The Moon is the only natural satellite of the Earth,[nb 4][7] and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary, having a quarter the diameter of Earth and 1⁄81 its mass. The Moon is the second densest satellite after Io, a satellite of Jupiter. Sony VAIO VPCW111XXW Battery
It is in synchronous rotation with Earth, always showing the same face; the near side is marked with dark volcanic maria among the bright ancient crustal highlands and prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a similar reflectance to coal. Sony VAIO VPCW115XG Battery
Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. Sony VAIO VPCW115XGP Battery
The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses.
The Moon is the only celestial body other than Earth on which humans have set foot. Sony VAIO VPCW115XW/P Battery
While the Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959, the United States' NASA Apollo programachieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. Sony VAIO VPCW115XW/T Battery
These missions returned over 380 kg of lunar rocks, which have been used to develop geological understanding of the Moon's origins, the formation of its internal structure, and its subsequent history. It is thought to have formed some 4.5 billion years ago. One formation theory is a giant impact event involving Earth. Sony VAIO VPCW115XW/W Battery
The impact theory was called into question in 2012, after re-analysis of Apollo samples.[8]
After the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft, notably by the finalSoviet Lunokhod rover. Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters. Sony VAIO VPCW117XC/P Battery
These spacecraft have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. Future manned missions to the Moon have been planned, including government as well as privately funded efforts. Sony VAIO VPCW117XC/T Battery
The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.
Name and etymology
The English proper name for Earth's natural satellite is "the Moon".Sony VAIO VPCW117XC/W Battery
The noun moon derives from moone (around 1380), which developed from mone (1135), which derives from Old English mōna (dating from before 725), which, like all Germanic language cognates, ultimately stems from Proto-Germanic *mǣnōn.[11]
The principal modern English adjective pertaining to the Moon is lunar, derived from the Latin Luna. Sony VAIO VPCW119XJ Battery
Another less common adjective is selenic, derived from the Ancient Greek Selene (Σελήνη), from which the prefix "seleno-" (as in selenography) is derived.[12]
Formation
Several mechanisms have been proposed for the Moon's formation 4.527 ± 0.010 billion years ago,[nb 6] some 30–50 million years after the origin of the Solar System. Sony VAIO VPCW119XJ/P Battery
These included the fission of the Moon from the Earth's crust through centrifugal force,[14] which would require too great an initial spin of the Earth,[15] the gravitational capture of a pre-formed Moon,[16] which would require an unfeasibly extended atmosphere of the Earth to dissipate the energy of the passing Moon, Sony VAIO VPCW119XJ/W Battery
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.[15] These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.[17]
The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact: Sony VAIO VPCW11AXJ Battery
a Mars-sized body hitting the newly formed proto-Earth, blasting material into orbit around it, which accreted to form the Moon.[18] 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 and the small size of the lunar core; Sony VAIO VPCW11S1E/P Battery
they also show that most of the Moon came from the impactor, not from the proto-Earth.[19] More recent tests suggest more of the Moon coalesced from the Earth and not the impactor. Meteorites show that other inner Solar System bodies such as Mars and Vesta have very different oxygen and tungsten isotopic compositions to the Earth, Sony VAIO VPCW11S1E/T Battery
while the Earth and Moon have near-identical isotopic compositions. Post-impact mixing of the vaporized material between the forming Earth and Moon could have equalized their isotopic compositions,[23] although this is debated.[24] Sony VAIO VPCW11S1E/W Battery
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 Earth, forming a magma ocean. The newly formed Moon would also have had its own lunar magma ocean; estimates for its depth range from about 500 km to the entire radius of the Moon. Sony VAIO VPCW121AX Battery
Published in 2012, an analysis of titanium isotopes in Apollo lunar samples showed that the Moon has the same composition as the Earth, which conflicts with the impact event hypothesis for its formation.
The Moon is a differentiated body: it has a geochemically distinct crust, mantle, and core. Sony VAIO VPCW126AG Battery
The Moon has a solid iron-rich inner core with a radius of 240 kilometers and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometers. Around the core is a partially molten boundary layer with a radius of about 500 kilometers. This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago. Sony VAIO VPCW127JC/P Battery
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.[30] Sony VAIO VPCW127JC/T Battery
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,[6] Sony VAIO VPCW127JC/W Battery
andmoon 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.[31] Sony VAIO VPCW127JC/WZ Battery
However, the inner 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 sulphur and nickel; Sony VAIO VPCW12AAJ Battery
analyses of the Moon's time-variable rotation indicate that it is at least partly molten.[32]
Surface geology
The topography of the Moon has been measured with laser altimetry and stereo image analysis.[34] The most visible topographic feature is the giant far side South Pole –Sony VAIO VPCW12AKJ Battery
Aitken basin, some 2,240 km in diameter, the largest crater on the Moon and the largest known crater in the Solar System.[35][36] At 13 km deep, its floor is the lowest elevation on the Moon.[35][37] The highest elevations are found just to its north-east, and it has been suggested that this area might have been thickened by the oblique formation impact of South Pole – Aitken. Sony VAIO VPCW12AVJ Battery
Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims.[35] The lunar far side is on average about 1.9 km higher than the near side.[1]
Volcanic features
The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are calledmaria (Latin for "seas"; singular mare), Sony VAIO VPCW12S1E/P Battery
since they were believed by ancient astronomers to be filled with water.[39] 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. Sony VAIO VPCW12S1E/T Battery
The majority of these lavas erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domesare found within the near side maria.[42]
Maria are found almost exclusively on the near side of the Moon, covering 31% of the surface on the near side,[43] Sony VAIO VPCW12S1E/W Battery
