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Moon

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

                                                             CAPTION: Moon

The Moon as seen by an observer from Earth

                                            The Moon as seen from Earth
                                                 Orbital characteristics
Semi-major axis                                               384,399 km
                                                            (0.00257 AU)
Perigee                                                       363,104 km
                                                             (0.0024 AU)
Apogee                                                        405,696 km
                                                             (0.0027 AU)
Orbital circumference                                       2,413,402 km
                                                              (0.016 AU)
Eccentricity                                                      0.0549
Revolution period
   ( Sidereal period)                                        27.321582 d
                                                     (27 d 7 h 43.1 min)
Synodic period                                               29.530588 d
                                                    (29 d 12 h 44.0 min)
Avg. Orbital Speed                                            1.022 km/s
Max. Orbital Speed                                            1.082 km/s
Min. Orbital Speed                                            0.968 km/s
Inclination                      between 18.29° and 28.58°
                                   to Earth equatorial plane;
                                                      5.145° to ecliptic
Longitude of the
       ascending node                                        regressing,
                                              1 revolution in 18.6 years
Argument of perigee                                         progressing,
                                              1 revolution in 8.85 years
Satellite of                                                       Earth
                                                Physical characteristics
Mean diameter
          Mean radius                         3,474.13 km (0.273 Earths)
                                              1,737.06 km (0.273 Earths)
Equatorial diameter
    Equatorial radius                         3,476.28 km (0.273 Earths)
                                              1,738.14 km (0.273 Earths)
Polar diameter
         Polar radius                         3,471.94 km (0.273 Earths)
                                              1,735.97 km (0.273 Earths)
Oblateness                                                       0.00125
Surface area                                            3.793×10^7 km²
                                                          (0.074 Earths)
Volume                                                2.1958×10^10 km³
                                                          (0.020 Earths)
Mass                                                    7.3477×10^22 kg
                                                         (0.0123 Earths)
Mean density                                              3,346.4 kg/m^3
Equatorial gravity                                           1.622 m/s^2
                                                              (0.1654 g)
Escape velocity                                                2.38 km/s
Rotation period                                              27.321582 d
                                                          ( synchronous)
Rotation velocity                                            16.655 km/h
                                                        (at the equator)
Axial tilt                                        6.688° to orbit plane
                                                      1.543° to ecliptic
Albedo                                                              0.12
Magnitude                                                         -12.74
Angular size                                              from 29′to 33′
Surface temp.


                      min  mean   max
                      40 K 250 K 396 K

                               Bulk silicate composition (estimated wt%)
SiO[2]                                                            44.4 %
Al[2]O[3]                                                         6.14 %
FeO                                                               10.9 %
MgO                                                               32.7 %
CaO                                                                4.6 %
Na[2]O                                                           0.092 %
K[2]O                                                             0.01 %
Cr[2]O[3]                                                         0.61 %
MnO                                                               0.15 %
TiO[2]                                                            0.31 %
                                             Atmospheric characteristics
Atmospheric density                           10^7 particles cm^-3 (day)
                                            10^5 particles cm^-3 (night)

   The Moon is Earth's only natural satellite. It has no formal English
   name other than "the Moon", although it is occasionally called Luna (
   Latin: moon), or Selene ( Greek: moon), to distinguish it from the
   generic term "moon" (referring to any of the various natural satellites
   of other planets). Its symbol is a crescent (☽). The related adjective
   for the Moon is lunar (again from the Latin root), but this is not
   found in combination with the forms seleno-/-selene (again from the
   Greek) and -cynthion (from the Lunar deity Cynthia).

   The average distance from the Earth to the Moon is 384,399  kilometres
   (238,854 mi), which is about 30 times the diameter of the Earth. At
   this distance, it takes sunlight reflected from the lunar surface
   approximately 1.3 seconds to reach Earth. The Moon's diameter is
   3,474 kilometres (2,159 mi), which is about 3.7 times smaller than the
   Earth, making it the Solar System's fifth largest moon, both by
   diameter and mass, ranking behind Ganymede, Titan, Callisto, and Io.

   The Soviet Union's ( USSR) Luna program was the first to reach the Moon
   with unmanned vehicles, or space probes: The first man-made object to
   escape Earth's gravity and pass near the Moon was Luna 1 in 1959. The
   first man-made object to impact the lunar surface was Luna 2, also in
   1959. The first photographs of the normally occluded far side of the
   Moon were made by Luna 3 in the same year. The first spacecraft to
   perform a successful lunar soft landing was Luna 9 in 1966. The first
   (unmanned) vehicle to orbit the Moon was Luna 10 in 1966.

