Moon The Moon is a familiar object in our sky because it is our nearest neighbour and is the only natural object to orbit the Earth. It is smaller than the Earth, but has all the important attributes of a terrestrial planet except that of being in independent orbit about the Sun. Some basic data are presented in Table 1.

Something that adds to the Moon's familiarity is that it always presents the same face towards us. This is because the Moon rotates exactly once per orbit, in what is described as synchronous rotation. This is a phenomenon that is also exhibited by most of the satellites of the giant planets. It is brought about because tidal forces between the planet and its satellite slow the satellite's rotation until it matches its orbital period.

Table 1 Basic data for the Moon

Missions to the Moon

The Moon was the first extraterrestrial target for space missions. Probes have been directed towards it since almost the very dawn of the space age (Table 2), and it was the main focus of the 1960s–1970s ‘space race’ between the USA and the then Soviet Union. In the end, only NASA (the National Aeronautics and Space Administration) attempted to put people on the Moon, and the six successful Apollo landings brought back at total of 382 kg of lunar rocks. Radiometric dating of these samples, together with 0.3 kg collected from other sites by unmanned Soviet sample return missions, enabled us to calibrate the cratering timescale (based on the number of impact areas per unit area) that is used for dating unsampled areas of the Moon and other surfaces in the inner Solar System. Geochemical analysis of these samples was immensely important in developing our current level of understanding of the Moon's origin and history.

After the budget for the Apollo programme was terminated in 1972, there was little further effort in lunar exploration until 1994, when the Clementine probe went into lunar orbit and collected a wealth of previously unknown information about the topography, crustal thickness, and variations in crustal composition across the whole lunar globe. This was followed in 1998 by another orbiter, Lunar Prospector, which provided even more insights and discoveries, such as the existence of ice dispersed within the regolith at both poles. The European Space Agency and Japan each plan lunar missions for the early years of the twenty-first century.

Origin and history of the Moon

The Moon apparently owes its origin to a giant impact between a Moon-sized or Mars-sized body and the primitive Earth towards the end of planet formation 4.5 billion years (4.5 Ga) ago. This was the last of several giant impacts by which the Earth grew to its current size, but it was unusual in that much of the debris from the fragmented impactor plus some ejecta from the Earth itself ended up in orbit about the Earth, where it collected together to form the Moon. Once the last giant impact in the inner Solar System had occurred, there were just four surviving terrestrial planets (Mercury, Venus, Earth, and Mars). The total is five if you count the Moon, which is a planet in a geological sense (because of its size and character) though not in an astronomical sense (because it orbits the Earth rather than the Sun).

Table 2 Some of the successful and anticipated missions to the Moon

Polar ice and the atmosphere

The Clementine mission returned our first clear views of the lunar poles, showing sites in particular near the south pole that are permanently in shadow, and could therefore be places where ice might accumulate. Clementine's simple radar gave the first indications that frozen water is present there, and this theory was dramatically backed up by measurements by Lunar Prospector that show a reduction in the average speed of neutrons (produced by cosmic radiation) over both lunar poles. The only reasonable explanation of this seems to be collisions between neutrons and the hydrogen atoms within ice molecules, and it now seems that there may be as much as three billion tonnes of ice mixed with the regolith at each pole. This is not a lot in terms of the size of the Moon (it is the equivalent of a 1.5 km ice cube at each pole), but it could be ample to supply the immediate needs of human habitation on the Moon.

The Moon's gravity is too slight for it to be able to retain a gaseous envelope, so its atmosphere is tenuous in the extreme. Atoms that have been detected in the Moon's atmosphere are listed in Table 1. These are all light enough to escape to space, and what has been detected must be a steady-state mixture that is continually replenished. Presumably these elements are either supplied continually by micrometeorites or are liberated from the surface under the influence of solar radiation or meteorite impact.

Although water has not been detected in the atmosphere, it is likely that some or most of the Moon's polar ice derives from molecules of atmospheric water (supplied by cometary impact or outgassing) that wandered into the cold polar regions and became incorporated into the surface, thus avoiding the more usual fate of escaping or becoming split into hydrogen and oxygen atoms by radiation.

The interior

The Moon is the only planetary body other than the Earth for which we have any seismic data that can tell us about its interior. Four seismic stations were established at the Apollo 12, 14,15, and 16 sites and these continued to send back data until September 1977, when operations were terminated for budgetary reasons. The Moon is seismically very quiet compared to the Earth, and moonquakes are many orders of magnitude less powerful than typical earthquakes. The Apollo seismometers also detected the vibrations from meteorite impacts (including one from the far side that gave crucial data on the maximum possible size of the core) and deliberate crashes of various expended units of the Apollo spacecraft.

The picture that has emerged from combining Apollo seismic data with topographical and gravitational mapping by Clementine and Lunar Prospector is that the Moon's crust is on average about 70 km thick. However, it exceeds 100 km in some highland regions and falls to about 20 km thick below some impact basins. The crust is thicker on the far side, with the result that the Moon's centre of mass is offset from its geometric centre by about 2 km towards the Earth.

The mantle is rigid to a depth of about 1000 km, which is also the greatest depth for the source of moonquakes. This depth therefore marks the base of the lunar lithosphere, below which it is probably slowly convecting. Like the Earth's mantle, that of the Moon is approximately peridotite in composition, though there are signs that it varies in detail from region to region.

The Moon's core is between about 220 and 450 km in radius (so the core–mantle boundary is at least 1290 km depth), and is probably iron-rich and solid. It makes up less than 4 per cent of the Moon's total mass, a much smaller fraction than for any of the other terrestrial planets. This can be explained by computer models of the giant impact in which the Moon was born, which show the core of the impacting body accreting on to the core of the proto-Earth, whereas the Moon formed from the mixture of fragments of the mantles of the two bodies that were thrown into space. The fact that no magnetic field is generated in the Moon's core today shows that it must be solid. There are, however, local weak remanent magnetic fields associated with some of the Moon's major impact basins that suggest that part of the core may still have been liquid and generating a global field when these formed 3.6 Ga ago.

