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Moon
The completely illuminated disk of the full Moon, in the darkness of the night sky. The disk is patterned with a mix of light-tone regions and darker, irregular blotches, and scattered with varying sizes of impact craters, circles surrounded by outthrown rays of bright ejecta.
A full Moon as seen from Earth's northern hemisphere
Designations
Adjectiveslunar
SymbolMoon symbol
Orbital characteristics
Perigee363,104 km  (0.002 4 AU)
Apogee405,696 km  (0.002 7 AU)
384,399 km  (0.002 57 AU[1])
Eccentricity0.054 9[1]
27.321 582 d  (27 d 7 h 43.1 min[1])
29.530 589 d  (29 d 12 h 44 min 2.9 s)
1.022 km/s
Inclination5.145° to the ecliptic[1]
(between 18.29° and 28.58° to Earth's equator)
regressing by one revolution in 18.6 years
progressing by one revolution in 8.85 years
Satellite ofEarth
Physical characteristics
1,737.10 km  (0.273 Earths)[1][2]
Equatorial radius
1,738.14 km  (0.273 Earths)[2]
Polar radius
1,735.97 km  (0.273 Earths)[2]
Flattening0.001 25
Circumference10,921 km (equatorial)
3.793 km²  (0.074 Earths)
Volume2.195 8 km³  (0.020 Earths)
Mass7.347 7 kg  (0.012 3 Earths[1])
Mean density
3,346.4 kg/m³[1]
1.622 m/s² (0.165 4 g)
2.38 km/s
27.321 582 d (synchronous)
Equatorial rotation velocity
4.627 m/s
1.542 4° (to ecliptic)
6.687° (to orbit plane)
Albedo0.136[3]
Surface temp. min mean max
equator 100 K 220 K 390 K
85°N[4] 70 K 130 K 230 K
−2.5 to −12.9[nb 1]
−12.74 (mean full Moon)[2]
29.3 to 34.1 arcminutes[2][nb 2]
Atmosphere[5][nb 3]
Surface pressure
10−7 Pa (day)
10−10 Pa (night)
Composition by volumeH, He, Na, K, Rn, Ar

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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 384,403 kilometres (238,857 mi), about thirty times the diameter of the Earth. The common centre of mass of the system (the barycentre) is located at about 1,700 kilometres (1,100 mi)—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[nb 4] (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[nb 5] (the synodic period).

The Moon's diameter is 3,474 kilometres (2,159 mi),[6] a little more than a quarter of Earth's. Thus, the Moon's surface area is less than a tenth of the Earth (about a quarter of Earth's land area), 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 United States' 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. had committed to return to the Moon by 2018,[7][8][9] however that commitment has been put into jeopardy by the proposed 2011 budget, which will cancel Constellation, NASA's project to send humans back to the moon by 2020. On November 13, 2009, NASA announced the discovery of proof that water exists on the Moon, based on data obtained from its LCROSS lunar impact mission.[10]

Name and etymology

The proper English name for Earth's natural satellite is, simply, the Moon (capitalized).[11][12] Moon is a Germanic word, related to the Latin [mensis] Error: {{Lang}}: text has italic markup (help) and Ancient Greek [μήνας] Error: {{Lang}}: text has italic markup (help) (mēnas) both meaning month, and to [Μήνη] Error: {{Lang}}: text has italic markup (help) (Mēnē), the alternate name for [σελήνη] Error: {{Lang}}: text has italic markup (help) (Selēnē), the Ancient Greek word for the Moon.[13][14] It is ultimately a derivative of the Proto-Indo-European root me-, also represented in measure[15] (time), with reminders of its importance in measuring time in words derived from it like Monday, month and menstrual. The related adjective is lunar, after the Latin name [Luna] Error: {{Lang}}: text has italic markup (help), as well as an adjectival prefix seleno- and suffix -selene, from the Ancient Greek name.[16] 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.[15] Subsequently, these objects were given distinct names to avoid confusion.[12] The Moon is occasionally referred to by its Latin name [Luna] Error: {{Lang}}: text has italic markup (help), primarily in science fiction.

