Moon, Sun Myung, 1920-, Korean religious leader. He was an engineering student and dock worker before founding (1954) the Unification Church with a doctrine loosely based on Christianity as interpreted by Moon, who has declared (2004) himself the "Messiah." Moon introduced the movement to the United States in the 1960s; its world headquarters are now in New York City. Moon has been accused of brainwashing converts and of various illegal activities; he was convicted (1982) of conspiracy to evade taxes.
Moon, Warren (Harold Warren Moon, Jr.), 1956-, African-American football player, b. Los Angeles. Moon quarterbacked the Univ. of Washington Huskies to a Rose Bowl title in 1978, but he went undrafted by the National Football League, whose scouts were wary of Washington's rollout offense and an African-American at quarterback. Urged to change positions, Moon instead signed with the Canadian Football League's Edmonton Eskimos. Starting quarterback there from 1980, he led the team to four straight Grey Cup games (1980-3), winning three times (1980-82). Moon joined the NFL's Houston Oilers in 1984, and his all-around skills at quarterback were well displayed in the run-and-shoot offense the Oilers adopted two years later. Moon was traded to the Minnesota Vikings in 1994, and joined the Seattle Seahawks in 1997; he finished his career as a backup quarterback with the Kansas City Chiefs (1999-2001). Moon, a pioneering African American quarterback in the NFL, ranks second in professional football (NFL and CFL combined) career passing yardage (70,553).
Moon, Mountains of the, Africa: see Ruwenzori.
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, but several space probes have found evidence of water in the soil. At its most extreme, the surface temperature can rise to above 125°C; (257°F;) at lunar noon at the equator and can sink below -245°C; (-409°F;) at night in the northern polar region. 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.


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).

Adrastea (, or as in Greek Αδράστεια), also known as , is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in Voyager 2 probe photographs taken in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than through telescopic photography. It was officially named after the mythological Adrastea, daughter of Greek god Zeus—the equivalent of Roman god Jupiter. Adrastea is one of the few moons in the Solar System known to orbit its planet in less than the length of that planet's day. It orbits at the edge of Jupiter's Main Ring and is thought to be the main contributor of material to the Rings of Jupiter. Despite observations made in the 1990s by the Galileo spacecraft, very little is known about the moon's physical characteristics outside its size and the fact that it is tidally locked to Jupiter.

Discovery and observations

Adrastea was discovered by David C. Jewitt and G. Edward Danielson in Voyager 2 probe photographs taken on July 8, 1979, and received the designation . Although it appeared only as a dot, it was the first moon to be discovered by an interplanetary spacecraft. Soon after its discovery, two other of the inner moons of Jupiter (Thebe and Metis) were observed in the images taken by a few weeks earlier by Voyager 1. The Galileo spacecraft was able to determine the moon's shape in 1998, but the images remain poor. In 1983, Adrastea was officially named after the Greek nymph Adrastea, the daughter of the Zeus and his lover Ananke.

Physical characteristics

Adrastea has an irregular shape and measures 20×16×14 km across. This makes Adrastea the smallest of the four inner moons. The bulk, composition and mass of Adrastea are not known, but assuming that its mean density is like that of Amalthea,(around 0.86 g/cm³) its mass can be estimated at about 2 kg. Amalthea's density implies that the moon is composed of water ice with a porosity of 10–15%, and Adrastea may be similar.

No surface details of Adrastea are known, due to the low resolution of available images.


Adrastea is the smallest and second closest member of the inner Jovian satellite family. It orbits Jupiter at a radius of about 129,000 km (1.806 Jupiter radii) at the exterior edge of the planet's Main Ring. Adrastea is only one of the three moons in the Solar System known to orbit its planet in less than the length of that planet's day—the other two being Jupiter's innermost moon Metis, and Mars' moon Phobos. The orbit has very small eccentricity and inclination—around 0.0015 and 0.03°, respectively. Inclination is relative to the equator of Jupiter.

Due to tidal locking, Adrastea rotates synchronously with its orbital period, keeping one face always looking toward the planet. Its long axis is aligned towards Jupiter, this being the lowest energy configuration.

The orbit of Adrastea lies inside Jupiter's synchronous orbit radius (as does Metis’s), and as a result, tidal forces are slowly causing its orbit to decay so that it will one day impact Jupiter. If its density is similar to Amalthea's then its orbit would actually lie within the fluid Roche limit. However, since it is not breaking up, it must still lie outside its rigid Roche limit.

Relationship with Jupiter's rings

Adrastea is the largest contributor to material in Jupiter's rings. This material appears to consist primarily of material that is ejected from the surfaces of Jupiter's four small inner satellites by meteorite impacts. It is easy for the impact ejecta to be lost from these satellites into space. This is because, due to the satellites' low density, their surfaces lie rather close to the edge of their Roche spheres.

It seems that Adrastea is the most copious source of this ring material, as evidenced by the densest ring (the Main Ring) being located at and within Adrastea's orbit. More precisely, the orbit of Adrastea lies near the outer edge of Jupiter's Main Ring. The exact extent of visible ring material depends on the phase angle of the images: in forward-scattered light Adrastea is firmly outside the Main Ring, but in back-scattered light (which reveals much bigger particles) there appears to also be a narrow ringlet outside Adrastea's orbit.


External links

Adrastea Profile by NASA's Solar System Exploration

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