The surface

Look at the Moon even with the unaided eye, and you will see that it has dark patches on a paler background. This simple observation picks out the two distinct types of crust on the Moon. The paler areas are the lunar highlands, and the darker areas are the lunar ‘seas’ or maria (singular: mare). On both the highlands and the maria there is very little bedrock exposed at the surface, because it is buried by several metres of regolith composed of ejecta from crater-forming impacts at all scales. In detail the regolith consists of a mixture of rock fragments, crystal fragments, and droplets of glass (which are congealed globules of melt produced by impacts). This forms a ‘soil’ capable of bearing the imprint of a booted foot (Fig. 1, left), but there are also scattered boulders of a variety of sizes (Fig. 1, right).

The highlands are the first lunar crust to have formed, probably by crystals rising to the surface of the ‘magma ocean’ that is thought to have covered the young Moon as a result of the heat generated as the Moon-forming fragments collided. Because of the preponderance of the mineral anorthite (a calcium-rich variety of the mineral plagioclase feldspar) this rock type is called anorthosite. The oldest anorthosite sample collected from the Moon has been dated at 4.5 Ga, but the highland surface suffered such heavy bombardment (until the rate of impact cratering declined towards its current level) that most highland rock samples consist of fragments of rock that were welded together between about 4.0 and 3.8 Ga ago.

The high degree of cratering suffered by the highland crust is notable in the view seen in Fig. 2. This also illustrates the dependence of crater morphology on size. Craters less than about 15 km across are simple bowl-shaped depressions with raised rims, but the largest crater in this view has the terraced inner walls and central peak that are characteristic of craters larger than this. At yet larger diameters the central peak is replaced by a ring of peaks. The largest impact structures of all are multi-ringed impact basins dating from 4.0 to 3.8 Ga ago. At 2500 km in diameter, the south pole Aitken basin, which lies on the far side, is the largest lunar example, but there are several others, notably the Imbrium basin, occupying the north-west of the Moon's near side.

All the Moon's multi-ringed impact basins are older than the Moon's second kind of crust, consisting of basalts that have flooded low-lying areas to form the lunar maria. Many of the maria occupy multi-ringed impact basins, and it was once thought that the melting to produce the mare-filling basalts was a direct response to the basin-forming impacts. However, the ages of samples of mare basalt that have been returned to Earth range from 3.8 to 3.1 Ga; so clearly the magma was from unrelated sources and ended up in the basins merely because it took advantage of the easiest routes to the surface. Inspection of detailed images of some mare areas that have yet to be sampled suggest that the mare-forming eruptions did not cease until about a billion years ago. The only extensive mare regions are on the lunar near side, and most of the far-side features (even the south pole Aitken basin) have escaped mare flooding, presumably because of the greater thickness of far-side crust.

The mare basalts probably came into place as series of extensive lava flows, amounting to some hundreds of metres in total thickness. In some places the margins of individual lava flows can still be seen, and elsewhere there is spectacular evidence of the lava having flowed in tubes, which are revealed by later collapse of their roofs. A good example of this is seen in Fig. 3, which shows Hadley Rille, the main focus of the Apollo 15 surface explorations. Most of the top left view shows the Apennine mountains, which are highland crust forming the rim of the Imbrium basin (a large near-side multi-ringed impact basin). Hadley Rille emerges from an unseen source at the edge of the mountains, and snakes across the much flatter mare basalt terrain, appearing to have acted as a channelway that fed the later stages of mare flooding. It probably became roofed over by chilled and solidified lava, and this insulating cap enabled a vast quantity of lava to continue to flow through the tube without congealing. The rille is now visible because the roof of the tube collapsed after it became drained of lava.

Living on the Moon?

The projected date for a permanent lunar base seems to have been receding into the future ever since the demise of the Apollo programme. However, unless we give up space exploration entirely it is bound to happen eventually. One major objection to lunar habitation, the lack of water, has been removed by the discovery of substantial quantities of ice near the poles. No convincing reasons have yet emerged for basing industries on the Moon, but apart from the advantages for lunar exploration itself the far side would offer an excellent place from which to do radio astronomy, shielded from the increasing interference from cellular telephones and the like.

David A. Rothery

Bibliography

Spudis, P. D. (1996) The once and future Moon. Smithsonian University Press, Washington D.C.
Rothery, D. A. (2000) Teach yourself planets. Hodder and Stoughton. London

Moon

Our solitary and prominent Moon orbits Earth at a mean distance of only 382,000 kilometers (236,840 miles). The nearest planet, Venus, is never closer than 40 million kilometers (25 million miles). The Moon's mass is just under one-eightieth that of Earth, its volume just over one-fiftieth; the difference mainly stems from the Moon lacking a large metallic iron core and therefore having a much lower overall density than Earth. Its low mass is responsible for the low surface gravity (one-sixth that at Earth's surface), popularly recognized in the jumping, bouncing gait of Apollo astronauts. The mass is much too low for the Moon to hold any significant atmosphere—it is essentially in a vacuum —or for its surface to have liquid water.

The surface area of the Moon is only about four times that of the land area of the United States. The Moon is not as large as any planet other than distant little Pluto but is of the same scale as the Galilean satellites of Jupiter. These moons are much smaller in comparison with the planet they orbit. Earth's Moon is very different in chemical composition and structure—and probably origin—from any other body in the solar system.