Physical characteristics

In a set of four images, the dark shadowed disk of the Moon moves from left to right across the face of the blue and brown quarter-phase Earth, covering only a small part of the cloud-swirled semicircle.
Comparative sizes of Earth and the Moon, as seen from Deep Impact, 50 million km distant

The Moon is exceptionally large relative to the Earth: a quarter the diameter of the planet and 1/81 its mass. It is the largest moon in the solar system relative to the size of its planet (although Charon is larger relative to the dwarf planet Pluto). The Moon's surface area is less than one-tenth that of the Earth; about a quarter of the Earth's land area. However, the Earth and Moon are still considered a planet-satellite system, rather than a double-planet system, as their barycentre, the common centre of mass, is located about 1,700 km (about a quarter of the Earth's radius) beneath the surface of the Earth.

Formation

Main article: Giant impact hypothesis

Several mechanisms have been historically proposed for the Moon's formation 4.527 ± 0.010 billion years ago, some 30–50 million years after the origin of the Solar System.[17] These include the fission of the Moon from the Earth's crust through centrifugal forces,[18] which would require too great an initial spin of the Earth,[19] the gravitational capture of a pre-formed Moon,[20] which would require an unfeasibly extended atmosphere of the Earth to dissipate the energy of the passing Moon,[19] and the co-formation of the Earth and the Moon together in the primordial accretion disk, which does not explain the depletion of metallic iron in the Moon.[19] These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.[21]

The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact: a Mars-sized body hit the proto-Earth, blasting material into orbit around the proto-Earth, which accreted to form the Moon.[6] Giant impacts are thought to have been common in the early Solar System. 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.[22] Unresolved questions regarding this theory are the relative sizes of the proto-Earth and impactor, and how much material from these two bodies formed the Moon. However, 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.[23]

The large amount of energy released in the giant impact event and the subsequent reaccretion of material in Earth orbit would have melted the outer shell of the Moon, forming a magma ocean. Estimates for its depth range from about 500 km to the entire radius of the Moon.[24]

Internal structure

Main article: Internal structure of the Moon
Elemental Composition of the Crust of the Moon[25][26]
Oxygen 41–46%
Silicon 21%
Magnesium 6%
Iron 13%
Calcium 8%
Aluminium 7%

The Moon is a differentiated body: it has a geochemically distinct crust, mantle, and core. This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.4 billion years ago.[27] 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 on top.[24] The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements.[1] Consistent with this, geochemical mapping from orbit shows the crust is mostly anorthosite,[5] and moon rock samples of the flood lavas erupted on the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron rich than that of Earth.[1] Geophysical techniques suggest that the crust is on average ~50 km thick.[1]

The Moon is the second densest satellite in the Solar System after Io. However, the core of the Moon is small, with a radius of about 350 km or less:[1] this is only ~20% the size of the Moon, in contrast to the ~50% of most other terrestrial bodies. Its composition is not well constrained, but it is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's time-variable rotation indicate that it is at least partly molten.[28]

Surface geology

Main articles: Geology of the Moon and Moon rocks
See also: Topography of the Moon and List of features on the Moon
The dark irregular mare lava plains are most prominent in the fully illuminated disk. A single bright star of ejecta, with rays stretching a third of the way across the disk, emblazons the lower centre: this is the crater Tycho.
Near side of the Moon
In contrast, the full disk is near-featureless, with no dark mare and a uniform grey surface. Many bright dots of impact craters are randomly arranged on the disk, almost all overlapping others.
Far side of the Moon. Note the lack of dark maria.[29]
The near side has flat, deeper mares and southern highlands; the far side has a huge southern circular depression, with a similarly sized, more irregular high region to its north-east. The relief on the far side is much more extreme than that on the near side.
Topography of the Moon, referenced to the lunar geoid.

