a geochemically distinct crust and mantle. The mantle is
inferred to have formed largely by the precipitation and sinking of
the minerals
olivine,
clinopyroxene, and
orthopyroxene. After about three-quarters of
magma ocean crystallisation was complete, the mineral
anorthite is inferred to have precipitated and
floated to the surface because of its low density, forming the
crust.
The final liquids to crystallise from the magma ocean would have
been initially sandwiched between the crust and mantle, and would
have contained a high abundance of incompatible and heat-producing
elements.
This geochemical component is referred to by
the acronym KREEP, for potassium (K), rare
earth elements (REE), and phosphorus
(P), and appears to be concentrated within the Procellarum KREEP Terrane, which is a small
geologic province that encompasses most of Oceanus
Procellarum and Mare
Imbrium on the near side of the Moon.
Geologic evolution
A large portion of the Moon's post–magma-ocean geologic evolution
was dominated by impact cratering.
The lunar geologic timescale is largely
divided in time on the basis of prominent basin-forming impact
events, such as Nectaris, Imbrium, and Orientale. These impact structures are characterised
by multiple rings of uplifted material, and are typically hundreds
to thousands of kilometres in diameter. Each multi-ring basin is
associated with a broad apron of ejecta deposits that forms a
regional stratigraphic horizon. While only a few multi-ring basins
have been definitively dated, they are useful for assigning
relative ages on the basis of
stratigraphic grounds. The continuous effects
of impact cratering are responsible for forming the
regolith.
The other major geologic process that affected the Moon's surface
was
mare volcanism. The enhancement of
heat-producing elements within the
Procellarum KREEP Terrane is thought to have
caused the underlying mantle to heat up, and eventually, to
partially melt. A portion of these magmas rose to the surface and
erupted, accounting for the high concentration of mare basalts on
the near side of the Moon. Most of the Moon's
mare basalts erupted during the Imbrian period in
this geologic province 3.0–3.5 billion years ago. Nevertheless,
some dated samples are as old as 4.2 billion years, and the
youngest eruptions, based on the method of
crater counting, are hypothesized to have
occurred only 1.2 billion years ago.
There has been controversy over whether features on the Moon's
surface undergo changes over time. Some observers have claimed that
craters either appeared or disappeared, or that other forms of
transient phenomena had occurred. Today, many of these claims are
thought to be illusory, resulting from observation under different
lighting conditions, poor
astronomical seeing, or the inadequacy
of earlier drawings. Nevertheless, it is known that the phenomenon
of
outgassing does occasionally occur,
and these events could be responsible for a minor percentage of the
reported
lunar transient
phenomena. Recently, it has been suggested that a roughly
3 km diameter region of the lunar surface was modified by a
gas release event about a million years ago.
Moon rocks
Moon rocks fall into two main categories, based on whether they
underlie the lunar highlands (terrae) or the maria. The lunar
highlands rocks are composed of three suites: the
ferroan
anorthosite suite, the
magnesian suite, and the
alkali suite (some consider the alkali suite to be a
subset of the mg-suite). The ferroan anorthosite suite rocks are
composed almost exclusively of the mineral
anorthite (a calic
plagioclase feldspar), and are
hypothesized to represent plagioclase flotation cumulates of the
lunar magma ocean. The ferroan anorthosites have been dated using
radiometric methods to have formed about 4.4 billion years
ago.
The mg- and alkali-suite rocks are predominantly mafic plutonic
rocks. Typical rocks are
dunites,
troctolites,
gabbros,
alkali
anorthosites, and more rarely,
granite. In contrast to the ferroan
anorthosite suite, these rocks all have relatively high Mg/Fe
ratios in their mafic minerals. In general, these rocks represent
intrusions into the already-formed highlands crust (though a few
rare samples appear to represent extrusive lavas), and they have
been dated to have formed about 4.4–3.9 billion years ago.
Many of these rocks have high abundances of, or are genetically
related to, the geochemical component
KREEP.
The lunar maria consist entirely of mare basalts. While similar to
terrestrial basalts, they have much higher abundances of iron, are
completely lacking in
hydrous
alteration products, and have a large range of titanium
abundances.
Orbit and relationship to Earth
The Moon makes a complete orbit around the Earth with respect to
the fixed stars about once every 27.3 days(its
sidereal period). However, since the Earth
is moving in its orbit about the Sun at the same time, it takes
slightly longer for the Moon to show its same
phase to Earth, which is about 29.5 days
(its
synodic period). Unlike most
satellites of other planets, the Moon orbits near the
ecliptic and not the Earth's
equatorial plane. It is the largest moon in
the solar system relative to the size of its planet. (
Charon is larger relative to the
dwarf planet Pluto.) The
natural satellites orbiting other
planets are called "moons", after Earth's Moon.