compared with a few scattered patches on the far side covering only 2%.[44] This is thought to be due to a concentration of heat-producing elements under the crust on the near side, seen on geochemical maps obtained by Lunar Prospector's gamma-ray spectrometer, which would have caused the underlying mantle to heat up, Sony VAIO VPCS11AGJ Battery
partially melt, rise to the surface and erupt.[30][45][46] Most of the Moon's mare basaltserupted during the Imbrian period, 3.0–3.5 billion years ago, although some radiometrically dated samples are as old as 4.2 billion years,[47] and the youngest eruptions, dated by crater counting, appear to have been only 1.2 billion years ago.[48] Sony VAIO VPCS11AHJ Battery
The lighter-coloured regions of the Moon are called terrae, or more commonly highlands, since they are higher than most maria. They have been radiometrically dated as forming 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean. Sony VAIO VPCS11AVJ Battery
In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[49]
Impact craters
The other major geologic process that has affected the Moon's surface is impact cratering,[50] with craters formed when asteroids and comets collide with the lunar surface. Sony VAIO VPCS11J7E/B Battery
There are estimated to be roughly 300,000 craters wider than 1 km on the Moon's near side alone.[51] Some of these are named for scholars, scientists, artists and explorers.[52] The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Sony VAIO VPCS11M1E/W Battery
Imbrium, and Orientale, structures characterized 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.[53] The lack of an atmosphere, weather and recent geological processes mean that many of these craters are well-preserved. Sony VAIO VPCS11V9E Battery
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.[53] The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: Sony VAIO VPCS11V9E/B Battery
this has been used to propose a Late Heavy Bombardmentof impacts.[54]
Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture like snow and smell like spent gunpowder.[55] Sony VAIO VPCS11X9E/B Battery
The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–20 m in the highlands and 3–5 m in the maria.[56] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometres thick.[57] Sony VAIO VPCS123FGB Battery
Presence of water
Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, Sony VAIO VPCS125EC Battery
and hydrogen from solar wind, leaving traces of water which could possibly survive in cold, permanently shadowed craters at either pole on the Moon.[58][59]Computer simulations suggest that up to 14,000 km2 of the surface may be in permanent shadow.[60] Sony VAIO VPCS128EC Battery
The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.[61]
In years since, signatures of water have been found to exist on the lunar surface. Sony VAIO VPCS129GC Battery
In 1994, the bistatic radar experimentlocated on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters.[63] Sony VAIO VPCS12C7E/B Battery
In 1998, the neutron spectrometer located on the Lunar Prospector spacecraft, indicated that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions.[64] In 2008, an analysis of volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water to exist in the interior of the beads.[65] Sony VAIO VPCS12L9E/B Battery
The 2008, Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. Sony VAIO VPCS12V9E/B Battery
The spacecraft showed that concentrations may possibly be as high as 1,000 ppm.[66] In 2009, LCROSS sent a 2300 kg impactor into a permanently shadowed polar crater, and detected at least 100 kg of water in a plume of ejected material. Another examination of the LCROSS data showed the amount of detected water, to be closer to 155 kilograms (± 12 kg). Sony VAIO VPCW111XX/P Battery
In May 2011, Erik Hauri et al. reported[70] 615–1410 ppm water in melt inclusions in lunar sample 74220, the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. Sony VAIO VPCW111XX/PC Battery
This concentration is comparable with that of magma in Earth's upper mantle. While of considerable selenological interest, Hauri's announcement affords little comfort to would-be lunar colonists—the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument. Sony VAIO VPCW111XX/T Battery
Gravity and magnetic fields
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, Sony VAIO VPCW111XX/W Battery
partly caused by the dense mare basaltic lava flows that fill these basins.[71] 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, and some mascons exist that are not linked to mare volcanism.[72] Sony VAIO VPCW111XXP Battery
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 global dipolar magnetic field, as would be generated by a liquid metal core geodynamo, and only has crustal magnetization, Sony VAIO VPCW111XXT Battery
probably acquired early in lunar history when a geodynamo was still operating.[73][74] Alternatively, some of the remnant magnetization may be from transient magnetic fields generated during large impact events, Sony VAIO VPCP118JC/W Battery
through the expansion of an impact-generated plasma cloud in the presence of an ambient magnetic field—this is supported by the apparent location of the largest crustal magnetizations near the antipodes of the giant impact basins.[75]
Atmosphere
The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 metric tons.[76] Sony VAIO VPC-P118KX/B Battery
The surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa); it varies with the lunar day. Its sources include outgassing and sputtering, the release of atoms from the bombardment of lunar soil by solar wind ions. Elements that have been detected include sodium and potassium, produced by sputtering, Sony VAIO VPC-P118KX/D Battery
which are also found in the atmospheres ofMercury and Io; helium-4 from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, Sony VAIO VPC-P118KX/G Battery
hydrogen and magnesium, which are present in theregolith, is not understood.[78] Water vapour has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith. Sony VAIO VPC-P118KX/P Battery
These gases can either return into the regolith due to the Moon's gravity, or be lost to space: either through solar radiation pressure, or if they are ionised, by being swept away by the solar wind's magnetic field.[78]
Seasons
The Moon's axial tilt with respect to the Ecliptic is only 1.54°, Sony VAIO VPC-P118KX/W Battery
much less than the 23.44° of the Earth. Because of this, the Moon's solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects.From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of Peary crater at the Moon's north pole remain illuminated for the entire lunar day, Sony VAIO VPCP119JC Battery
creating peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters,[60] and these dark craters are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C), Sony VAIO VPCP119JC/BI Battery
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.