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 nearly always keeping the same face turned towards the Earth. 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.[30] 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.[31]

The topography of the Moon has been measured with laser altimetry and stereo image analysis. The most visible topographic feature is the giant far side South Pole-Aitken basin, some 2,240 km in diameter, the largest crater on the Moon and one of the largest known craters in the Solar System.[32] At 13 km deep, its floor is the lowest elevation on the Moon.[33] The highest elevations are found just to its north-east, 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. The lunar far side is on average about 1.9 km higher than the near side.[1]

Volcanic features

Main article: Lunar mare

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. They are now known to be vast solidified pools of ancient basaltic lava. While similar to terrestrial basalts, the mare basalts have much higher abundances of iron and are completely lacking in minerals altered by water.[34][35] The majority of these lavas erupted or flowed into the depressions associated with impact basins. Several provinces containing shield volcanoes and volcanic domes are found within the near side maria.[36] Oceanus Procellarum is a notable exception: it does not correspond to a known impact basin.

Maria are found almost exclusively on the near side of the Moon, covering 31% of the surface on the near side[6], compared with a few scattered patches on the far side covering only 2%.[37] 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 the Lunar Prospector gamma-ray spectrometer, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt.[24][38] 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,[39] and the youngest eruptions, dated by crater counting, appear to have been only 1.2 billion years ago.[40]

Impact topography

See also: List of craters on the Moon
A grey, many-ridged surface viewed obliquely from an indeterminate height: there is no sense of scale. The largest feature is a circular ringed structure with high walled sides and a lower central peak: the entire surface out to the horizon is filled with similar structures that are smaller and overlapping.
Lunar crater Daedalus on the Moon's far side

The other major geologic process that has affected the Moon's surface is impact cratering[41], with craters formed 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.[citation needed] The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale, structures characterised by multiple rings of uplifted material, typically hundreds to 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. While only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Since impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface. The radiometric ages of impact melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment of impacts.

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 regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 3–5 m in the maria, and 10–20 m in the highlands.[42] This lunar soil of silicon dioxide glass is extremely fine, with a texture like snow and smell like spent gunpowder.[43] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometres thick.[44]

Although the Moon's minute axial tilt (1.54 degrees) means that seasonal variation is minimal, it is just enough to create a 3-degree variation in the Sun's elevation at the poles, resulting in a very slight "summer" and "winter".[45] From images taken by the Clementine mission in 1994, it appears that four mountainous regions on the rim of Peary crater at the Moon's north pole remain illuminated for the entire lunar day, creating peaks of eternal light. No similar regions of eternal light exist at the south pole. Similarly, there are regions that remain in permanent shadow at the bottoms of many polar craters,[46] and these dark craters are extremely cold: the Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C),[47] and just 26 K 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.[45]

The lighter-colored regions of the Moon are called terrae, or more commonly highlands, since they are higher than most maria. They have been dated radiometrically as forming 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean.[39][40] Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins, and may be the surviving remnants of the impact basin's outer rims.[48] In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[49]

Presence of water

Main article: Lunar water
A full twenty degrees of latitude of the Moon's disk, northwards of the South Pole, completely covered in the overlapping circles of craters. The illumination angles are from all directions, keeping almost all the crater floors in sunlight, but a set of merged crater floors right at the south pole are completely shadowed.
Lunar south pole as imaged by Clementine: note permanent polar shadow.

Although liquid water cannot persist at the Moon's surface, and water vapour is quickly decomposed by sunlight and lost to space, scientists have thought since the 1960s that water ice, deposited by impacting comets or produced by the reaction of oxygen-rich lunar rocks and hydrogen in the solar wind, could survive in the cold, permanently shadowed craters at the Moon's poles.[50] These craters have been in shadow for the past two billion years,[51] and computer simulations suggest that up to 14,000 km² might be in permanent shadow.[46] 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.