Most of the tidal effects seen on the Earth are caused by the
Moon's gravitational pull, with the Sun making a somewhat smaller
contribution. Tidal drag slows the Earth's rotation by about 0.002
seconds per day per century. As a result of the
conservation of angular
momentum, the slowing of Earth's rotation is accompanied by an
increase of the mean Earth-Moon distance of about 3.8 m per
century, or 3.8 cm per year.The Moon is exceptionally large
relative to the Earth, being a quarter the diameter of the planet
and 1/81 its mass. However, the Earth and Moon are still commonly
considered a planet-satellite system, rather than a double-planet
system, since the common centre of mass of the system (the
barycentre) is located about 1,700 km
beneath the surface of the Earth, or about a quarter of the Earth's
radius. The surface of the Moon is less than one-tenth that of the
Earth, and only about a quarter the size of the Earth's land area
(or about as large as Russia, Canada, and the U.S. combined).
The current
obliquity of the Moon means
that the Sun never rises above 1.85° at the poles. The axial tilt
of the Moon has remained at its present orientation for the past
two billion years, allowing the craters at the poles to remain in
permanent shadow for that length of time. Prior to that point, the
Moon had much larger values for its obliquity, possibly reaching
angles as high as 77° for periods of several hundred thousand
years.
In 1997, the asteroid
3753 Cruithne
was found to have an unusual Earth-associated
horseshoe orbit. However, astronomers do not
consider it to be a second moon of Earth, and its orbit is not
stable in the long term. Three other
near-Earth asteroids,
54509 YORP,
1998
UP1 and
2002 AA29, which exist in
orbits similar to Cruithne's, have since been discovered.
The relative sizes and separation of
the Earth–Moon system are shown to scale above.
The beam of light is depicted travelling between the Earth and
the Moon in the same time it actually takes light to scale the real
distance between them: 1.255 seconds at its mean orbital
distance.
The light beam helps provide the sense of scale of the
Earth-Moon system relative to the Sun, which is 8.28 light-minutes
away (photosphere to Earth surface).
Diagram illustrating various phases of
the Moon in their order of appearance stating from the New Moon and
progressing through Crescent, First Quarter, and Gibbous to reach
the Full Moon.
It is followed by Gibbous, Last Quarter and Crescent to
complete full circle at the New Moon again.
Ocean tides
Earth's
ocean tides are initiated by the
tidal force (a gradient in
intensity) of Moon's gravity and are magnified by a host of effects
in Earth's oceans. The gravitational tidal force arises because the
side of Earth facing the Moon (nearest it) is attracted more
strongly by the Moon's gravity than is the center of the Earth
and—even less so—the Earth's far side. The gravitational tide
stretches the Earth's oceans into an ellipse with the Earth in the
center. The effect takes the form of two
bulges—elevated
sea level relative to the Earth; one nearest the Moon and one
farthest from it. Since these two bulges rotate around the Earth
once a day as it spins on its axis, ocean water is continuously
rushing towards the ever-moving bulges. The effects of the two
bulges and the massive ocean currents chasing them are magnified by
an interplay of other effects; namely frictional coupling of water
to Earth's rotation through the ocean floors, inertia of water's
movement, ocean basins that get shallower near land, and
oscillations between different ocean basins. The magnifying effect
is a bit like water sloshing high up the sloped end of a bathtub
after a relatively small disturbance of one's body in the deep part
of the tub.
Gravitational coupling between the Moon and the ocean bulge nearest
the Moon affects its orbit. The Earth rotates on its axis in the
very same direction, and roughly 27 times faster, than the Moon
orbits the Earth. Thus, frictional coupling between the sea floors
and ocean waters, as well as water's
inertia, drags the peak of the near-Moon tidal bulge
slightly forward of the imaginary line connecting the centers of
the Earth and Moon. From the Moon's perspective, the center of mass
of the near-Moon tidal bulge is perpetually slightly
ahead
of the point about which it is orbiting. Precisely the opposite
effect occurs with the bulge farthest from the Moon; it lags
behind the imaginary line. However it is 12,756 km
farther away and has slightly less gravitational coupling to the
Moon. Consequently, the Moon is constantly being gravitationally
attracted forward in its orbit about the Earth. This gravitational
coupling drains
kinetic energy and
angular momentum from the Earth's
rotation (see also,
Day and
Leap second). In turn, angular momentum is added to
the
Moon's orbit, which lifts the
Moon into a higher orbit with a longer period. The effect on the
Moon's orbital radius is a small one, just 0.10
ppb/year, but results in a measurable
3.82 cm annual increase in the Earth-Moon distance.
