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 and Phases
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.
As the moon orbits the earth, the amount of its illuminated surface that can be seen from the earth changes. When none of the lighted half can be seen, because the moon is between the earth and sun, the moon is said to be new. For a few days before and after a new moon we can see a small part, or crescent, of the lighted half. When the moon has completed half its orbit from new moon to new moon, it is on the opposite side of the earth from the sun and we see the entire lighted half, or the full moon. When the moon has completed either one quarter or three quarters of its orbit from new moon to new moon, half the lighted side, the half-moon, is visible. The half-moon between the new and full moon is the first quarter and that between the full and new moon is the last quarter. Between a full moon and half-moon we see more than half the lighted side, or a gibbous moon. A blue moon is a second full moon in a calendar month; a black moon is a second new moon in a calendar month, or a calendar month with no full moon.
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.
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.
The lunar surface is divided into the mountainous highlands and the large, generally roughly circular plains called maria (sing. mare; from Lat.,=sea) by early astronomers, who erroneously believed them to be bodies of water. The largest of the mare, Oceanus Procellarum or the Ocean of Storms, is rectangular in shape, however. 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.
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 maria. The mascons may have been created by the implantation of very dense, iron-rich meteorites, whose impact formed the overlying mare basins.
Formation and Evolution
It is now most commonly believed that moon formed when an object (sometimes called Theia after the mother of Selene, goddess of the moon) collided with the young earth. One theory holds that when a Mars-sized body impacted the earth 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. Another theory holds that the body was larger and faster, delivering a glancing blow and contributing relatively little material to the earth-moon system that it created. 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.
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).