Many different signatures of lunar water have since been found. In 1994, Clementine's bistatic radar experiment found indications of small, frozen pockets of water close to the surface (though later Arecibo radar observations suggested these might be rocks ejected from young impact craters);[52] the Lunar Prospector's neutron spectrometer indicated in 1998 that high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions;[53] in 2008, new analysis found small amounts of water in the interior of volcanic lava beads brought to Earth by Apollo 15.[54] In September 2009, Chandrayaan-1, the first Indian lunar mission, detected water and hydroxyl absorption lines in reflected sunlight using NASA's Moon Mineralogy Mapper instrument, evidence of large quantities of water on the Moon's surface, possibly as high as 1,000 ppm .[55] Weeks after, NASA's LCROSS lunar orbiter announced that "not just water, but lots of water" had been detected near the southern pole.[56]

Gravity and magnetic fields

Main articles: Gravity of the Moon and Magnetic field 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 these basins.[57] These 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, some mascons exist that are not linked to mare volcanism,[58] and the Oceanus Procellarum mare does not show a mascon.

The Moon has an external magnetic field of the order of one to a hundred nanoteslas, less than one-hundredth that of the Earth. It does not currently have a dipolar magnetic field, as would be generated by a geodynamo in its core, and only has crustal magnetization,[59] possibly 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 the airless Moon, transient magnetic fields could be generated during large impact events, from 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.[60]

Atmosphere

Main article: Atmosphere of the Moon

The Moon has an atmosphere so thin as to be near-negligible, with a total atmospheric mass of less than 104 kg.[61] The effective surface pressure of this small mass is around 3 atm (0.3 nPa); it varies with the lunar day. Its sources are outgassing, the release of gases such as radon produced by radioactive decay within the crust and mantle,[62] and sputtering, the bombardment of lunar soil with solar wind ions.[5] Gases released by sputtering can either return into the regolith due to the Moon's gravity, or be lost to space through solar radiation pressure or by being swept away by the solar wind's magnetic field if they are ionised. Elements detected include sodium, potassium, radon-222, polonium-210,[63] argon-40, helium-4, oxygen and/or methane, nitrogen and/or carbon monoxide, and carbon dioxide.[64]

Orbit and relationship to Earth

Main articles: Orbit of the Moon and Lunar theory

The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days[nb 4](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 the same phase to Earth, which is about 29.5 days[nb 5] (its synodic period).[6] Unlike most satellites of other planets, the Moon orbits near the ecliptic and not the Earth's equatorial plane.

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.[65] Three other near-Earth asteroids with orbits similar to Cruithne's have since been discovered: 54509 YORP, (85770) 1998 UP1 and 2002 AA29.[66]

A line is emitted from the full Earth, and moves across dark space until it reaches the considerably smaller full Moon, where it shortens and disappears into the Moon's surface.
The relative sizes and separation of the Earth–Moon system. The yellow line travels from the Earth to the Moon in the same time that an actual beam of light would take to reach the Moon from the Earth: 1.255 seconds at the mean orbital distance.

Tidal effects

The tides on the Earth are mostly generated by the gradient in intensity of the Moon's gravitational pull from one side of the Earth to the other, the tidal forces, with a smaller contribution by the Sun's gravity. These tidal forces form two tidal bulges on the Earth, most clearly seen in elevated sea level, one in the direction of the Moon and one in the opposite direction. Since the Earth spins about 27 times faster than the Moon moves around it, the tidal bulges are dragged along with the Earth's surface faster than the Moon moves, rotating around the Earth once a day as it spins on its axis. The ocean tides, effects of the two ever-moving bulges and the massive ocean currents chasing them, are magnified by an interplay of other effects: frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that get shallower near land, and oscillations between different ocean basins. The gravitational attraction of the Sun on the Earth's oceans is almost half that of the Moon, and this interplay is responsible for spring and neap tides.