Orbit and Rotation

The 29.53-day orbit provides us with the lunar phases, as well as the occasional eclipses of the Sun and the more frequent eclipses of the Moon. The orbit is tilted only slightly (5.1°) from the plane of the ecliptic , but because Earth itself has a tilted axis of rotation (23.5°), the Moon's orbit is tilted substantially with respect to Earth's equator. The Moon's own axial rotation period is exactly the same as its orbital period, and so it shows almost the same face to Earth continuously. It is not exactly the same face because of the tilt of the Moon's rotational axis (1.5°) to its orbital plane around Earth, and the slight ellipticity of that orbit (the position of the observer on Earth also has a slight effect). Altogether, only 41 percent of the Moon's surface is permanently invisible to observers on Earth.

The gravitational pull of the Moon provides the twice-daily tides on Earth as Earth spins under the Moon. The Moon is gradually receding because of the tidal effects. As the Moon recedes, its angular momentum increases, compensated by a decrease in the spin rate of Earth. Thus, Earth's day is increasing in length; 600 million years ago it was only about eighteen hours long. The Moon stabilizes the tilt of Earth's own axis of rotation over long periods of time, and this has been important for stabilizing climate and thus life habitats.

The Exploration of the Moon

Even to the naked eye the Moon's face has darker and lighter patches. Italian mathematician and astronomer Galileo Galilei used a telescope in 1610 to discover its rugged, varied, and essentially unchanging features. He distinguished the brighter areas as higher and more rugged, the darker as lower, flatter, and smoother. He called the former "terra" (meaning "land"; pl. "terrae") and the latter "mare" (meaning "sea"; pl. "maria"), although that is not what they are.

For three centuries the Moon remained an object of astronomical study, with the collection of data about its shape, size, movements, and surface physical properties, as well as mapping. Not until the middle of the twentieth century were observations and a combination of natural and terrestrial analogs advanced enough that the volcanic origin of its dark plains and the impact origin of its craters and basins could be considered as settled. In the 1960s, a program of geological mapping, using techniques such as crater counting and overlapping relationships, confirmed and elucidated the nature of geological units and the order in which they were produced.

The study of the Moon reached peak activity in the space age, when spacecraft sent back detailed information from orbiters, hard-landers , and soft-landers (mainly from 1959 to 1970), and Apollo astronauts conducted experiments and made observations from equatorial orbit and at the surface (from 1968 to 1972). Six Apollo missions and three robotic samplereturn

vehicles collected samples of the Moon (from 1970 to 1976). Samples are particularly useful for understanding the processes that created the rocks and for the dating of events using radiogenic isotope techniques . Two flybys by the Galileo mission^* to Jupiter (in 1990 and 1992), the Clementine lunar polar orbiter (in 1994) and the Lunar Prospector polar orbiter (in 1998) have provided substantially more global imaging, topographic, chemical, and mineralogical data.

Global and Interior Characteristics

The Moon is nearly homogeneous, as shown by its motions in space, and by the fact that rocks near the surface are not much different in density from the Moon as a whole. Nonetheless, samples show that the Moon was thoroughly heated at its birth about 4.5 billion years ago, possibly to the point of total melting, and then quickly solidified to produce a comparatively thin (60 to 100 kilometers [37 to 62 miles]) crust of slightly lighter material. This structure was confirmed by seismic experiments performed on the early Apollo missions. There may be an iron core, but if so it is very tiny, and there is no significant magnetic field.

Samples show that the Moon is very depleted in volatile elements (those that form gases and low-temperature boiling-point liquids), to the extent that it lacks any water of its own at all, even bonded into rocks. Water delivered to the Moon by cometary impact might exist, frozen in crater floors near the poles. The Moon is very reduced chemically, such that iron metal exists, but rust (oxidized, ferric iron) does not. The Moon is very depleted in the siderophile elements ("iron-loving") that go with metallic iron into a core, except for the surface rubble to which such elements have been delivered by eons of meteorite impact.

The Uppermost Surface of the Moon

The Moon has been bombarded by meteorites ranging in size from numerous tiny dust particles to rare objects hundreds of kilometers in diameter. The surface is covered everywhere with a thin fragmental layer (known as soil, or "regolith") that consists mainly of ground-up and remelted lunar rocks, with an average grain size of less than 0.1 millimeters (0.004 inch). This soil contains pebbles, cobbles, and even boulders of lunar rocks. A small percentage of the regolith consists of the meteoritic material that did the bombarding. The regolith is about 5 meters (16.5 feet) thick on basalts that were poured out about 3 billion years ago, while older surfaces have even thicker regoliths. This regolith layer, exposed to cosmic radiation and the solar wind , contains materials, such as hydrogen, that do not reach the surface of Earth because of its protection by both a magnetic field and an atmosphere.

The Older Crust of the Moon

Much of the crust consists of material that formed within a few tens of millions of years of the Moon's origin, partly by the floating of light (in both density and color) feldspar minerals , which crystallized from a vast ocean of silicate magma. The magma formed because of the Moon's rapid formation, and because of the generation of radioactive heat, which was greater then than now. Continued melting and remelting added to the crust, and the final dregs of the crystallizing magma ocean, richer in those elements that do not easily fit into common crystallizing minerals (feldspar, pyroxene, and olivine), also ended up in the crust. The rocks from the dregs are commonly called "KREEP"-rich because they are richer in potassium (K), rare Earth elements (REE) such as lanthanum, and phosphorus (P) than are typical rocks. Most, though not all, of this crust was in place by 4.3 billion years ago.

At its birth and at about 3.9 billion years ago (what happened in the time between remains somewhat unknown) the Moon was subjected to enormous bombardments that created deep basins as well as numerous small craters, partly disrupting the crust. This crust is somewhat thinner on the front side (about 60 kilometers [37 miles]) than on the farside (about 100 kilometers [62 miles]).