   The United States' Apollo program achieved the first (and only) manned
   missions to the Moon: The first manned mission to orbit the Moon was
   Apollo 8 in 1968, and the first people to land and walk on the Moon
   came aboard Apollo 11 in 1969. The unsuccessful Soviet manned Moon
   project was abandoned in 1974. The Moon is the only celestial body
   other than the Earth upon which humans have set foot.

The two sides of the Moon

   The Moon is in synchronous rotation, meaning that it keeps nearly the
   same face turned toward Earth at all times. Small variations resulting
   from the finite eccentricity of the lunar orbit, termed optical
   librations, allow up to about 59% of the lunar surface to be visible
   from Earth. The side of the Moon that faces Earth is called the near
   side, and the opposite side is called the far side. The far side should
   not be confused with the dark side, as the unilluminated hemisphere
   only corresponds to the far side during full moon. Spacecraft are cut
   off from direct radio communication with Earth when behind the Moon
   since electromagnetic waves propogate in straight lines (see
   line-of-sight propagation).

   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 (singular: mare), which are the dark
   albedo features. Only about 2% of the surface of the far side is
   covered by maria, compared to 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.
     90° W  Near side
   PIA00305 PIA00302
   PIA00303 PIA00304
     90° E  Far side

Orbit and relationship to Earth

   The Moon makes a complete orbit about the Earth with respect to the
   fixed stars (its sidereal period) approximately once every 27.3 days.
   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.

   The Earth and Moon have many physical effects upon one another,
   including the tides. Most of the tidal effects seen on the Earth are
   caused by the Moon's gravitational pull, with the Sun making only a
   small contribution. Tidal effects result in an increase of the mean
   Earth-Moon distance, over long periods of time, of about 4 meters per
   century. As a result of the conservation of angular momentum, the
   increasing semimajor axis of the Moon is accompanied by a gradual
   slowing of the Earth's rotation.

   The Earth-Moon system may be considered to be a double planet rather
   than a planet-moon system. This is due to the exceptionally large size
   of the Moon relative to its host planet; the Moon is one-fourth the
   diameter of Earth and 1/81 the mass. However, as the barycenter is
   located within the Earth, the Earth-Moon system does not meet the
   official IAU definition of a double planet. The surface of the Moon is
   less than 1/10th that of the Earth, and only about one quarter the size
   of Earth's land area (about as large as Russia, Canada, and the United
   States combined).

   In 1997 the asteroid 3753 Cruithne was found to have an unusual
   Earth-associated horseshoe orbit, and has been dubbed by some to be a
   second moon of Earth. It is not considered a moon by astronomers,
   however, and its orbit is not stable in the long term. Three other
   near-Earth asteroids (NEAs), (54509) 2000 PH5, (85770) 1998 UP1 and
   2002 AA29, which exist in orbits similar to Cruithne's, have since been
   discovered.
   The Earth and Moon to scale.
   Enlarge
   The Earth and Moon to scale.

Origin and geologic evolution

   Early speculation proposed that the Moon broke off from the Earth's
   crust due to centrifugal forces, leaving an ocean basin (presumed to be
   the Pacific Ocean) behind as a scar. This fision concept requires too
   great an initial spin of the Earth, and besides, the presumption of a
   Pacific origin is not compatible with the relatively young age of the
   oceanic crust at this locale. Others speculated that the Moon formed
   elsewhere and was captured into Earth's orbit. However, the conditions
   required for this capture mechanism to work (such as an extended
   atmosphere of the Earth for dissipating energy) are not too probable.
   The coformation hypothesis posits that the Earth and the Moon formed
   together at the same time from the primordial accretion disk. In this
   theory, the Moon forms from material surrounding the proto-Earth,
   similar to the way in which the planets formed around the Sun. Some
   suggest that this hypothesis fails to adequately explain the depletion
   of metallic iron in the Moon. A major deficiency with all of these
   hypotheses is that they can not easily account for the high angular
   momentum of the Earth-Moon system.