Gravitational coupling between the Moon and the bulge nearest the Moon affects its orbit. 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. In the same way, the bulge farthest from the Moon lags behind, but as it is 12,756 km farther away, it has slightly less gravitational coupling to the Moon. This gravitational asymmetry of forces attracts the Moon forward in its orbit. This gravitational coupling acts like a torque on the Earth's rotation, and drains angular momentum and rotational kinetic energy from the Earth's spin. In turn, angular momentum is added to the Moon's orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period. As a result, the distance between the Earth and Moon is increasing, and the Earth's spin slowing down. Measurements from lunar ranging experiments with laser reflectors left during the Apollo missions have showed that, at present, the Moon's distance to the Earth increases by 38 mm per year[67] (though this is only 0.10 ppb/year of the radius of the Moon's orbit). Cumulatively, this effect becomes ever more significant over time. Atomic clocks also show that the Earth's day lengthens by about 15 microseconds every year,[68] requiring the occasional addition of a leap second to the calendar. This tidal drag will continue until the spin of the Earth has slowed to match the orbital period of the Moon, at which time the tidal effect of the Sun will dominate, further slowing the Earth and thereafter causing the orbit of the Moon to steadily shrink. However, long before this could happen, the sun will have become a red giant.

The libration of the Moon over a single lunar month.

The lunar surface also experiences tides, of amplitude ~10 cm over 27 days, with two components: a fixed one due to the Earth, as they are in synchronous rotation, and a varying component from the Sun. The Earth-induced component arises from libration, 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 the Moon is seen, allowing about 59% of its surface to be seen from the Earth (but only half at any instant).[6] The magnitude of the Moon's tides corresponds to a Love number of 0.0266, and supports the idea of a partially melted zone around its core. Moonquake waves lose energy below 1,000 kilometres in depth, and this may also show that the deep material is at least partially melted. The Earth’s Love number is 0.3, corresponding to a movement of 0.5 metres per day.[69]

Eclipses

Main articles: Solar eclipse and Lunar eclipse
The fiercely bright disk of the Sun is completely obscured by the exact fit of the disk of the dark, non-illuminated Moon, leaving only the radial, fuzzy, glowing coronal filaments of the Sun visible around the edge.
The 1999 solar eclipse
The bright disk of the Sun, showing many coronal filaments, flares and grainy patches in the wavelength of this image, is partly obscured by a small dark disk: here, the Moon covers less than a fifteenth of the Sun.
The Moon passing in front of the Sun, from the STEREO-B spacecraft. [70]
From the Earth, the Moon and Sun appear the same size. From a satellite in an Earth-trailing orbit, the Moon appears smaller than the Sun.

Eclipses can only occur 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. 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.[71] 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 would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, about 600 million years from now (if the angular diameter of the Sun does not change), the Moon will no longer cover the Sun completely, and only annular eclipses will occur.[72]

Because the Moon's orbit around the Earth is inclined by about 5° 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.[72] 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).[73]

As the Moon is continuously blocking our view of a half-degree-wide circular area of the sky, the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to the Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Due to the precession of the lunar orbit, each year different stars are occulted.[74]

Observation

See also: Lunar phase, Earthshine, Observing the Moon, and Moon illusion

During its brightest phase, at "full Moon", the Moon has an apparent magnitude of about −12.7. By comparison, the Sun has an apparent magnitude of −26.7. 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.

The orbital positions of the Moon and Earth, with illumination from the right. As the Moon travels anticlockwise around the Earth, each geometric arrangement of Moon, Earth and illumination are matched to the corresponding phase of the Moon: dark circle, sliver of a crescent, half circle, three-quarters circle, fully illuminated circle, and the same in reverse.
The phases of the Moon in their order of appearance: New Moon through Crescent, First Quarter, and Gibbous to reach Full Moon. This is followed by Gibbous, Last Quarter and Crescent to complete full circle at the New Moon again.