The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days[nb 7] (itssidereal period). Sony VAIO VPCP11S1E/B Battery
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 8] (its synodic period).[43] Unlike most satellites of other planets, the Moon orbits nearer the ecliptic plane than to the planet's equatorial plane. Sony VAIO VPC-P11S1E/B Battery
The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbital motion gradually rotates, which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws.[84] Sony VAIO VPCP11S1E/D Battery
Relative size
The Moon is exceptionally large relative to the Earth: a quarter the diameter of the planet and 1/81 its mass.[43] It is the second largest moon orbiting an object in the Solar System relative to the size of its planet. Charon is larger relative to the dwarf planet Pluto, at slightly more than 1/9 (11.6%) of Pluto's mass.[86] Sony VAIO VPC-P11S1E/D Battery
However, the Earth and Moon are still considered a planet–satellite system, rather than a double-planetsystem, as their barycentre, the common centre of mass, is located 1,700 km (about a quarter of the Earth's radius) beneath the surface of the Earth.[87] Sony VAIO VPCP11S1E/G Battery
Appearance from Earth
The Moon is in synchronous rotation: it rotates about its axis in about the same time it takes to orbitthe Earth. This results in it nearly always keeping the same face turned towards the Earth. Sony VAIO VPC-P11S1E/G Battery
The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth.[88] The side of the Moon that faces Earth is called the near side, and the opposite side the far side. Sony VAIO VPCP11S1E/P Battery
The far side is often called the "dark side", but in fact, it is illuminated 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.[89]
The Moon has an exceptionally low albedo, giving it a similar reflectance to coal. Sony VAIO VPCP11S1E/W Battery
Despite this, it is the second brightest object in the sky after the Sun.[43][nb 9] This is partly due to the brightness enhancement of theopposition effect; at quarter phase, the Moon is only one-tenth as bright, rather than half as bright, as at full moon.[90]
Additionally, colour constancy in the visual system recalibrates the relations between the colours of an object and its surroundings, Sony VAIO VPC-P11S1E/W Battery
and since the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the centre, with no limb darkening, due to the reflective properties of lunar soil, which reflects more light back towards the Sun than in other directions. Sony VAIO VPCP11Z9E/B Battery
The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the Moon illusion, first described in the 7th century BC.[91] The full moon subtends an arc of about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (seeeclipses). Sony VAIO VPCS111FM/S Battery
The highest altitude of the Moon in the sky varies: while it has nearly the same limit as the Sun, it alters with the lunar phase and with the season of the year, with the full moon highest during winter. The 18.6-year nodes cycle also has an influence: when the ascending node of the lunar orbit is in the vernal equinox, the lunar declination can go as far as 28° each month. Sony VAIO VPCS115EC Battery
This means the Moon can go overhead at latitudes up to 28° from the equator, instead of only 18°. 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 smile-shaped crescent Moon.[92]
The distance between the Moon and the Earth varies from around 356,400 km to 406,700 km at the extreme perigees (closest) and apogees (farthest). Sony VAIO VPCS115FG Battery
On 19 March 2011, it was closer to the Earth while at full phase than it has been since 1993.[93] Reported as a "super moon", this closest point coincides within an hour of a full moon, and it thus appeared 30 percent brighter, and 14 percent larger than when at its greatest distance. Sony VAIO VPCS117GG Battery
There has been historical controversy over whether features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, Sony VAIO VPCS117GGB Battery
outgassing does occasionally occur, and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km diameter region of the lunar surface was modified by a gas release event about a million years ago.[97][98] The Moon's appearance, like that of the Sun, can be affected by Earth's atmosphere: Sony VAIO VPCS118EC Battery
common effects are a 22° halo ring formed when the Moon's light is refracted through the ice crystals of high cirrostratus cloud, and smaller coronal rings when the Moon is seen through thin clouds.[99]
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. Sony VAIO VPCS119FJ/B Battery
This forms two tidal bulges on the Earth, which are most clearly seen in elevated sea level as ocean tides.[100] Since the Earth spins about 27 times faster than the Moon moves around it, the 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. Sony VAIO VPCS119GC Battery
The ocean tides are magnified by other effects: frictional coupling of water to Earth's rotation through the ocean floors, the inertiaof 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 their gravitational interplay is responsible for spring and neap tides. Sony VAIO VPCS11AFJ Battery
Gravitational coupling between the Moon and the bulge nearest the Moon acts as a torque on the Earth's rotation, drainingangular 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. Sony VAIO VPC-P113KX/B Battery
As a result, the distance between the Earth and Moon is increasing, and the Earth's spin slowing down.[102] Measurements from lunar ranging experimentswith laser reflectors left during the Apollo missions have found that the Moon's distance to the Earth increases by 38 mm per year(though this is only 0.10 ppb/year of the radius of the Moon's orbit). Sony VAIO VPCP113KX/D Battery
Atomic clocks also show that the Earth's day lengthens by about 15 microseconds every year,[104] slowly increasing the rate at which UTC is adjusted by leap seconds. Left to run its course, this tidal drag would continue until the spin of the Earth and the orbital period of the Moon matched. Sony VAIO VPC-P113KX/D Battery
However, the Sun will become a red giant long before that, engulfing the Earth.
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. Sony VAIO VPCP113KX/G Battery
The Earth-induced component arises fromlibration, a result of the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would only be solar tides.[102] 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). Sony VAIO VPC-P113KX/G Battery
The cumulative effects of stress built up by these tidal forces producesmoonquakes. Moonquakes are much less common and weaker than earthquakes, although they can last for up to an hour – a significantly longer time than terrestrial earthquakes – because of the absence of water to damp out the seismic vibrations. Sony VAIO VPCP113KX/P Battery
The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972.
Eclipses
Eclipses can only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Sony VAIO VPC-P113KX/P Battery
Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when the Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. Sony VAIO VPCP113KX/W Battery
The Sun is much larger than the Moon but it is the precise vastly greater distance that coincidentally gives it the same apparent size as the much closer and much smaller Moon from the perspective of the Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. Sony VAIO VPC-P113KX/W Battery
This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses.[109] In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Sony VAIO VPC-P114KX/B Battery
Since the distance between the Moon and the Earth is very slowly increasing over time,[100] 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. Sony VAIO VPC-P114KX/D Battery
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.[110]
Because the Moon's orbit around the Earth is inclined by about 5° to the orbit of the Earth around the Sun, Sony VAIO VPC-P114KX/G Battery
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.[110] 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 18 years.[111] Sony VAIO VPC-P114KX/P Battery
As the Moon is continuously blocking our view of a half-degree-wide circular area of the sky,[nb 10][112] 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. Sony VAIO VPC-P114KX/W Battery
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. Because of the precession of the lunar orbit, each year different stars are occulted.
Early studies
Understanding of the Moon's cycles was an early development of astronomy: Sony VAIO VPCP115JC Battery
by the 5th century BC, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses,[114] and Indian astronomers had described the Moon’s monthly elongation.[115] The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses. Sony VAIO VPCP115JC/B Battery
Later, the physical form of the Moon and the cause of moonlight became understood. The ancient Greekphilosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. Sony VAIO VPCP115JC/D Battery
Although the Chinese of the Han Dynasty believed the Moon to be energy equated toqi, their 'radiating influence' theory also recognized that the light of the Moon was merely a reflection of the Sun, andJing Fang (78–37 BC) noted the sphericity of the Moon.In 499 AD, the Indian astronomer Aryabhata mentioned in hisAryabhatiya that reflected sunlight is the cause of the shining of the Moon. Sony VAIO VPCP115JC/G Battery
The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions.Shen Kuo (1031–1095) of the Song Dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, Sony VAIO VPCP115JC/P Battery
when doused with white powder and viewed from the side, would appear to be a crescent.