The Younger Crust of the Moon

Impacts decreased substantially after 3.8 billion years ago, to a level close to that of today by about 3.2 billion years ago. The Moon's deep basins, partly filled with overlapping thin flows of mare basalt, formed from the melting of small amounts of the lunar interior. These basins (150 kilometers [93 miles] to perhaps 500 kilometers [310 miles] deep) are prominent as the dark plains—the maria—of the Moon and show many signs of volcanic flow. Some of the volcanic lava erupted as fiery fountains, forming heaps of glass spherules . These lavas comprise only about 1 percent of the crust, but as the latest, topmost rocks, least affected by impacts, they remain clearly visible. They are much less abundant on the lunar farside, and everywhere their formation had ceased by 2 billion years ago. The Moon is now magmatically dead, and its uppermost crust is being continually gardened and converted into regolith.

The Origin of the Moon

Earth and the Moon show an identical relationship of oxygen isotope ratios (oxygen being the most common element in both planets), a relationship that is different from all other measured solar system objects (including Mars) except yEH chondrites. This indicates that Earth and the Moon formed in the same part of the solar system and gives credence to ideas that the Moon formed from Earth materials.

The pre-Apollo ideas of either capture, fission from Earth (by rapid spinning), or formation together as a double planet are not consistent with what scientists now know from geological or sample studies, nor with the orbital and angular momentum constraints. Thus a new concept was developed in the 1980s: Earth collided during its growth with an approximately Mars-sized object, producing an Earth-orbiting disk of material that accumulated to form the Moon. This idea can account for many features, including the chemistry of the Moon, its magma ocean, and even the tilt of Earth's axis. It is compatible with concepts of how planets develop by accumulation of solid objects. One of the implications of this theory is that the Moon actually must have accumulated very rapidly, on the order of days to years, rather than older ideas of tens of millions of years, and this explains the early melting of the Moon.

see also Apollo (volume 3); Apollo Lunar Landing Sites (volume 3); Exploration Programs (volume 2); Galilei, Galileo (volume 2); Lunar Bases (volume 4); Lunar Outposts (volume 3); Lunar Rovers (volume 3); Nasa (volume 3); Planetary Exploration, Future of (volume 2); Robotic Exploration of Space (volume 2); Shoemaker, Eugene (volume 2).

Graham Ryder

Bibliography

Heiken, Grant H., David Vaniman, Bevan M. French, and Jack Schmidt, eds. The Lunar Sourcebook: A User's Guide to the Moon. Cambridge, UK: Cambridge University Press, 1991.

Ryder, Graham. "Apollo's Gift: The Moon." Astronomy 22, no. 7 (1994):40-45.

Spudis, Paul D. "An Argument for Human Exploration of the Moon and Mars."American Scientist 80, no. 3 (1992):269-277.

——. The Once and Future Moon. Washington, DC, and London: Smithsonian Institution Press, 1996.

——. "The Moon." In The New Solar System, eds. J. Kelly Beatty, Carolyn C. Peterson, and Andrew Chaikin. Cambridge, UK: Cambridge University Press, 1999.

Taylor, G. Jeffrey. "The Scientific Legacy of Apollo." Scientific American 271, no. 1(1994):26-33.

Wilhelms, Donald E. To a Rocky Moon: A Geologist's History of Lunar Exploration. Tucson: University of Arizona Press, 1993.

National Aeronautics and Space Administration See NASA (Volume 3).

^*The Galileo mission successfully used robots to explore the outer solar system. This mission used gravity assists from Venus and Earth to reach Jupiter, where it dropped a probe into the atmosphere and studied the planet for nearly seven years.

moon natural satellite of a planet (see satellite, natural ) or dwarf planet, in particular, the single natural satellite of the earth .

The Earth-Moon System

The moon is the earth's nearest neighbor in space. In addition to its proximity, the moon is also exceptional in that it is quite massive compared to the earth itself, the ratio of their masses being far larger than the similar ratios of other natural satellites to the planets they orbit (though that of Charon and the dwarf planet Pluto exceeds that of the moon and earth). For this reason, the earth-moon system is sometimes considered a double planet. It is the center of the earth-moon system, rather than the center of the earth itself, that describes an elliptical orbit around the sun in accordance with Kepler's laws . It is also more accurate to say that the earth and moon together revolve about their common center of mass, rather than saying that the moon revolves about the earth. This common center of mass lies beneath the earth's surface, about 3,000 mi (4800 km) from the earth's center.

The Lunar Month

The moon was studied, and its apparent motions through the sky recorded, beginning in ancient times. The Babylonians and the Maya, for example, had remarkably precise calendars for eclipses and other astronomical events. Astronomers now recognize different kinds of months, such as the synodic month of 29 days, 12 hr, 44 min, the period of the lunar phases , and the sidereal month of 27 days, 7 hr, 43 min, the period of lunar revolution around the earth.

The Lunar Orbit

As seen from above the earth's north pole, the moon moves in a counterclockwise direction with an average orbital speed of about 0.6 mi/sec (1 km/sec). Because the lunar orbit is elliptical, the distance between the earth and the moon varies periodically as the moon revolves in its orbit. At perigee, when the moon is nearest the earth, the distance is about 227,000 mi (365,000 km); at apogee, when the moon is farthest from the earth, the distance is about 254,000 mi (409,000 km). The average distance is about 240,000 mi (385,000 km), or about 60 times the radius of the earth itself. The plane of the moon's orbit is tilted, or inclined, at an angle of about 5° with respect to the ecliptic . The line dividing the bright and dark portions of the moon is called the terminator.

Retarded Lunar Motion

Due to the earth's rotation, the moon appears to rise in the east and set in the west, like all other heavenly bodies; however, the moon's own orbital motion carries it eastward against the stars. This apparent motion is much more rapid than the similar motion of the sun. Hence the moon appears to overtake the sun and rises on an average of 50 minutes later each night. There are many variations in this retardation according to latitude and time of year. In much of the Northern Hemisphere, at the autumnal equinox , the harvest moon occurs; moonrise and sunset nearly coincide for several days around full moon. The next succeeding full moon, called the hunter's moon, also shows this coincidence.