Cumulatively, this effect becomes ever more significant over time;
since
astronauts first landed on the Moon
approximately years ago, it is metres farther
away.
Eclipses
The 1999 solar eclipse
Eclipses can occur only when the Sun, Earth, and Moon are all in a
straight line.
Solar eclipses occur
near a
new Moon, when the Moon is between
the Sun and Earth. In contrast,
lunar
eclipses occur near a
full Moon, when
the Earth is between the Sun and Moon.
Because the Moon's orbit around the Earth is inclined by about 5°
with respect to the
orbit of the 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 the Earth, is described by the
saros cycle, which has a period of approximately
6 585.3 days (18 years 11 days 8 hours).
The angular diameters of the Moon and the Sun as seen from Earth
overlap in their variation, so that both
total and
annular solar eclipses are possible. In a
total eclipse, the Moon completely covers the disc of the Sun and
the solar
corona becomes visible to the
naked eye. Since the distance between the
Moon and the Earth is very slightly increasing over time, the
angular diameter of the Moon is decreasing. This means that
hundreds of millions of years ago the Moon could always completely
cover the Sun on solar eclipses so that no annular eclipses were
possible. Likewise, about 600 million years from now (assuming
that the angular diameter of the Sun will not change), the Moon
will no longer cover the Sun completely and only annular eclipses
will occur.
A phenomenon related to eclipse is
occultation. The Moon is continuously blocking
our view of the sky by a 1/2 degree-wide circular area. When a
bright star or planet
passes behind the Moon it is
occulted or hidden from view. A solar eclipse is an
occultation of the Sun. Because the Moon is close to Earth,
occultations of individual stars are not visible everywhere, nor at
the same time. Because of the precession of the lunar orbit, each
year different stars are occulted.
Observation
During its brightest phase, at "full Moon", the Moon has an
apparent magnitude of about
−12.6. By comparison, the Sun has an apparent magnitude of −26.8.
When the Moon is in a quarter phase, its brightness is not half of
a full Moon, but only about a tenth. This is because the lunar
surface is not a perfect
Lambertian reflector. When the Moon
is full the
opposition effect
makes it appear brighter, but away from full there are shadows
projected onto the surface which diminish the amount of reflected
light.
On average, the Moon covers an area of 0.21078 square degrees on
the night sky.
The Moon appears larger when close to the horizon. This is a purely
psychological effect (see
Moon
illusion). It is actually about 1.5% smaller when the Moon is
near the horizon than when it is high in the sky (because it is
farther away by up to one Earth radius).
The Moon appears as a relatively bright object in the sky, in spite
of its low
albedo. The Moon is about the
poorest
reflector in the
solar system and reflects only about 7% of the
light incident upon it (about the same proportion as is reflected
by a lump of
coal). However, the Moon is not a
Lambertian
scatterer and reflects more light back towards the Sun (albedo
of 12%) than in other directions because of the spherical glass
beads in the
moondust. This increases the
brightness of a full Moon. It also has the effect of making the
edges of a full Moon seem about as bright as the centre. Besides
this,
color constancy in the
visual system recalibrates the relations
between the colours of an object and its surroundings, and since
the surrounding sky is comparatively dark the sunlit Moon is
perceived as a bright object.
The highest
altitude of the
Moon on a day varies and has nearly the same limits as the Sun. It
also depends on the Earth season and lunar phase, with the full
Moon being highest in winter. Moreover, the 18.6 year nodes cycle
also has an influence, as when the ascending node of the lunar
orbit is in the vernal equinox, the lunar declination can go as far
as 28° each month (which happened most recently in 2006). This
results that the Moon can go overhead on latitudes up to 28 degrees
from the equator (e.g.
Florida, Canary
Islands or in the southern hemisphere Brisbane). Slightly more than 9 years later (next
time in 2015) the declination reaches only 18° N or S each
month.The orientation of the Moon's crescent also depends on the
latitude of the observation site. Close to the equator, an observer
can see a
boat Moon.