In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries. Sony VAIO VPCP115JC/W Battery
However, in the 2nd century BC, Seleucus of Seleuciacorrectly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun.[124] In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the Earth radius for the distance. Sony VAIO VPCP115KG Battery
These figures were greatly improved by Ptolemy (90–168 AD): his values of a mean distance of 59 times the Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively.[125] Archimedes (287–212 BC) invented a planetarium calculating motions of the Moon and the known planets.[126] Sony VAIO VPCP116KG Battery
During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth".[127] 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. Sony VAIO VPC-P116KX/B Battery
Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834-6 Mappa Selenographica of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book Der Mond, the first trigonometrically accurate study of lunar features, Sony VAIO VPC-P116KX/D Battery
included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography.[128] Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions.[43] Sony VAIO VPC-P116KX/G Battery
This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s,[129] leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology. Sony VAIO VPC-P116KX/P Battery
Soviet missions
The Cold War-inspired Space Race between the Soviet Union and the U.S. led to an acceleration of interest in exploration of the Moon. Once launchers had the necessary capabilities, these nations sent unmanned probes on both flyby and impact/lander missions. Sony VAIO VPC-P116KX/W Battery
Spacecraft from the Soviet Union's Luna program were the first to accomplish a number of goals: following three unnamed, failed missions in 1958,[130] 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, Sony VAIO VPCP118JC Battery
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.[43] Sony VAIO VPCP118JC/B Battery
Rock and soil samples were brought back to Earth by three Luna sample return missions (Luna 16in 1970, Luna 20 in 1972, and Luna 24 in 1976), which returned 0.3 kg total.[131] Two pioneering robotic rovers landed on the Moon in 1970 and 1973 as a part of Soviet Lunokhod programme. Sony VAIO VPCP118JC/P Battery
United States missions
American lunar exploration began with robotic missions aimed at developing understanding of the lunar surface for an eventual manned landing: the Jet Propulsion Laboratory's Surveyor program landed its first spacecraft four months after Luna 9. Sony VAIO VPCF11AGJ Battery
NASA's manned Apollo program was developed in parallel; after a series of unmanned and manned tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar flight, in 1968 Apollo 8 made the first crewed mission to lunar orbit.Sony VAIO VPCF11AHJ Battery
The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race.[132] Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969. Sony VAIO VPCF11JFX/B Battery
The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) returned 382 kg of lunar rock and soil in 2,196 separate samples.[134] The American Moon landing and return was enabled by considerable technological advances in the early 1960s, in domains such as ablationchemistry, Sony VAIO VPCF11M1E Battery
software engineering and atmospheric re-entry technology, and by highly competent management of the enormous technical undertaking.
Scientific instrument packages were installed on the lunar surface during all the Apollo missions. Sony VAIO VPCF11M1E/H Battery
Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17landing sites. Direct transmission of data to Earth concluded in late 1977 due to budgetary considerations,Sony VAIO VPCF11MFX/B Battery
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.[139] Sony VAIO VPCF11S1E Battery
Current era: 1990–present
Post-Apollo and Luna, many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft. The spacecraft released a smaller probe,Hagoromo,Sony VAIO VPCF11S1E/B Battery
in lunar orbit, but the transmitter failed, preventing further scientific use of the mission.[140] 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.[141] Sony VAIO VPCF11Z1E Battery
This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.[142] Sony VAIO VPCF11Z1E/BI Battery
The European spacecraft SMART-1, the second ion-propelled spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface.[143] China has expressed ambitious plans for exploring the Moon, Sony VAIO VPCF11ZHJ Battery
and successfully orbited its first spacecraft, Chang'e-1, from 5 November 2007 until its controlled lunar impact on 1 March 2008.[144] In its sixteen-month mission, it obtained a full image map of the Moon. Between 4 October 2007 and 10 June 2009, the Japan Aerospace Exploration Agency's Kaguya (Selene) mission, Sony VAIO VPCF127HGBI Battery
a lunar orbiter fitted with a high-definition video camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit.[145][146] India's first lunar mission, Chandrayaan I, orbited from 8 November 2008 until loss of contact on 27 August 2009, Sony VAIO VPCF137HG/BI Battery
creating a high resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil.[147] The Indian Space Research Organisation plans to launch Chandrayaan II in 2013, which is slated to include a Russian robotic lunar rover. Sony VAIO VPC-P111KX/B Battery
The U.S. co-launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor and follow-up observation orbiter on 18 June 2009; LCROSS completed its mission by making a planned and widely observed impact in the craterCabeus on 9 October 2009, Sony VAIO VPC-P111KX/D Battery
while LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery. In November 2011, the LRO passed over the Aristarchus crater, which spans 40 kilometres and sinks more than 3.5 kilometres deep. The crater is one of the most visible ones from Earth. Sony VAIO VPC-P111KX/G Battery
"The Aristarchus plateau is one of the most geologically diverse places on the Moon: a mysterious raised flat plateau, a giant rille carved by enormous outpourings of lava, fields of explosive volcanic ash, and all surrounded by massive flood basalts", said Mark Robinson, Sony VAIO VPC-P111KX/P Battery
principal investigator of the Lunar Reconnaissance Orbiter Camera at Arizona State University. NASA released photos of the crater on 25 December 2011.[151]
Two GRAIL spacecraft begin orbiting the Moon around 1 January 2012.[152] Sony VAIO VPC-P111KX/W Battery
Other upcoming lunar missions include Russia's Luna-Glob: an unmanned lander, set of seismometers, and an orbiter based on its Martian Fobos-Grunt mission, which is slated to launch in 2012.[153][154] Privately funded lunar exploration has been promoted by the Google Lunar X Prize, announced 13 September 2007, Sony VAIO VPC-P112KX/B Battery
which offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria.[155] Shackleton Energy Company is building a program to establish operations on the south pole of the Moon to harvest water and supply their Propellant Depots.[156] Sony VAIO VPC-P112KX/D Battery
NASA began to plan to resume manned missions following the call by U.S. President George W. Bush on 14 January 2004 for a manned mission to the Moon by 2019 and the construction of a lunar base by 2024.[157] The Constellation program was funded and construction and testing begun on a manned spacecraft and launch vehicle, Sony VAIO VPC-P112KX/G Battery
and design studies for a lunar base.[159] However, that program has been cancelled in favour of a manned asteroid landing by 2025 and a manned Marsorbit by 2035.[160] India has also expressed its hope to send a manned mission to the Moon by 2020.[161]
Astronomy from the Moon
For many years, the Moon has been recognized as an excellent site for telescopes. Sony VAIO VPC-P112KX/P Battery
It is relatively nearby; astronomical seeing is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth.[163] The lunar soil, Sony VAIO VPC-P112KX/W Battery
although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes andepoxies in the construction of mirrors up to 50 meters in diameter.[164] A lunar zenith telescope can be made cheaply with ionic liquid.[165]
Legal status
Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. Sony VAIO VPCP113KX/B Battery
flags were symbolically planted at their landing sites by the Apollo astronauts, no nation currently claims ownership of any part of the Moon's surface.[166] Russia and the U.S. are party to the 1967 Outer Space Treaty,[167] which defines the Moon and all outer space as the "province of all mankind".[166] Sony VAIO VPCEB1MFX Battery
This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction.[168] The 1979 Moon Agreement was created 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. Sony VAIO VPCEB1MFX/BI Battery
While several individuals have made claims to the Moon in whole or in part, none of these are considered credible.