   Today, the giant impact hypothesis for forming the Earth-Moon system is
   widely accepted by the scientific community. In this theory, the impact
   of a Mars-sized body (which has been referred to as Theia or Orpheus)
   into the proto-Earth is postulated to have put enough material into
   circumterrestrial orbit to form the Moon. Given that planetary bodies
   are believed to have formed by the hierarchical accretion of smaller to
   larger sized bodies, it is now recognized that giant impact events such
   as this should be expected to have occurred for some planets. Computer
   simulations modeling this impact can account for the angular momentum
   of the Earth-Moon system, as well as the small size of the lunar core.
   Unresolved questions concerning this theory are (1) the relative sizes
   of the proto-Earth and impactor, and (2) whether the material that
   makes up the Moon was derived principally from the proto-Earth or
   impactor.

   The formation of the Moon is believed to have occurred at 4.527 ± 0.01
   billion years, which would imply that it formed only 30 to 50 million
   years after the origin of the solar system. The subsequent geologic
   evolution of the Moon was dominated principally by impact cratering,
   but also by mare volcanism. The lunar geologic timescale is divided in
   time based on a few prominent impact events, such as Nectaris, Imbrium,
   Eratosthenes, and Copernicus. While not all of these impacts have been
   definitively dated (and some ages are still being debated), they are
   useful for assigning relative ages based on stratigraphic grounds.

   Most of the Moon's mare basalts erupted during the Imbrian period,
   around 3 to 3.5 billion years ago. Nevertheless, some dated samples are
   as old as 4.2 billion years old, and the youngest eruptions, based on
   the method of crater counting, are believed to have occurred only 1.2
   billion years ago. 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.

Physical characteristics

Structure

   Schematic illustration of the internal structure of the Moon.
   Enlarge
   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 and the compositional
   variations observed from orbit and among the samples are believed to
   have resulted from the fractional crystallization of a magma ocean
   about 4.5 billion years ago.

   A large amount of energy would have been liberated during both the
   giant impact event that formed the Earth-Moon system and the
   reaccretion of material in Earth orbit. It is widely believed that this
   energy would have been sufficient to melt a large portion of the outer
   portion of the Moon, with depths of this magma ocean estimated to be
   between about 500 km to the entire radius of the Moon. Fractional
   crystallization of this magma would have led to a mantle composed
   largely of the minerals olivine, clinopyroxene, and orthopyroxene, and
   after about three-quarters of crystallization was complete, the mineral
   anorthosite would have precipated and floated to the surface because of
   its low density. Estimates for the average thickness of the crust are
   about 50 km, and both lunar samples and geochemical mapping from orbit
   are consistent with the crust being predominantly anorthositic in
   composition.. The final liquids to crystallize 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 potasium (K), rare earth elements (REE), and
   phosphorous (P). KREEP 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.

   In terms of elements, the lunar crust is composed primarily of oxygen,
   silicon, magnesium, iron, calcium, and aluminium. Important minor and
   trace elements include titanium, uranium, thorium, potassium, and
   hydrogen. A complete global mapping of the Moon for the abundance of
   the major and minor elements has not yet been performed. However, some
   spacecraft have done so for portions of the Moon, or for certain
   elements. In particular, a gamma-ray spectrometer onboard the
   spacecraft Lunar Prospector has determined near-global abundances of
   iron, calcium, aluminium, magnesium, titanium, potassium, thorium,
   uranium, and hydrogen. The Clementine spacecraft has obtained
   near-global abundances for iron and titanium, but at a much higher
   spatial resolution.

   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 (which include the Moon's mean density, moment of inertia,
   rotation, and magnetic induction) imply that the lunar core is small,
   with a radius of about 350 km. The radius of the lunar core is only
   about 20% its surface radius, in contrast to most other terrestrial
   bodies that have a core radius close to 50% of their size. The
   composition of the lunar core is not well constrained, but most believe
   that it is composed of metallic iron with a small amount of sulfur,
   though a dense titanium-rich silicate magma is also permissible.

Landscape

   Lunar crater Daedalus.
   Enlarge
   Lunar crater Daedalus.
   The Copernicus impact crater.
   Enlarge
   The Copernicus impact crater.