On average, the Moon covers an area of 0.21078 square degrees on the night sky.[75] 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 13.6%[3]) than in other directions because of the spherical glass beads in the moondust. This increases the brightness of a full Moon.[76] 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 Moon does appear larger when close to the horizon, but this is a purely psychological effect known as the Moon illusion.

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.[77] The Moon's appearance, like that of the Sun, can be affected by Earth's atmosphere. Such atmospheric effects include a 22° halo ring, and the smaller coronal rings seen more often through thin clouds.

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.[78][79]

Exploration

Main article: Exploration of the Moon
See also: Robotic exploration of the Moon, List of current and future lunar missions, Colonization of the Moon, List of Apollo astronauts, and List of man-made objects on the Moon
The small blue-white semicircle of the Earth, almost glowing with colour in the blackness of space, rising over the limb of the desolate, cratered surface of the Moon from lunar orbit. Africa is at the sunset terminator, both Americas are under cloud, and Antarctica is at the left end of the terminator.
Earth as viewed from the Moon during the Apollo 8 mission, Christmas Eve, 1968

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.

First direct exploration: 1959-1980s

Main articles: Apollo program and Moon landing

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 once allowed by launcher capabilities. 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.[6] Rock samples were 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).

An astronaut in an American Apollo-program spacesuit, standing on the flat, heavily footprinted landing area, with the utterly black sky of space above the horizon. His shadow is cast partly towards the photographer, and his reflective gold visor shows the similarly suited astronaut taking the photograph.
Astronaut Buzz Aldrin photographed by Neil Armstrong during the first Moon landing on July 20, 1969

The landing of the first humans on the Moon in 1969 is seen by many as the culmination of the space race.[80] 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 the early 1960s, in domains such as ablation chemistry and atmospheric re-entry technology.

Today, 51 years, 259 days have 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): no one has set foot on it since.

Scientific instrument packages were installed on the lunar surface during all the Apollo missions. 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,[81][82] 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.[83]

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); the last was Luna 24, in 1976. Only 18 of these were controlled Moon landings, with nine completing a round trip from Earth and returning rock samples. The Soviet Union then turned its primary attention to Venus and space stations, and the U.S. to Mars and beyond.[citation needed]

Current era: 1990-present

Post-Apollo and Luna, many more countries have become involved in direct exploration of the Moon.

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, preventing further scientific use of the mission.

In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. 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. 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).[84] NASA is now planning for the construction of a permanent outpost at one of the lunar poles.[85]

The European spacecraft Smart 1 was launched September 27, 2003 and was in lunar orbit from November 15, 2004 to September 3, 2006. 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. On September 14, 2007 the Japan Aerospace Exploration Agency launched SELENE, also known as Kaguya, a lunar orbiter fitted with a high-definition camera and two small satellites.[86] On October 22, 2008 India launched the Chandrayaan I (a Sanskrit word meaning 'Moon-craft') lunar orbiter. The country plans to launch Chandrayaan II in 2010 or 2011, which is slated to include a robotic lunar rover, and has also expressed its hope for a manned mission to the Moon by 2020.[87] The U.S. co-launched the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite (LCROSS) on June 18, 2009.

Russia also announced intentions to resume its previously frozen project Luna-Glob, consisting of an unmanned lander and orbiter, which is slated to land in 2012.[88] 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.[citation needed]

Human understanding

See also: Moon in art and literature, Lunar calendar, Lunar deity, Lunar effect, and Blue moon
On an open folio page is a carefully drawn and coloured disk of the full Moon, delineating the dark mare and many large impact craters. In the upper corners of the page are inscribed waving banners held aloft by pairs of winged cherubs. In the lower left page corner a cherub assists another to measure distances with a pair of compasses; in the lower right corner a cherub views the main map through a handheld telescope, while another, kneeling, peers at the map from over a low cloth-draped table.
Map of the Moon by Johannes Hevelius from his Selenographia (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 5000-year-old rock carving at Knowth, Ireland may represent the Moon, which would be the earliest depiction discovered.[89] 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.