Solar and Lunar Eclipses

Although an optical illusion causes the moon to appear larger when it is near the horizon than when it is near the zenith, the true angular size of the moon's diameter is about 1/2 °, which also happens to be the sun's apparent diameter. This coincidence makes possible total eclipses of the sun in which the solar disk is exactly covered by the disk of the moon. An eclipse of the moon occurs when the earth's shadow falls onto the moon, temporarily blocking the sunlight that causes the moon to shine. Eclipses can occur only when the moon, sun, and earth are arranged along a straight line—lunar eclipses at full moon and solar eclipses at new moon.

Tidal Influence of the Moon

The gravitational influence of the moon is chiefly responsible for the tides of the earth's oceans, the twice-daily rise and fall of sea level. The ocean tides are caused by the flow of water toward the two points on the earth's surface that are instantaneously directly beneath the moon and directly opposite the moon. Because of frictional drag, the earth's rotation carries the two tidal bulges slightly forward of the line connecting earth and moon. The resulting torque slows the earth's rotation while increasing the moon's orbital velocity. As a result, the day is getting longer and the moon is moving farther away from the earth. The moon also raises much smaller tides in the solid crust of the earth, deforming its shape. The tidal influence of the earth on the moon was responsible for making the moon's periods of rotation and revolution equal, so that the same side of the moon always faces earth.

Physical Characteristics

The study of the moon's surface increased with the invention of the telescope by Galileo in 1610 and culminated in 1969 when the first human actually set foot on the moon's surface. The physical characteristics and surface of the moon thus have been studied telescopically, photographically, and more recently by instruments carried by manned and unmanned spacecraft (see space exploration ). The moon's diameter is about 2,160 mi (3,476 km) at the moon's equator, somewhat more than 1/4 the earth's diameter. The moon has about 1/81 the mass of the earth and is 3/5 as dense. On the moon's surface the force of gravitation is about 1/6 that on earth. It has been established that the moon completely lacks an atmosphere and, despite some tantalizing hints that there might be ice under the surface dust in shaded portions of Shackleton Crater (near the moon's south pole), there is no definite evidence of water. The surface temperature rises above 100°C (212°F) at lunar noon and sinks below -155°C (-247°F) at night. The gross surface features of the moon are visible to the unaided eye and were first studied telescopically in 1610 by Galileo.

Surface Features

The lunar surface is divided into the mountainous highlands and the large, roughly circular plains called maria (sing. mare; from Lat.,=sea) by early astronomers, who erroneously believed them to be bodies of water. The smooth floors of the maria, varying from flat to gently undulating, are covered by a thin layer of powdered rock that darkens them and accounts for the moon's low albedo (only 7% of the incident sunlight is reflected back, the rest being absorbed). The brighter regions on the moon are the mountainous highlands, where the terrain is rough and strewn with rocky rubble. The lunar mountain ranges, with heights up to 25,000 ft (7800 m), are comparable to the highest mountains on earth but in general are not very steep. The highlands are densely scarred by thousands of craters—shallow circular depressions, usually ringed by well-defined walls and often possessing a central peak. Craters range in diameter from a few feet to many miles, and in some regions there are so many that they overlap or several smaller craters lie within a large crater. Craters are also found on the maria, although there are nowhere near as many as in the lunar highlands. Other prominent surface features include the rilles and rays. Rilles are sinuous, canyonlike clefts found near the edges of mountain ranges. Rays are bright streaks radiating outward from certain craters, such as Tycho.

Mare and highland rocks differ in both appearance and chemical content. For example, mare rocks are richer in iron and poorer in aluminum than highland rocks. The maria consist largely of basalt, i.e., igneous rock formed from magma. In the highlands the majority of the rocks are breccias— conglomerates formed from basaltic rock and often studded with small, green, glassy spheres. These spheres probably were formed as the spray of molten rock, originally melted by the heat of meteorite impact, recongealed in midflight. The exposure ages of some rocks (the time their surfaces have been exposed to the action of cosmic rays that produce radioactive isotopes) are as short as 50 million years, much shorter than their crystallization ages. These rocks may have been shifted in position by meteorite impact or seismic activity (moonquakes). However, present lunar seismic activity is very low, corroborating the image of the moon as an essentially static, nonevolving world.

Internal Structure

Diffraction of seismic waves provided the first clear-cut evidence for a lunar crust, mantle, and core analogous to those of the earth. The lunar crust is about 45 mi (70 km) thick, making the moon a rigid solid to a greater depth than the earth. The inner core has a radius of about 600 mi (1,000 km), about 2/3 of the radius of the moon itself. The internal temperature decreases from 830°C (1,530°F) at the center to 170°C (340°F) near the surface. The heat traveling outward near the lunar surface is about half that of the earth but still twice that predicted by current theory. This heat flow is directly related to the rate of internal energy production, so that the internal temperature profile provides information about long-lived radio isotopes and the moon's thermal evolution. The heat-flow measurements indicate that the moon's radioactive content is higher than that of the earth. The moon's magnetic field is a million times weaker than that of the earth, but it varies by a factor of 20 from point to point on the surface. Certain rocks retain a high magnetization, indicating that they crystallized in the presence of magnetic fields much higher than those presently existing on the moon. Mascons are large concentrations of unusually high density that are located below certain of the circular maria. The mascons may have been created by the implantation of very dense, iron-rich meteorites, whose impact formed the mare basins themselves.

