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 probe and 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 ice 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

It is now most commonly believed that moon formed when a Mars-sized object collided with the young earth. This so-called giant impact hypothesis states the cores of the earth and object merged in the earth while material from the crust and mantle was blasted into orbit around the earth and later accreted to form the moon. After the moon's crust formed, 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); B. Brunner, Moon: A Brief History (2010).

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 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).