Like the Sun, the Moon can give rise to atmospheric effects,
including a 22°
halo ring,
and the smaller
coronal rings
seen more often through thin clouds. For more information on how
the Moon appears in Earth's sky, see
lunar
phase.
Exploration
The first leap in lunar observation was prompted by the invention
of the telescope.
Galileo Galilei
made good use of this new instrument and observed mountains and
craters on the Moon's surface.
The
Cold War-inspired
space race between the Soviet Union and the U.S.
led to an acceleration of interest in the Moon. Unmanned probes,
both flyby and impact/lander missions, were sent almost as soon as
launcher capabilities would allow. The Soviet Union's
Luna program was the first to reach the Moon
with unmanned
spacecraft. The first
man-made object to escape Earth's gravity and pass near the Moon
was
Luna 1, the first man-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. Moon samples
have been brought back to Earth by three Luna missions (
Luna 16,
20, and
24) and the Apollo missions 11 to 17 (except
Apollo 13, which aborted its planned lunar
landing).
The 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 July 21, 1969. The American
Moon landing and return was enabled by
considerable technological advances, in domains such as
ablation chemistry and
atmospheric re-entry technology, in the
early 1960s.
Scientific instrument packages were installed on the lunar surface
during all of the
Apollo missions. Long-lived
ALSEP stations (Apollo lunar surface experiment
package) were installed at the
Apollo 12,
14,
15,
16, and
17 landing sites,
whereas a temporary station referred to as EASEP (Early Apollo
Scientific Experiments Package) was installed during the Apollo 11
mission. The ALSEP stations contained, among others, heat flow
probes, seismometers, magnetometers, and corner-cube
retroreflectors. Transmission of data to Earth was terminated on
September 30, 1977 because of budgetary considerations. Since the
lunar laser ranging
(LLR) corner-cube arrays are passive instruments, they are still
being used. Ranging to the LLR 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.
have now passed since Eugene Cernan and Harrison Schmitt, as part of the mission Apollo 17, left the surface of the Moon on December 14, 1972 (Cernan being the last to enter the LM) and no one has set foot on it since.
From the mid-1960s to the mid-1970s, there were 65 instances of
artificial objects reaching the Moon (both manned and robotic, with
ten in 1971 alone), with the last being
Luna
24 in 1976. Only 18 of these were controlled
Moon landings, with nine completing a round
trip from Earth and returning samples of
Moon
rocks. The Soviet Union then turned its primary attention to
Venus and
space
stations, and the U.S. to
Mars and beyond.
In 1990, Japan orbited the Moon with the
Hiten spacecraft, becoming the third country to
place a spacecraft into lunar orbit. The spacecraft released a
smaller probe,
Hagormo, in lunar orbit, but the
transmitter failed, thereby preventing further scientific use of
the mission.
In 1994, the U.S. finally returned to the Moon, robotically at
least, sending the Joint Defense Department/NASA spacecraft
Clementine. 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 by the
Lunar Prospector mission in 1998. The
neutron spectrometer on
Lunar Prospector
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 European spacecraft
Smart
1 was launched September 27, 2003 and was in lunar orbit
from November 15, 2004 to September 3, 2006.
On January 14, 2004, U.S. President
George W. Bush
called for a plan to resume manned missions to the Moon by 2020
(see
Vision for Space
Exploration). NASA is now planning for the construction of a
permanent outpost at one of the lunar poles. The People's Republic
of China has expressed ambitious plans for exploring the Moon and
has started the
Chang'e program for
lunar exploration, successfully launching its first spacecraft,
Chang'e-1, on October 24, 2007. Like NASA,
China hopes to land people on the Moon by 2020. The U.S. launched
the
Lunar
Reconnaissance Orbiter and the
Lunar Crater Observation and
Sensing Satellite on June 18, 2009 (the two missions were
co-manifested). Russia also announced to resume its previously
frozen project
Luna-Glob,
consisting of an unmanned lander and orbiter, which is slated to
land in 2012.
The
Google Lunar X Prize,
announced September 13, 2007, hopes to boost and encourage
privately funded lunar exploration. The
X Prize Foundation is offering anyone
US$20 million who can land a robotic rover on the Moon and meet
other specified criteria.
On September 14, 2007 the
Japan Aerospace Exploration
Agency launched
SELENE, also known as
Kaguya, a lunar orbiter which is fitted with a
high-definition camera and two small
satellites. The mission is expected to last one year.