In Mesopotamia, Babylonian astronomers by the early first millenium BC had discovered a repeating 18-year cycle of lunar eclipses. They had also known by this time that 19 solar years is about equal to 235 lunar months. In the 2nd century BC, Seleucus of Seleucia correctly theorized that tides were caused by the Moon, although he believed that the interaction was mediated by the Earth's atmosphere. According to Strabo (1.1.9), Seleucus was the first to state that the tides are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.[90]

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.[91] 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 Aristotelian physics for many centuries after.[92] Aristarchus went a step further and computed the distance from Earth, together with its size, obtaining a value of 20 times the Earth radius for the distance (the real value is 60; the Earth radius was roughly 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.[93] 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).[94] 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).[94] Jing Fang noted the sphericity of the Moon, while Zhang Heng accurately described a lunar eclipse and solar eclipse.[94][95] 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.[96] 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.[96]

Between 825 and 835 AD, the Persian astronomer, Habash al-Hasib al-Marwazi, conducted various observations at the Al-Shammisiyyah observatory in Baghdad.[97] Using these observations, he estimated the Moon's diameter as 3,037 km (equivalent to 1,519 km radius) and its distance from the Earth as 215,209 miles, which come close to the currently accepted values.[98] In 1021, the Islamic physicist, Alhazen, accurately explained the Moon illusion in the Book of Optics, which stated that judging the distance of an object depends on there being an uninterrupted sequence of intervening bodies between the object and the observer. With the Moon, there are no intervening objects, therefore since the size of an object depends on its observed distance, which is in this case inaccurate, the Moon appears larger on the horizon. Through Alhazen's work, the Moon illusion gradually came to be accepted as a psychological phenomenon.[99] He also investigated moonlight, which he proved through experimentation that it originates from sunlight and correctly concluded that it "emits light from those portions of its surface which the sun's light strikes."[100]

By the Middle Ages, before the invention of the telescope, an increasing number of people began to recognise the Moon as a sphere, though many believed that it was "perfectly smooth".[101] In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book [Sidereus Nuncius] Error: {{Lang}}: text has italic markup (help) 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.[102] Almost at the same time however (during 1834–1836), Wilhelm Beer and Johann Heinrich Mädler were publishing their four-volume [Mappa Selenographica] Error: {{Lang}}: text has italic markup (help) and the book [Der Mond] Error: {{Lang}}: text has italic markup (help) in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere.

Main article: Space law

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 ([res communis] Error: {{Lang}}: text has italic markup (help)). This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction.[103]

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.[104]

Notes

  1. ^ The maximum value is given based on scaling of the brightness from the value of −12.74 given for an equator to Moon-centre distance of 378 000 km in the NASA factsheet reference to the minimum Earth-Moon distance given there, after the latter is corrected for the 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).
  2. ^ The range of angular size values given are based on simple scaling of the following values given in the fact sheet reference: at an Earth-equator to Moon-centre distance of 378 000 km, the angular size is 1896 arcseconds. The same fact sheet gives extreme Earth-Moon distances of 407 000 km and 357 000 km. For the maximum angular size, the minimum distance has to be corrected for the Earth's equatorial radius of 6 378 km, giving 350 600 km.
  3. ^ 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.
  4. ^ a b More accurately, the Moon's mean sidereal period (fixed star to fixed star) is 27.321661 days (27d 07h 43m 11.5s), and its mean tropical orbital period (from equinox to equinox) is 27.321582 days (27d 07h 43m 04.7s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
  5. ^ a b More accurately, the Moon's mean synodic period (between mean solar conjunctions) is 29.530589 days (29d 12h 44m 02.9s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).

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Footnotes

Further reading
Cartographic resources
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