In culture
The Moon's regular phases make it a very convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. Sony VAIO VPCEB1NFX Battery
Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon.The ~30-day month is an approximation of the lunar cycle. The English noun monthand its cognates in other Germanic languages stem from Proto-Germanic *mǣnṓth-,Sony VAIO VPCEB1NFX/B Battery
which is connected to the above mentioned Proto-Germanic *mǣnōn, indicating the usage of a lunar calendar among the Germanic peoples (Germanic calendar) prior to the adoption of a solar calendar.The same Indo-European root as moon led, via Latin, to measure and menstrual, Sony VAIO VPCEB1NFX/L Battery
words which echo the Moon's importance to many ancient cultures in measuring time (see Latin mensis and Ancient Greek μήνας (mēnas), meaning "month").
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. Sony VAIO VPCEB1NFX/P Battery
A 5,000-year-old rock carving at Knowth, Ireland, may represent the Moon, which would be the earliest depiction discovered.[179] 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. Sony VAIO VPCEB1NFX/W Battery
In many prehistoric and ancient cultures, the Moon was personified as a deity or other supernatural phenomenon, and astrological views of the Moon continue to be propagated today.
The Moon has a long association with insanity and irrationality; Sony VAIO VPCEB1PFX Battery
the words lunacy and lunatic (popular shorteningloony) are derived from the Latin name for the Moon, Luna. Philosophers such as Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, Sony VAIO VPCEB1PFX/B Battery
but the Moon's gravity is too slight to affect any single person.[180] Even today, people insist that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, although there is no scientific evidence to support such claims.
The Moon illusion is an optical illusion in which the Moon appears larger near the horizon than it does while higher up in the sky. Sony VAIO VPCEB1QGX Battery
This optical illusion also occurs with the sun and star constellations. It has been known since ancient times, and recorded by numerous different cultures. The explanation of this illusion is still debated.
Proof of illusion
A popular belief, stretching back at least to Aristotle in the 4th century B.C., Sony VAIO VPCEB1QGX/BI Battery
holds that the Moon appears larger near the horizon due to a real magnificationeffect caused by the Earth's atmosphere. This is not true: although the atmosphere does change the perceived color of the Moon, it does not magnify or enlarge it.[3] In fact, the Moon appears about 1.5% smaller when it is near the horizon than when it is high in the sky, Sony VAIO VPCEB1RGX Battery
because it is farther away by up to one Earth radius and also because of atmospheric refraction, which makes the image of the Moon slightly smaller in the vertical direction. (Note that between differentfull moons, the Moon's angular diameter can vary from 33.5 arc minutes at perigee to 29.43 arc minutes at apogee—a difference of over 10%.[4] Sony VAIO VPCEB1RGX/BI Battery
This is because of the ellipticity of the Moon's orbit.)
The angle that the full Moon subtends at an observer's eye can be measured directly with a theodolite to show that it remains constant as the Moon rises or sinks in the sky (discounting the very small variations due to the physical effects mentioned). Sony VAIO VPCEB20 Battery
Photographs of the Moon at different elevations also show that its size remains the same.
A simple way of demonstrating that the effect is an illusion is to hold a small object (say, 1/4 inch wide) at arm's length (25 inches) with one eye closed, positioning it next to the seemingly large Moon. Sony VAIO VPCEC20 Battery
When the Moon is higher in the sky, positioning the same object near the Moon reveals that there is no change in size.
Possible explanations
Ptolemy is credited with attempts to explain the Moon illusion through atmospheric refraction, although this is disputed.[5] Sony VAIO VPCEE20 Battery
In the Book of Optics (1011-1022 C.E.), Ibn al-Haytham (Alhazen) argued against refraction as an explanation, instead proposing an explanation based in human perception. His argument was that judging the distance of an object depends on there being an uninterrupted sequence of intervening bodies between the object and the observer; Sony VAIO VPCEF20 Battery
however, since there are no intervening objects between the Earth and the Moon, the observed distance is inaccurate and the Moon appears larger on the horizon. Through additional works (by Roger Bacon, John Pecham, Witelo, and others) based on Ibn al-Haytham's explanation, the Moon illusion came to be accepted as a psychological phenomenon in the 17th century.[1] Sony VAIO VPCF112FX/B Battery
Most recent research on the Moon illusion has been conducted by psychologists specializing in human perception. After reviewing the many different explanations in their 2002 book The Mystery of the Moon Illusion, Ross and Plug conclude "No single theory has emerged victorious".[6] Sony VAIO VPCF115FG/B Battery
The same conclusion is reached in the 1989 book, The Moon Illusion edited by Hershenson, which offers about 24 chapters written by different illusion researchers.