   When observed with Earth based telescopes, the Moon can be seen to have
   some 30,000 craters having a diameter of at least 1 km, but close up
   observation from lunar orbit reveals a multitude of ever smaller
   craters. Most are hundreds of millions or billions of years old, and
   the lack of an atmosphere, weather and recent geological processes
   ensures that many of them have remained relatively well preserved in
   comparison to their terrestrial counterparts. In many places, it is
   indeed impossible to form a crater without obliterating portions of
   another. The largest crater on the Moon, which also has the distinction
   of being the largest known crater in the solar system, is the South
   Pole-Aitken basin. This impact basin is located on the far side,
   between the South Pole and equator and is some 2,240 kilometres in
   diameter and 13 kilometres in depth.

   The dark and relatively featureless lunar plains are called maria,
   Latin for seas, since they were believed by ancient astronomers to be
   filled by water. They are actually vast ancient basaltic lava flows,
   many of which filled the topographic depressions associated with large
   impact basins ( Oceanus Procellarum is a major exception in that it
   does not correspond to any known impact basin). The lighter-colored
   highlands are called terrae. Maria are found almost exclusively on the
   lunar nearside, with the lunar far side having only a few scattered
   patches (see lunar mare for a discussion of the distribution of mare
   basalts).

   Blanketed atop the Moon's crust is a highly comminuted and "impact
   gardened" surficial 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 he
   regolith varies in thickness from about 3 to 5 metres (10 to 16 ft) in
   the maria to about 10 to 20 metres (33 to 66 ft) 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 or tens of kilometers) and consists of highly fractured bedrock.

   Using images taken by the Clementine mission, 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
   unnamed mountains of eternal light are possible due to the Moon's
   extremely small axial tilt to the ecliptic plane. Since Clementine's
   images were taken during the northern lunar hemisphere's summer season,
   it remains unknown whether these mountains are shaded at any point
   during their local winter season. No similar regions of eternal light
   exist at the south pole, although the rim of Shackleton crater is
   illuminated for 80% of the lunar day. Another consequence of the Moon's
   small axial tilt is that there are many regions that remain in
   permanent shadow at the bottoms of many polar craters.

Topography

   Topography of the Moon, referenced to the lunar geoid.
   Enlarge
   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 during the
   Clementine mission. The most visible 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 1.9 km higher on the far side than the near
   side. If it is assumed that the crust is in isostatic equilibrium, and
   that the density of the crust is everywhere the same, then the higher
   elevations would be associated with a thicker crust. Using gravity,
   topography, and seismic data, the crust is thought to be on average
   about 50±15 km thick, with the far-side crust being on average thicker
   than the near side by about 15 km. .

Gravity field

   Radial gravitational anomaly at the surface of the Moon.
   Enlarge
   Radial gravitational anomaly at the surface of the Moon.

   The gravitational field of the Moon has been determined by the tracking
   of radio signals emitted by orbiting spacecraft. The principle used is
   based on that of the doppler effect, whereby the line-of-sight
   spacecraft acceleration can be measured by small shifts in frequency of
   the radio signal, as well as by measuring the distance to the
   spacecraft from the travel time of the signal between the spacecraft
   and a station on Earth. Since the gravitational field of the body
   affects the orbit of the spacecraft, it is possible to use these
   tracking data to invert for gravitational anomalies. However, because
   of the Moon's synchronous rotation it is not possible to track
   spacecraft much over the limbs of the Moon. Nevertheless, it is
   possible to make inferences about the farside gravity field (though
   with a lower precision) as their existence does influence the
   spacecraft orbit.

   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 neccesary in the planning of both
   manned and unmanned missions. They were initially discoved by the
   analysis of Lunar Orbiter tracking data,, since pre-Apollo navigational
   tests were experiencing landing position errors much larger than
   mission specifications.

   The origin of 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 gravitional
   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. It should be noted that the huge expanse of mare
   basaltic volcanism associated with Oceanus Procellarum does not possess
   a positive gravitational anomaly.

Magnetic field

   Total magnetic field strength at the surface of the Moon as derived
   from the Lunar Prospector electron reflectometer experiment.
   Enlarge
   Total magnetic field strength at the surface of the Moon as derived
   from the Lunar Prospector electron reflectometer experiment.

   The Moon has only a very weak external magnetic field in comparison to
   the Earth. 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 orgin. 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.