Formation and Evolution

The moon probably formed by the cold accretion of small particles about 4.6 billion years ago at the same time that the rest of the solar system formed; thus, it is now believed that the moon was never in an entirely molten state. The crust, showing pronounced chemical differentiation, formed early. Subsequent impact of very large meteorites depressed the mare basins, at the same time thrusting up the surrounding crust to form the highlands. The mare basins later filled with lava flow, which in turn was covered by a thin layer of lunar "soil" —fine rock dust pulverized by the very slow mechanisms of lunar erosion (thermal cycling, solar wind, and micrometeorites). The craters were probably also formed by meteorite bombardment rather than by internal volcanic action as once believed. The rays surrounding the craters are material ejected during the impacts that formed the craters. The moon's rock types are correlated with its major geological periods.

Bibliography

See P. Moore and P. J. Cattermole, The Craters of the Moon (1967); D. Thomas, ed., Moon (1970); G. Gamow, The Moon (rev. ed. 1971); S. R. Taylor, Lunar Science (1975); B. M. French, The Moon Book (1977); W. K. Hartmann, ed., The Origins of the Moon (1986).

Moon

The Moon is Earth's only natural satellite . Reflecting light from the Sun , the Moon is often the brightest object in the night sky.

The Moon orbits Earth at an average distance of approximately 240,000 miles (385,000 km). With revolution and rotation periods of approximately 27.32 Earth days, the Moon is in synchronous orbit about the earth. This synchronous orbit maintains a "near side" and "far side" of the Moon. The "near side" faces Earth, while the far side is not visible from Earth. Although Russian space probes—and later many American probes—took the first pictures of the far side of the Moon years earlier, it was not until the flight of Apollo 8 that United States astronauts became the first humans to directly view the far side of the Moon.

Orbital dynamics between the Sun, Moon, and Earth cause different patterns of illumination on the surface of the Moon as seen from Earth. As the Moon revolves about the earth, it appears to go through a series of illumination phases. The Sun constantly illuminates one-half of the lunar surface. The changing orientation in the three body system (Sun, Earth, and Moon), changes to what extent that solar illumination covers areas on the surface of the Moon that are visible from Earth.

Because the earth is revolving about the Sun, the displacement of the earth along it's orbital path establishes the time it takes to complete a cycle of lunar phases—a synodic month—and return the Sun, Earth, and Moon to the same starting alignment. This synodic month is approximately 29.5 days, and is longer than the 27.32-day sideral month.

A waxing moon is one where the area illuminated increases each night. A waning moon describes a decreasing area of illumination.

The Moon's phases are a cyclic repetition of illumination patterns described as: new moon, waxing crescent moon, waxing half moon, waxing gibbous moon, full moon, waning gibbous moon, waning half moon, waning crescent moon, followed by a return to the new moon phase.

A new moon occurs when the Moon's orbital path places it between the earth and the Sun. Only the side of the Moon not visible to Earth is illuminated and the Moon is lost in the bright sunlight. Occasionally when the Moon is also in the proper plane of alignment, it may provide a full or partial solar eclipse over portions of Earth's surface.

Relative to the Sun and starfield, the Moon appears to move eastward. Following the new moon, the next night, a small sliver or crescent becomes illuminated. The waxing crescent moon is low on the western horizon and is visible just after sunset (i.e., the Moon "sets" shortly after sunset). As the orbital dynamics shift, the crescent grows larger—and the Moon sets later—each night following sunset. Approximately one week following the new moon, the Moon is one quarter of the way through it's orbital revolution of Earth, and one half of the lunar surface is illuminated as a waxing half moon. Depending upon latitude , the waxing half moon appears nearly directly overhead (at the zenith of the celestial meridian) at sunset. The waxing half moon will set about midnight local time. During the next week, the area of the Moon reflecting sunlight to Earth covers more than half of the visible lunar surface, and is described as a waxing gibbous moon.

Approximately two weeks after the new moon, the visible surface of the Moon becomes fully illuminated because the Moon is on the opposite side of Earth relative to the Sun. If the earth and Moon are in the proper plane, Earth may actually block the Sun's light over a portion of the lunar surface and cause a partial to full lunar eclipse. The full moon rises at sunset and sets at dawn.

Following the full moon, the Moon begin to progressively darken through waning gibbous phases until about a week following the full moon it forms a waning half moon. The waning half moon rises about midnight and sets about noon the next day. Continued darkening over the last week of the lunar cycle provides a waning crescent moon that finally returns full cycle to the new moon state, where the Moon and Sun, on the same side of Earth's orbit about the Sun, appear to rise and set together.

The phases of the Moon proved one of the most fundamental astronomical calendars for ancient peoples and the ancient Greek astronomers asserted that the Moon reflected the Sun's light. Phases of the Moon remain critical in determining the date and timing of many religious observances (e.g., Passover, Easter, Ramadan, Visakha Puja, etc.)

Because the earth is larger than the Moon and relatively close to the Moon, it casts a large shadow that causes lunar eclipses. Solar eclipses (where the Moon blocks the Sun) are less frequent and are only possible because, although the Sun is much larger than the Moon, the Moon is much closer to Earth. The present set of orbital dynamics and distances allow solar eclipses because the Sun and Moon have the same angular size (approximately 0.5°) when viewed from Earth. The average human thumb, held out at arm's length obscures approximately 0.5° degrees and will thus, block both the Sun and Moon. (Warning: Direct viewing of the Sun may cause blindness or optic injury and should not be attempted. Solar observation requires special protective goggles that filter and reduce the intensity of sunlight. )

The Moon appears to shift its position eastward on the celestial sphere by approximately 13° per night (i.e., appears to move 13° to the east from its prior position if observed at the same time on successive nights).

The Moon is nearly spherical with polar and equatorial radii varying by about a mile. The equatorial radius of the Moon is approximately 1,080 miles (1,738 km). The diurnal temperatures (the day/night temperatures) on the Moon range from approximately −280°F to +260°F (−173°C to +126°C). Contrary to popular belief, the Moon does have a thin atmosphere that consists of helium, argon, methane, minute amounts of oxygen , and other trace elements. The density of the lunar atmosphere is only approximately 2 × 10⁵ particles/cm³ and results in a lunar atmospheric pressure of only 8.86 × 10⁻¹⁴ inHg (3 × 10⁻¹² mb) in contrast to Earth's average surface atmospheric pressure of 29.92 inHg (1,014 mb).