Angular size and physical size
The "size" of an object in our view can be measured either as angular size Sony VAIO VPCF116FGBI Battery
(the angle that it subtends [is in opposition to] at the eye, corresponding to the proportion of the field of vision that it occupies) or physical size (its real size measured in, say, metres). As far as human perception is concerned, these two concepts are quite distinct. For example, if two small, Sony VAIO VPCF117FJ/W Battery
identical, and familiar objects are placed at distances of five and ten metres respectively, then the more distant object subtends approximately half the angle of the nearer object, but we do not normally perceive that it is half the size. Conversely, if the more distant object did subtend the same angle as the nearer object then we would normally perceive it to be twice as big. Sony VAIO VPCF117HG/BI Battery
A central question pertaining to the Moon illusion, therefore, is whether the horizon moon appears larger because its perceived angular size seems greater, or because its perceived physical size seems greater, or some combination of both. There is currently no firm consensus on this point. Sony VAIO VPCF118FJ/W Battery
Apparent distance hypothesis
An apparent distance theory evidently was first clearly described by Cleomedes around AD 200. The theory proposes that the horizon moon looks larger than the zenith moon because it looks farther away. Sony VAIO VPCF119FC Battery
When we see objects such as clouds, birds and airplanes in the sky, those near the horizon are typically further away from us than those overhead. This may result in the perception of the sky itself as a comparatively flat or only gently curving surface in which objects moving towards the horizon always recede away from us. Sony VAIO VPCF119FC/BI Battery
If we perceive the Moon to be in the general vicinity of those other things we see in the sky, we would expect it to also recede as it approaches the horizon, which should result in a smaller retinal image. But since its retinal image is approximately the same size whether it is near the horizon or not, Sony VAIO VPCF119FJ/BI Battery
our brains, attempting to compensate for perspective, assume that a low moon must be physically larger.
Extensive experiments in 1962 by Kaufman and Rock showed that a crucial causative factor in the illusion is a change in the pattern of cues to distance. (See Ponzo illusion, Depth perception, linear perspective, Texture gradient.) Sony VAIO VPCF11AFJ Battery
The horizon moon is perceived to be at the end of a stretch of terrain receding into the distance, accompanied by distant trees, buildings and so forth, all of which indicate that it must be a long way away, while these cues are absent from the zenith moon. Experiments by many other researchers have found the same result; Sony VAIO VPCEB1AFX Battery
namely, when pictorial cues to a great distance are subtracted from the vista of the large-looking horizon moon it looks smaller. When pictorial cues to an increased distance are added into the vista of the zenith moon, it appears larger.
A potential problem for the apparent distance theory has been that very few people Sony VAIO VPCEB1AFX/B Battery
(perhaps about 5%) perceive the horizon moon as being both larger and farther away. Indeed most people (perhaps 90%) say the horizon moon looks both larger and closer than the zenith moon (Boring, 1962, Hershenson, 1982, McCready, 1965, 1986, Restle, 1970). Most of the rest say it looks larger and about the same distance away as the zenith moon, Sony VAIO VPCEB1AGX Battery
with a few people reporting no Moon illusion at all. Nevertheless, the apparent distance explanation is the one most often found in textbooks.
Those advocating the apparent distance hypothesis might counter that in the Ponzo illusion, people do not perceive one line to actually be farther away than another either, Sony VAIO VPCEB1AGX/BI Battery
or that everything on the horizon is perceived as further away, out of the habit that this is true for every object ever seen there (average-altitude clouds are about 100 times further away at the horizon than when overhead).
Relative size hypothesis
Historically, the best-known alternative to the "apparent distance" theory has been a "relative size" theory. Sony VAIO VPCEB1BGX Battery
This states that the perceived size of an object depends not only on its retinal size, but also on the size of objects in its immediate visual environment. In the case of the Moon illusion, objects in the vicinity of the horizon moon (that is, objects on or near the horizon) exhibit a fine detail that makes the Moon appear larger, Sony VAIO VPCEB1BGX/BI Battery
while the zenith moon is surrounded by large expanses of empty sky that make it appear smaller.[7]
The effect is illustrated by the classic Ebbinghaus illusion shown at the right. The lower central circle surrounded by small circles might represent the horizon moon accompanied by objects of smaller visual extent, Sony VAIO VPCEB1CGX Battery
while the upper central circle represents the zenith moon surrounded by expanses of sky of larger visual extent. Although both central circles are actually the same size, many people think the lower one looks larger. Sony VAIO VPCEB1CGX/BI Battery
Angle of regard hypothesis
According to the angle of regard hypothesis, the Moon illusion is produced by changes in the position of the eyes in the head accompanying changes in the angle of elevation of the moon. Though once popular, this explanation no longer has much support.Sony VAIO VPCEB1DGX Battery
Immanuel Kant refers to the moon illusion in his 1781 text the Critique of Pure Reason when he writes that "the astronomer cannot prevent himself from seeing the moon larger at its rising than some time afterwards, although he is not deceived by this illusion." Sony VAIO VPCEB1DGX/BI Battery
The giant impact hypothesis states that the Moon was formed out of the debris left over from a collision between the Earth and a Mars-sized body, sometime around 4.5 Ga (four and a half billion years ago). The colliding body is sometimes called Theia, for the mythical Greek Titan who was the mother of Selene, the goddess of the moon. Sony VAIO VPCEB1EGX Battery
The giant impact hypothesis is the currently favoured scientific hypothesis for the formation of the Moon.[3]Supporting evidence includes: the identical direction of the Earth's spin and the Moon's orbit,[4] Moon samples which indicate the surface of the Moon was once molten, the Moon's relatively small iron core, Sony VAIO VPCEB1EGX/BI Battery
lower density compared to the Earth, evidence of similar collisions in other star systems (which result in debris disks), and giant collisions are consistent with the leading theories of the formation of the solar system.
There remain several questions concerning the best current models of the giant impact hypothesis. Sony VAIO VPCEB1FGX Battery
The energy of such a giant impact is predicted to heat Earth to produce a global 'ocean' of magma; yet there is no evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle. At present, there is no self-consistent model that starts with the giant impact event and follows the evolution of the debris into a single Moon. Sony VAIO VPCEB1FGX/BI Battery
Other remaining questions include: when did the Moon lose its share of volatile elements; and why Venus, which also experienced giant impacts during its formation, does not host a similar Moon.
In 1898, George Darwin made the suggestion that the Earth and Moon had once been one body. Sony VAIO VPCEB1GGX Battery
Darwin's hypothesis was that a molten Moon had been spun from the Earth because of centrifugal forces, and this became the dominant academic explanation.[5] Using Newtonian mechanics, he calculated that the Moon had actually orbited much closer in the past and was drifting away from the Earth. Sony VAIO VPCEB1GGX/BI Battery
This drifting was later confirmed by American and Soviet experiments using laser ranging targets placed on the Moon.