Presence of water

   Over time, comets and meteoroids continuously bombard the Moon, some of
   which contain a significant component of water. Energy from sunlight
   usually splits much of this water into its constituent elements
   hydrogen and oxygen, both of which generally escape to space. Attesting
   to the dryness of lunar rocks, it is noted that the samples collected
   by Apollo astronauts near the equator contained only traces of water.
   However, because of the very slight axial tilt of the Moon's spin axis
   to the ecliptic plane (only 1.5°), some deep craters near the poles
   never receive any light from the Sun, and are permanently shadowed.
   Thus, any water molecules that eventually ended up in these craters
   could be stable for long periods of time.

   Clementine has mapped craters at the lunar south pole which are
   shadowed in this way, and computer simulations suggest that up to
   14,000 km^2 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 meter of the
   regolith near the polar regions . Estimates for the total quantity of
   water ice are close to one cubic kilometer. Recently, radar
   observations with the Arecibo planetary radar showed that some of the
   near polar Clementine radar returns might instead be associated with
   rocks ejected from young craters. If true, this would indicate that the
   neutron results are primarily from hydrogen in forms other than ice,
   such as trapped hydrogen molecules or organics. Nevertheless, the
   interprettation of these data are non unique (ice or surface roughness
   could give rise to the observed signature), and it appears that these
   results do not exclude the possibility of water ice in permanently
   shadowed craters.

   Water ice can be mined and then split into hydrogen and oxygen by solar
   panel-equipped electric power stations or a nuclear generator. The
   presence of usable quantities of water on the Moon is an important
   factor in rendering lunar habitation cost-effective, since transporting
   water (or hydrogen and oxygen) from Earth would be prohibitively
   expensive.

Atmosphere

   The Moon has a relatively insignificant and tenuous atmosphere. One
   source of this 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. Gasses that are released by
   sputtering can either reimplant into the regolith as a cause 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 ionized. The elements sodium (Na) and potassium (K) have been
   detected using earth-based spectroscopic methods, whereas the element
   radon has been inferred from data obtained from the Lunar Prospector
   alpha particle spectrometer.

Eclipses

   The French 1999 solar eclipse
   Enlarge
   The French 1999 solar eclipse

   Eclipses happen only if Sun, Earth, and Moon are all in a straight
   line. Solar eclipses can only occur near a new moon, whereas lunar
   eclipses can only occur near a full moon. 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 total eclipses will not 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 of the Moon

   The Moon as illuminated by Earthshine. The brightest crescent is in
   direct sunlight; the upper portion is lit by light reflected from
   Earth.
   Enlarge
   The Moon as illuminated by Earthshine. The brightest crescent is in
   direct sunlight; the upper portion is lit by light reflected from
   Earth.
   Halo around Moon
   Enlarge
   Halo around Moon

   During the brightest full moons, the Moon can have an apparent
   magnitude of about −12.6. For comparison, the Sun has an apparent
   magnitude of −26.8. When the Moon is in a quarter phase, its brightness
   is not one half of a full moon, but instead is only about 1/10. This is
   because the lunar surface is not a perfect Lambertian reflector and
   because shadows projected onto the surface also diminish the amount of
   reflected light.

   The Moon appears larger when close to the horizon. This is a purely
   psychological effect (see Moon illusion). The angular diameter of the
   Moon from Earth is about one half of one degree, and 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 1 Earth radius).

   Another quirk of the visual system causes us to see the Moon as almost
   pure white, when in fact it reflects only about 7% of the light falling
   on it (about as dark as a lump of coal). It has a very low albedo.
   Colour constancy in the visual system recalibrates the relations
   between colors of an object and its surroundings; however, there is
   nothing next to the Moon to reflect the light falling on the Moon,
   therefore it is perceived as the brightest object visible. We have no
   standard to compare it to. An example of this is that, if you used a
   narrow beam of light to illuminate a lump of coal in a dark room, it
   would look white. If you then broadened the beam of the light source to
   illuminate the surroundings, it would revert to black.

   The highest altitude of the Moon on a day varies and has nearly the
   same limits as the Sun. It also depends on season and lunar phase with
   the full moon being highest in winter. The orientation of the Moon's
   crescent also depends on the latitude of the observing site. Close to
   the equator an observer can see a boat Moon.

   Like the Sun, the Moon can also give rise to the atmospheric effects
   including a 22 degree 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 of the Moon

   Apollo 17 astronaut Harrison Schmitt standing next to boulder at
   Taurus-Littrow during third EVA (extravehicular activity). NASA photo.
   Enlarge
   Apollo 17 astronaut Harrison Schmitt standing next to boulder at
   Taurus-Littrow during third EVA (extravehicular activity). NASA photo.
   The first time an "Earth-rise" was seen from the moon.
   Enlarge
   The first time an "Earth-rise" was seen from the moon.