The thin and dry lunar atmosphere provides no substantial weathering agents (e.g., wind, water , etc.) and so erosional processes are greatly slowed—essentially reduced to heating, cooling, and slow geochemical changes. The thin atmosphere also offers no protection from meteor impacts and the combination of lack of protection and lack of Earth-like erosion produces a heavily cratered lunar landscape that preserves billions of years of accumulated impact craters.

Although the Moon is a quarter of Earth's size, it has only approximately 1.2% of Earth's mass. The gravitational attraction at the surface of the Moon is about one-sixth that of the gravitational attraction at Earth's surface. Accordingly, neglecting air friction (something easily accomplished on the Moon but not on Earth) an object in freefall near Earth's surface accelerates at 9.8 m/s², but near the lunar surface, the acceleration due to gravity is approximately 1.62 m/s².

See also Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Diurnal cycles; Earth (planet); History of manned space exploration; Gravity and the gravitational field; Solar system

Moon

The Moon is a roughly spherical, rocky body orbiting Earth at an average distance of 240,00 miles (385,000 kilometers). It measures about 2,160 miles (3,475 kilometers) across, a little over one-quarter of Earth's diameter. Earth and the Moon are the closest in size of any known planet and its satellite, with the possible exception of Pluto and its moon Charon.

The Moon is covered with rocks, boulders, craters, and a layer of charcoal-colored soil from 5 to 20 feet (1.5 to 6 meters) deep. The soil consists of rock fragments, pulverized rock, and tiny pieces of glass. Two types of rock are found on the Moon: basalt, which is hardened lava; and breccia, which is soil and rock fragments that have melted together.

Elements found in Moon rocks include aluminum, calcium, iron, magnesium, titanium, potassium, and phosphorus. In contrast with Earth, which has a core rich in iron and other metals, the Moon appears to contain very little metal. The apparent lack of organic compounds rules out the possibility that there is, or ever was, life on the Moon.

The Moon has no weather, no wind or rain, and no air. As a result, it has no protection from the Sun's rays or meteorites and no ability to retain heat. Temperatures on the Moon have been recorded in the range of 280°F (138°C) to −148°F (−100°C).

Formation of the Moon

Both Earth and the Moon are about 4.6 billion years old, a fact that has led to many theories about their common origin. Before the 1970s, scientists held to one of three competing theories about the origin of the Moon: the fission theory, the simultaneous creation theory, and the capture theory.

The fission theory stated that the Moon spun off from Earth early in its history. The Pacific basin was the scar left by the tearing away of the Moon. The simultaneous creation theory stated that the Moon and Earth formed at the same time from the same planetary building blocks that were floating in space billions of years ago. The capture theory stated that the Moon was created somewhere else in the solar system and captured by Earth's gravitational field as it wandered too close to the planet.

After scientists examined the age and composition of lunar rocks brought back by Apollo astronauts, they discarded these previous theories and accepted a new one: the giant impact theory (also called the Big Whack model). This theory states that when Earth was newly formed, it was sideswiped by a celestial object that was at least as massive as Mars. (Some scientists contend the object was two to three times the mass of Mars.) The collision spewed a ring of crustal matter into space. While in orbit around Earth, that matter gradually combined to form the Moon.

The evolution of the Moon has been completely different from that of Earth. For about the first 700 million years of the Moon's existence, it was struck by great numbers of meteorites. They blasted out craters of all sizes. The sheer impact of so many meteorites caused the Moon's crust to melt. Eventually, as the crust cooled, lava from the interior surfaced and filled in cracks and some crater basins. These filled-in basins are the dark spots we see when we look at the Moon.

To early astronomers, these dark regions appeared to be bodies of liquid. In 1609, Italian astronomer Galileo Galilei became the first person to observe the Moon through a telescope. He named these dark patches "maria," Latin for "seas."

In 1645, Polish astronomer Johannes Hevelius, known as the father of lunar topography, charted 250 craters and other formations on the Moon. Many of these were later named for philosophers and scientists, such as Danish astronomer Tycho Brahe, Polish astronomer Nicolaus Copernicus, German astronomer Johannes Kepler, and Greek philosopher Plato.

Humans on the Moon

All Earth-based study of the Moon has been limited by one factor: only one side of the Moon ever faces Earth. The reason is that the Moon's rotational period is equal to the time it takes the Moon to complete one orbit around Earth. It wasn't until 1959, when the former Soviet Union's space probe Luna 3 traveled to the far side of the Moon that scientists were able to see the other half for the first time.

Then in 1966, the Soviet Luna 9 became the first object from Earth to land on the Moon. It took television footage showing that lunar dust, which scientists had anticipated finding, did not exist. The fear of encountering thick layers of dust was one reason both the Soviet Union and the United States hesitated sending a man to the moon.

Just three years later, on July 20, 1969, U.S. astronauts Neil Armstrong and Edwin "Buzz" Aldrin aboard Apollo 11 became the first humans to walk on the Moon. They collected rock and soil samples, from which scientists learned the Moon's elemental composition. There were five more lunar landings in the Apollo program between 1969 and 1972. To this day, the Moon remains the only celestial body to be visited by humans.

Water on the Moon?

In late 1996, scientists announced the possibility that water ice existed on the Moon. Clementine, a U.S. Defense Department spacecraft, had been launched in January 1994 and orbited the Moon for four months. It surveyed a huge depression in the south polar region called the South Pole-Aitken basin. Nearly four billion years ago, a massive asteroid had gouged out the basin. It stretches 1,500 miles (2,415 kilometers) and in places is as deep as 8 miles (13 kilometers), deeper than Mount Everest is high.