However, Darwin's calculations could not resolve the mechanics required to trace the Moon backwards to the surface of the Earth.Sony VAIO VPCEB1HGX Battery
In 1946, Reginald Aldworth Dalyof Harvard University challenged Darwin's explanation, adjusting it to postulate that the creation of the Moon was caused by an impact rather than centrifugal forces.[6] Little attention was paid to Professor Daly's challenge until a conference on satellites in 1974 where the idea was reintroduced and later published and discussed in Icarus in 1975 by Drs.Sony VAIO VPCEB1HGX/BI Battery
William K. Hartmann and Donald R. Davis. Their models suggested that, at the end of the planet formation period, several satellite-sized bodies had formed that could collide with the planets or be captured. They proposed that one of these objects may have collided with the Earth, ejecting refractory, volatile-poor dust that could coalesce to form the Moon. Sony VAIO VPCEB1JFX Battery
This collision could potentially explain the unique geological and geochemical properties of the Moon.[7]
A similar approach was taken by Canadian astronomer Alastair G. W. Cameron and American astronomer William R. Ward, who suggested that the Moon was formed by the tangential impact of a body the size of Mars. Sony VAIO VPCEB1JFX/B Battery
The outer silicates of the colliding body would mostly be vaporized, whereas a metallic core would not. Hence, most of the collisional material sent into orbit would consist of silicates, leaving the coalescing Moon deficient in iron. The more volatile materials that were emitted during the collision would probably escape the Solar System, whereas silicates would tend to coalesce. Sony VAIO VPCEB1JFX/G Battery
The name of the hypothesized protoplanet is derived from the mythical Greek titan Theia, who gave birth to the Moon goddess Selene. This designation was first proposed by the English geochemist Alex N. Halliday in 2000 and has since become accepted in the scientific community. Sony VAIO VPCEB1JFX/L Battery
According to modern theories of planet formation, Theia was part of a population of Mars-sized bodies that existed in the Solar System 4.5 billion years ago. Indeed, one of the attractive features of the giant impact hypothesis is that the formation of the Moon fits into the context of the formation of the Earth itself: Sony VAIO VPCEB1JFX/P Battery
during the course of its formation, the Earth is thought to have experienced dozens of collisions with such planet-sized bodies. The Moon-forming collision was only one such "giant impact" and perhaps the last.
Basic model of impact
Astronomers think the collision between Earth and Theia happened about 4.53 Ga; Sony VAIO VPCEB1JFX/W Battery
about 30-50 million years after the Solar System began to form. In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck the Earth at an oblique angle when the latter was nearly fully formed. Computer simulations of this "late-impact" scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s. Sony VAIO VPCEB1KGX Battery
Theia's iron coresank into the young Earth's core, and most of Theia's mantle accreted onto the Earth's mantle. However, a significant portion of the mantle material from both Theia and the Earth was ejected into orbit around the Earth. This material quickly coalesced into the Moon (possibly within less than a month, but in no more than a century). Sony VAIO VPCEB1KGX/B Battery
Estimates based on computer simulations of such an event suggest that some twenty percent of the original mass of Theia ended up as an orbiting ring of debris, and about half of this matter coalesced into the Moon. The Earth would have gained significant amounts of angular momentum and mass from such a collision. Sony VAIO VPCEB1KGX/W Battery
Regardless of the speed and tilt of the Earth's rotation before the impact, it would have had a day some five hours long after the impact, and the Earth's equator and the Moon's orbit would have become coplanar in the aftermath of the giant impact.[11] Sony VAIO VPCEB1LFX Battery
Energetic aftermath
In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks.[2] To their surprise, the team found that the Apollo rocks carried an isotopic signature that was identical with rocks from Earth, Sony VAIO VPCEB1LFX/BI Battery
and different from almost all other bodies in the Solar System. Since most of the material that goes into orbit to form the Moon was thought to come from Theia, this observation was unexpected. In 2007, researchers from the California Institute of Technology showed that the likelihood of Theia having an identical isotopic signature was very small (<1 percent). Sony VAIO VPCEB1LFX/WI Battery
Instead, they proposed that in the aftermath of the giant impact, while the Earth and the proto-lunar disk were molten and vaporized, the two reservoirs were connected by a common silicate vapour atmosphere, and that the Earth-Moon system was homogenized by convective stirring while the system existed in the form of a continuous fluid. Sony VAIO VPCEB12FX/BI Battery
Such an "equilibration" between the post-impact Earth and the proto-lunar disk is the only scenario capable of explaining the isotopic similarities of the Apollo rocks with rocks from the Earth's interior. However, for this scenario to be viable, the proto-lunar disk must exist for a time period of ~100 years. Work is ongoing to determine whether or not this is possible. Sony VAIO VPCEB12FX/BIC Battery
Evidence
Indirect evidence for the giant impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygen isotope ratios identical to those of Earth, which can be explained if the Earth-Moon system experienced turbulent mixing in the aftermath of the giant impact, as discussed above. Sony VAIO VPCEB12FX/T Battery
In addition, the highly anorthositic composition of the lunar crust, as well as the existence of KREEP-rich samples, gave rise to the idea that a large portion of the Moon was once molten, and a giant impact scenario could easily have supplied the energy needed to form such a magma ocean. Sony VAIO VPCEB14FX Battery
Several lines of evidence show that if the Moon has an iron-rich core, it must be small. In particular, the mean density, moment of inertia, rotational signature, and magnetic induction response all suggest that the radius of the core is less than about 25% the radius of the Moon, in contrast to about 50% for most of the other terrestrial bodies. Sony VAIO VPCEB14FX/BI Battery
Impact conditions can be found that give rise to a Moon that formed mostly from the mantles of the Earth and impactor, with the core of the impactor accreting to the Earth, and which satisfy the angular momentum constraints of the Earth-Moon system.[3]
Warm silica-rich dust and abundant SiO gas, products of high velocity (> 10 km/s) Sony VAIO VPCEB14FX/T Battery
impacts between rocky bodies has been detected around the nearby (29 pc distant) young (~12 My old) Beta Pic Moving Group star HD172555 by the Spitzer Space Telescope.[13] A belt of warm dust in a zone between 0.25AU and 2AU from the young star HD 23514 in the Pleiades cluster appears similar to the predicted results of Theia's Sony VAIO VPCEB14FX/WI Battery
collision with the embryonic Earth, and has been interpreted as the result of planet-sized objects colliding with each other.[14] This is similar to another belt of warm dust detected around the star BD +20°307 (HIP 8920, SAO 75016).[15]
Difficulties
This lunar origin hypothesis has some difficulties that have yet to be fully resolved. Sony VAIO VPCEB15FM Battery
For example, the giant impact hypothesis implies that a surface magma ocean would have formed following the impact. Yet there is no evidence that the Earth ever had such a magma ocean and it is likely there exists material that has never been processed by a magma ocean.[16] Sony VAIO VPCEB15FM/BI Battery
Composition
There are a number of compositional inconsistencies that need to be addressed.