On October 22, 2008 India successfully launched the
Chandrayaan I (a
Sanskrit word literally meaning the 'Moon-craft')
unmanned mission to the Moon and intends to launch several further
unmanned missions. The country plans to launch
Chandrayaan
II in 2010 or 2011, which is slated to include a robotic lunar
rover. India also has expressed its hope for a manned mission to
the Moon by 2020.
Human understanding
The Moon has been the subject of many works of art and literature
and the inspiration for countless others. It is a motif in the
visual arts, the performing arts, poetry, prose and music.
A
5000-year-old rock carving at Knowth, Ireland may represent the Moon, which would be the earliest
depiction discovered. In many prehistoric and ancient
cultures, the Moon was thought to be a
deity or other
supernatural phenomenon, and
astrological views of the Moon continue to
be propagated today.
Among the first in the Western world to offer a scientific
explanation for the Moon was the
Greek philosopher
Anaxagoras (d. 428 BC), who reasoned that the Sun
and Moon were both giant spherical rocks, and that the latter
reflected the light of the former. His atheistic view of the
heavens was one cause for his imprisonment and eventual
exile.
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. This separation
was held to be part of physics for many centuries after.
Aristarchus went a step further and
computed the
distance from earth, together with its size, obtaining a value of
20 earth radius for the distance (the real value is 60. The earth
radius was known since
Eratosthenes)
During the
Warring States of China,
astronomer
Shi Shen (fl. 4th century BC)
gave instructions for predicting solar and lunar eclipses based on
the relative positions of the Moon and Sun. Although the Chinese of
the
Han Dynasty (202 BC–202 AD) 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 (mentioned by Anaxagoras
above). This was supported by mainstream thinkers such as
Jing Fang (78–37 BC) and
Zhang Heng (78–139 AD), but it was also opposed
by the influential philosopher
Wang Chong
(27–97 AD). Jing Fang noted the sphericity of the Moon, while Zhang
Heng accurately described a lunar eclipse and solar eclipse. These
assertions were supported by
Shen Kuo
(1031–1095) of the
Song Dynasty
(960–1279) who 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. He also noted that the reason for the Sun and Moon not
eclipsing every time their paths met was because of a small
obliquity in their orbital paths.
By the
Middle Ages, before the invention
of the telescope, more and more people began to recognise the Moon
as a sphere, though they believed that it was "perfectly smooth".
In 1609,
Galileo Galilei drew one of
the first telescopic drawings of the Moon in his book and noted
that it was not smooth but had mountains and craters. Later in the
17th century,
Giovanni
Battista Riccioli and
Francesco Maria Grimaldi drew a map
of the Moon and gave many craters the names they still have
today.
On maps, the dark parts of the Moon's surface were called
maria (singular
mare) or seas, and the light
parts were called
terrae or continents.The possibility
that the Moon contains vegetation and is inhabited by selenites was
seriously considered by major astronomers even into the first
decades of the 19th century. The contrast between the brighter
highlands and darker maria create the patterns seen by different
cultures as the
Man in the Moon, the
rabbit and the buffalo, among
others.
In 1835, the
Great Moon Hoax fooled
some people into thinking that there were exotic animals living on
the Moon. Almost at the same time however (during 1834–1836),
Wilhelm Beer and
Johann Heinrich Mädler were
publishing their four-volume and the book in 1837, which firmly
established the conclusion that the Moon has no bodies of water nor
any appreciable atmosphere.
The far side of the Moon remained completely unknown until the
Luna 3 probe was launched in 1959, and it was
extensively mapped by the
Lunar
Orbiter program in the 1960s.
Legal status
Although
several pennants of the Soviet Union were scattered by Luna 2 in
1959 and by later landing missions, and U.S.
flags have been symbolically planted on the Moon, no nation
currently claims ownership of any part of the Moon's
surface. Russia and the U.S. are party to the
Outer Space Treaty, which places the Moon
under the same jurisdiction as
international waters ( ). This treaty
also restricts the use of the Moon to peaceful purposes, explicitly
banning military installations and
weapons of mass destruction
(including
nuclear weapons).
A second treaty, the
Moon Treaty, was
proposed to restrict the exploitation of the Moon's resources by
any single nation, but it has not been signed by any of the
space-faring nations. Several
individuals have made
claims to the Moon in whole or
in part, although none of these are generally considered
credible.
See also
Notes
References
Footnotes
External links
- Images and maps
- Exploration
- Moon phases
- Others
- Cartographic resources
- Movies
- Movie of the Moon at National Oceanic and Atmospheric
Administration