Areas of this basin are never exposed to sunlight, and temperatures there are estimated to be as low as −387°F (−233°C). While scanning these vast areas with radar signals, Clementine discovered what appeared to be ice crystals mixed with dirt. Scientists speculated that the crystals made up no more the 10 percent of the material in the region. They believe the ice is the residue of moisture from comets that struck the Moon over the last three billion years.

To learn more about the Moon and this possible ice, the National Aeronautics and Space Administration (NASA) launched the Lunar Prospector in January 1998. This was NASA's first mission back to the Moon in 25 years. As the name of this small, unmanned spacecraft implied, its nineteen-month mission was to "prospect" the surface composition of the Moon, providing a detailed map of minerals, water ice, and certain gases. It also took measurements of magnetic and gravity fields, and tried to provide scientists with information regarding the size and content of the Moon's core. For almost a year, Lunar Prospector orbited the Moon at an altitude

of 62 miles (100 kilometers). Then, in December 1998, NASA lowered its orbit to an altitude of 25 miles (40 kilometers). On July 31, 1999, in a controlled crash, the spacecraft settled into a crater near the south pole of the Moon. If there were water at the crash site, the spacecraft's impact would have thrown up a huge plume of water vapor that could have been seen by spectroscopes at the Keck Observatory on Mauna Kea, Hawaii, and other telescopes like the orbiting Hubble Space Telescope. However, no such plume was observed. For scientists, the question of whether there is hidden ice on the Moon, delivered by impacting comets, is still open. It is estimated that each pole on the Moon may contain up to 1 billion tons (900 million metric tons) of frozen water ice spread throughout the soil.

[See also Orbit; Satellite; Spacecraft, manned ]

Moon The only natural satellite of the Earth, orbiting at an average distance of 384 400 km. The magnitude of the full Moon is -12.7, but its surface is actually dark, with a mean geometric albedo of only 0.12, lower than for all the planets except Mercury. It is the fifth-largest satellite in the Solar System (diameter 3475 km), over a quarter the diameter of the Earth and about 1/81 the Earth's mass. Being so similar in size, the Earth and Moon are often considered a double planet. The Moon's sidereal period of axial rotation, 27.322 days, is the same as its orbital period, so that it keeps the same face towards the Earth. Its equator is inclined by 1° .53 to the plane of the ecliptic. Surface temperatures vary from extremes of 123°C during the day down to -233°C at night; typical values are 107°C (day) and -175°C (night). Polar regions of the Moon contain craters with permanently shadowed floors, where ice may exist.

The Moon shows two distinctly different types of terrain with very different densities of impact craters: the brighter highlands and the darker lowland mare areas. The lunar highlands have an albedo of 0.11–0.18, and are saturated with large craters of 50 km diameter and greater; the maria have an albedo of 0.07–0.10, and consist of younger plains of basaltic lava with few large craters. The mare basalts are enriched in

moon Natural satellite of a planet; in particular the natural satellite of the planet Earth. Apart from the Sun it is the brightest object in the sky as seen from the Earth, being at a mean distance of only 384,000km (239,000mi). Its diameter is 3476km (2160mi). As the Moon orbits the Earth, it goes through a sequence of phases. Its surface features may be broadly divided into the darker maria, which are low-lying volcanic plains, and the brighter highland regions (sometimes called terrae), which are found predominantly in the s part of the Moon's near side and over the entire far side. The origin of the Moon is uncertain. A current theory is that a Mars-sized body collided with the newly formed Earth, and debris from the impact formed the Moon. On July 20, 1969, Neil Armstrong became the first person to walk on the Moon. The chemical composition of material from the Moon consisted mainly of silica, iron oxide, aluminium oxide, calcium oxide, titanium dioxide, and magnesium oxide. Lunar rocks are igneous rocks. The Moon has only the most tenuous of atmospheres; Apollo instruments detected traces of gases, such as helium, neon and argon. The surface temperature variation is extreme, from 100 to 400K. In 1998, a probe discovered water-ice near the Moon's poles.

moon / moōn/ • n. (also Moon) the natural satellite of the earth, visible (chiefly at night) by reflected light from the sun. ∎  a natural satellite of any planet. ∎  (the moon) fig. anything that one could desire: you must know he'd give any of us the moon. ∎  a month, esp. a lunar month: many moons had passed since he brought a prospective investor home. • v. 1. [intr.] behave or move in a listless and aimless manner: lying in bed eating candy, mooning around. ∎  act in a dreamily infatuated manner: Timothy's mooning over her like a schoolboy. 2. [tr.] inf. expose one's buttocks to (someone) in order to insult or amuse them: Dan had whipped around, bent over, and mooned the crowd. PHRASES: many moons ago inf. a long time ago. over the moon inf. extremely happy; delighted. DERIVATIVES: moon·less adj. moon·like / -ˌlīk/ adj.

phases of the moon

280. Moon

See also 25. ASTRONOMY ; 318. PLANETS ; 387. SUN .

selenography

the branch of astronomy that deals with the charting of the moon’s surface. —selenographer, selenographist , n. —selenographic, selenographical , adj.

selenolatry

the worship of the moon.

selenology

the branch of astronomy that studies the physical characteristics of the moon. —selenologist , n. —selenological , adj.

selenomancy

a form of divination involving observation of the moon.

Moon The Earth's satellite, with a mass 1/81 that of the Earth, density 3344 kg/m³, and radius 1738 km. The average Moon—Earth distance is 384500 km. The Moon has no atmosphere and surface temperature extremes range from 127 to –173 °C. A feldspathic lunar highland crust 60–120 km thick overlies a silicate mantle. Basaltic lavas cover 17% of the surface. There is probably a small iron core of 300–400 km radius (2–3% of lunar volume).