The ratios of the Moon's volatile elements are not explained by the giant impact hypothesis. If the giant impact hypothesis is correct, they must be due to some other cause.[16] Sony VAIO VPCEB15FM/T Battery
- The presence of volatiles such as water trapped in lunar basalts is more difficult to explain if the Moon was caused by an impact which would entail a catastrophic heating event.[17]
- The iron oxide (FeO) content (13%) of the Moon, which is intermediate between Mars (18%) and the terrestrial mantle (8%), rules out most of the source of the proto-lunar material from the Earth's mantle.[18]
If the bulk of the proto-lunar material had come from the impactor, Sony VAIO VPCEB15FM/WI Battery
- the Moon should be enriched in siderophilic elements, when it is actually deficient in those.[19]
The Moon's oxygen isotopic ratios are essentially identical to those of Earth.[2] Oxygen isotopic ratios, which can be measured very precisely, yield a unique and distinct signature for each solar system body. Sony VAIO VPCEB15FX Battery
- If Theia had been a separate proto-planet, it would probably have had a different oxygen isotopic signature than Earth, as would the ejected mixed material.[21]
The Moon's titanium isotope ratio (50Ti/47Ti) appears so close to the earth's (within 4 ppm) that little if any of the colliding body's mass could have likely been part of the Moon.[22] Sony VAIO VPCEB15FX/BI Battery
Lack of a Venusian moon
If Earth's moon formed by such an impact, it is likely that other inner planets would also have been subjected to comparable impacts. A moon that formed around Venus by this process would have been unlikely to escape, so an explanation is needed to show why the planet does not have such a moon. Sony VAIO VPCEB15FX/T Battery
One possibility is that a second collision occurred that countered the angular momentum from the first impact.[23] Another is that the strong tidal forces from the Sun would tend to destabilize the orbits of moons around close-in planets. For this reason, if Venus's slow rotation rate began early in its history, any satellites larger than a few kilometres in size would likely have spiralled into the planet. Sony VAIO VPCEB15FX/WI Battery
Simulations of the chaotic period of terrestrial planet formation suggest that impacts, such as those hypothesized to form the Moon, are common. For typical terrestrial planets with a mass of 0.5–1 Earth masses, such an impact typically result in a single moon containing 4% of the host planet's mass. Sony VAIO VPCEB16FX Battery
The inclination of this moon's orbit is random, but this tilt affects the subsequent dynamic evolution of the system. For example, some orbits may cause the moon to spiral back into the planet. Likewise, the proximity of the planet to the star will also affect the orbital evolution. Sony VAIO VPCEB16FX/B Battery
The net effect is that it is more likely for impact-generated moons to survive when they orbit more distant terrestrial planets and to be aligned with the planetary orbit.[25]
Possible origin of Theia
In 2004, Princeton University mathematician Edward Belbruno and astrophysicist J. Sony VAIO VPCEB16FX/G Battery
Richard Gott III proposed that Theia coalesced at the L4 or L5 Lagrangian point relative to Earth (in about the same orbit and about 60° ahead or behind),[26][27]similar to a trojan asteroid.[4] Two-dimensional computer models suggest that the stability of Theia's proposed trojan orbitwould have been affected Sony VAIO VPCEB16FX/L Battery
when its growing mass exceeded a threshold of about 10% of the Earth's mass.[26] In this scenario, gravitational perturbations by planetesimals caused Theia to depart from its stable Lagrangian location, and subsequent interactions with proto-Earth led to a collision between the two bodies.[26] Sony VAIO VPCEB16FX/P Battery
In 2008, evidence was presented that suggests that the collision may have occurred later than the accepted value of 4.53 Ga, at about 4.48 Ga.[28]
It has been suggested that other significant objects may have been created by the impact, which could have remained in orbit between the Earth and Moon, stuck in Lagrangian points. Sony VAIO VPCEB16FX/W Battery
Such objects may have stayed within the Earth-Moon system for up to 100 million years, until the gravitational tugs of other planets destabilized the system enough to free the objects.[29] A study published in 2011 suggested that a subsequent collision between the Moon and one of these smaller bodies caused the notable differences in physical characteristics between the two hemispheres of the Moon. Sony VAIO VPCEB17FX Battery
This collision, simulations have supported, would have been at a low enough velocity so as not to form a crater; the material from the smaller body would instead have spread out across the Moon (in what would become its far side), adding a thick layer of highlands crust.[31] Sony VAIO VPCEB17FX/B Battery
The resulting mass irregularities would subsequently produce a gravity gradient that helped to rotationally lock the Moon so that today only the near side remains visible from Earth.
Alternative hypotheses
Other mechanisms which have been suggested at various times for the Moon's origin are that the Moon was spun off from the Earth's molten surface by centrifugal force,[5] Sony VAIO VPCEB17FX/G Battery
that it was formed elsewhere and later captured by the Earth's gravitational field,[32] or that the Moon formed at the same time and place as the Earth from the same accretion disk. Each of these hypotheses is claimed to lack a mechanism to account for the high angular momentum of the Earth–Moon system.[11] Sony VAIO VPCEB17FX/L Battery
A more recent proposal is that the moon formed much later than previously thought from the impact of a large asteroid creating the moon primarily from debris from earth. The formation of the moon in this hypothesis occurs 60-140 million years after the formation of the solar system. Sony VAIO VPCEB17FX/P Battery
The age of the moon had been previously thought to be 4.527 ± 0.010 billion years ago.[33] The impact would have created a magma ocean on Earth and the proto-moon with both bodies sharing a common plasma metal vapour atmosphere. Sony VAIO VPCEB17FX/W Battery
The shared metal vapour bridge would have allowed material from the Earth and proto-moon to exchange and equilibrate into a more common composition. Sony VAIO VPCEB190X Battery,Sony VAIO VPCEB19FX Battery,Sony VAIO VPCEB19GX Battery
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