   The first leap in lunar observation was caused by the invention of the
   telescope. Galileo Galilei made especially 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
   United States of America 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. What was the next
   big step depends on the political viewpoint: In the US (and the West in
   general) the landing of the first humans on the Moon in 1969 is seen 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 last person (as of 2006) to stand on the Moon was Eugene
   Cernan, who as part of the mission Apollo 17 walked on the Moon in
   December 1972. The USA Moon landing and return was enabled by several
   technologies where the US surpassed the Russians; for example, the US
   achieved considerable advances in ablation chemistry and atmospheric
   re-entry technology in the early 1960s. On the other hand, many
   scientifically important steps, such as the first photographs of the
   until then unseen far side of the Moon in 1959, were first achieved by
   the Soviet Union. Moon samples have been brought back to Earth by three
   Luna missions ( Luna 16, 20, and 24) and the Apollo missions 11 through
   17 (excepting Apollo 13, which aborted its planned lunar landing).
   Astronaut Alan Shepard raises the Flag of the United States on the
   surface of the Moon.
   Enlarge
   Astronaut Alan Shepard raises the Flag of the United States on the
   surface of the Moon.

   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 to today. Ranging to the LLR stations is routinely
   performed from earth-based stations with an accuracy of a few
   centimeters, and data from this experiment are being used to place
   constraints on the size of the lunar core..

   From the mid-1960s to the mid-1970s, there were a total of 65 Moon
   landings (both manned and robotic, with 10 in 1971 alone), but after
   Luna 24 in 1976 they stopped. The Soviet Union started focusing on
   Venus and space stations and the US on 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
   rendering the mission scientifically useless.

   In 1994, the US 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, as
   well as 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 due to 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, US President George W. Bush called for a plan to
   return manned missions to the Moon by 2020. The People's Republic of
   China has expressed ambitious plans for exploring the Moon and has
   started the Chang'e program for lunar exploration. Japan has two
   planned lunar missions, LUNAR-A and Selene. India is to launch an
   unmanned mission Chandrayaan-1 in February 2008. The US will launch the
   Lunar Reconnaissance Orbiter in 2008.

Human understanding of the Moon

   Map of the Moon by Johannes Hevelius (1647).
   Enlarge
   Map of the Moon by Johannes Hevelius (1647).

   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 5,000 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. For further details, see the Moon in mythology.
   Moon over red and blue haze
   Enlarge
   Moon over red and blue haze

   Among the first in the Western world to offer a scientific explanation
   for the Moon was the Greek philosopher Anaxagoras, 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. By the Middle
   Ages, before the invention of the telescope, more and more people began
   to recognize 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 Sidereus Nuncius 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.
   Still from silent movie "Le voyage dans la lune" (1902) by Georges
   Méliès.
   Enlarge
   Still from silent movie "Le voyage dans la lune" (1902) by Georges
   Méliès.

   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 could contain vegetation and
   be inhabited by selenites was seriously considered by some 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, amongst 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 Mappa Selenographica and the book Der
   Mond in 1837, which firmly established the conclusion that the Moon has
   no bodies of water nor any appreciable atmosphere.

   There remained some controversy over whether features on the Moon could
   undergo changes. Some observers claimed that some small craters had
   appeared or disappeared, or that other forms of transient phenomena had
   occured. Today, many of these claims are thought to be illusory,
   resulting from observing under different lighting conditions, poor
   astronomical seeing, or the inadequacy of earlier drawings. It is
   however known that the phenomenon of outgassing occasionally occurs,
   and these might be responsible for a minor percentage of the reported
   lunar transient phenomena.

   The far side of the Moon remained completely unknown until the Luna 3
   probe was launched in 1959, and was extensively mapped by the Lunar
   Orbiter program in the 1960s.

Legal status

   Though several flags of the Soviet Union and the United States have
   been symbolically planted on the Moon, the Russian and U.S. governments
   make no claims to 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 ( res communis). This treaty
   also restricts use of the Moon to peaceful purposes, explicitly banning
   weapons of mass destruction (including nuclear weapons) and military
   installations of any kind. 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, though none of these claims are generally considered credible
   (see Extraterrestrial real estate).

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