MOON

MOON

MOON, name given to the natural satellite of the Earth, and sometimes applied to the satellites of the other planets in the solar system. The diameter of the moon is about 3480 km (about 2160 mi), or about one-fourth that of earth, and the moon’s volume is about one-fiftieth that of the earth. The mass of the earth is 81 times greater than the mass of the moon. Thus the average density of the moon is only three-fifths, and the pull of gravity at the lunar surface only one-sixth that of the earth. The moon has no free water and essentially no atmosphere, and no weather exists to change its surface; yet it is not totally inert.

The moon moves about the earth at an average distance of 384,403 km (238,856 mi), and at an average speed of 3700 km per hr (about 2300 mph). It completes one revolution in an elliptical orbit about the earth in 27 days, 7 hr, 43 min, and 11.5 sec with reference to the stars. For the moon to go from one phase to the next similar phase, or one lunar month, requires 29 days, 12 hr, 44 min, and 2.8 sec. The moon rotates on its axis once in about the same period of time that elapses for its sidereal period of revolution, accounting for the fact that virtually the same portion of the moon is always turned toward the earth. Although the moon appears bright to the eye, it reflects into space only 7 percent of the light that falls on it. The reflectivity, or albedo, of 0.07 is similar to that of coal dust.

Appearance from Earth

At any one time, an observer can see only 50 percent of the moon’s entire surface. However, an additional 9 percent can be seen from time to time around the apparent edge because of the relative motion called libration. This is caused by slightly different angles of view from the earth at different relative positions of the moon along its inclined elliptical orbit.

Phases

The moon shows progressively different phases as it moves along its orbit around the earth. Half the moon is always in sunlight, just as half the earth has day while the other half has night. The phases of the moon depend on how much of the sunlit half can be seen at any one time. In the phase called the new moon, the face is completely in shadow. About a week later, the moon is in first quarter, resembling a luminous half-circle; another week later, the full moon shows its fully lighted surface; a week afterward, in its last quarter, the moon appears as a half-circle again. The entire cycle is repeated each lunar month. The moon is full when it is farther away from the sun than the earth; it is new when it is closer. When it is more than half-illuminated, it is said to be in gibbous phase. The moon is said to be waning when it progresses from full to new, and to be waxing as it proceeds again to full. Temperatures on its surface are extreme, ranging from a maximum of 127° C (261° F) at lunar noon to a minimum of –173° C (–279° F) just before lunar dawn.

Harvest moon and hunter’s moon

The full moon nearest the autumnal equinox ushers in a period of several successive days when it rises soon after sunset. Because this phenomenon gives farmers in temperate latitudes extra hours of light in which to harvest their crops before the first frost, it is known as the harvest moon. The hunter’s moon, the next full moon after the harvest moon, is accompanied by a similar but less marked phenomenon.

Lunar Exploration

Throughout the 19th and 20th centuries, visual exploration through powerful telescopes has yielded a fairly comprehensive picture of the visible side of the moon. The hitherto unseen far side of the moon was first revealed to the world in October 1959 through photographs made by the Soviet Lunik III spacecraft. These photographs showed that the far side of the moon is similar to the near side except that large lunar maria are absent. Craters are now known to cover the entire moon, ranging in size from huge, ringed maria to those of microscopic size. Photographs from U.S. spacecraft—Ranger 7, 8, and 9 and Orbiter 1 and 2—launched by the National Aeronautics and Space Administration (NASA) in 1964 and 1966 further supported these conclusions. The entire moon has about 3 trillion craters larger than about 1 m (3.32 ft) in diameter.

The successful landings of unmanned spacecraft of the Surveyer series by the U.S. and the Luna series by the USSR in the 1960s, and, finally, the manned landings on the lunar surface as part of the U.S. Apollo program made direct measurement of the physical and chemical properties of the moon a reality. The Apollo astronauts collected rocks, took thousands of photographs, and set up instruments on the moon that sent information back to earth by radio telemetry. These instruments measured temperature and gas pressure at the lunar surface; the heat flow from the moon’s interior; molecules and ions of hot gases streaming out from the atmosphere of the sun, called the solar wind; the magnetic field and gravity of the moon; seismic vibrations of the lunar surface caused by so-called moonquakes, landslides, and meteoroid impacts; and, through laser beams, the precise distance between the earth and the moon.

All six manned landings on the moon—Apollo 11, 12, and 14–17—returned samples of rock and soil to earth, weighing 384 kg (846 lb) in all. It was not until the final mission, Apollo 17, that the astronaut crew included a geologist, H. H. Schmitt (1935–    ). The scientist spent 22 hours exploring the Taurus-Littrow Valley region, covering 35 km (22 mi) in a lunar roving vehicle.

In January 1998 the Lunar Prospector probe sent into orbit around the moon—NASA’s first lunar mission in 25 years—detected water ice at the moon’s poles, confirming an indication by the U.S. Defense Department’s Clementine probe in 1994. The amount estimated was seen as a potential resource for future lunar research colonies.

Surface and Composition of the Moon

Ancient observers of the moon believed that the dark regions on its face were oceans, giving rise to the Latin name mare (“sea”), which is still used today; the brighter regions were likewise held to be continents. Modern observation and exploration of the moon has yielded far more comprehensive and specific knowledge. Since the Renaissance, telescopes have revealed a wealth of lunar detail, and lunar spacecraft have contributed further to this knowledge. Features discernible on the surface of the moon include craters, mountain ranges, plains or maria, faults, domes, rilles, and rays. The largest distinct crater, called Bailly, is about 295 km (about 183 mi) wide and 3960 m (about 13,000 ft) deep. The largest mare or sea is Mare Imbrium (Sea of Rains), about 1200 km (about 750 mi) wide. The highest mountains, in the Leibnitz and Doerfel ranges near the south pole of the moon, have peaks up to 6100 m (about 20,000 ft) in height, comparable to the Himalayas on earth. Craters as small as 1.6 km (1 mi) across have been defined in telescopic observations. The origin of lunar craters was long debated; the latest evidence indicates that nearly all craters were formed by explosive impacts of high-velocity meteorites or small asteroids, mostly during the early part of lunar history, when the solar system still contained many such fragments. Some craters, rilles, and domes, however, display characteristics of indisputable volcanic origin.

It is now known, from measuring the ages of lunar rocks retrieved during lunar missions, that the moon is about 4,600,000,000 years old, or about the same age as the earth and probably the rest of the solar system. Rocks from the lunar maria were formed when molten rock solidified between 3,160,000,000 and 3,960,000,000 years ago. These rocks most nearly resemble terrestrial basalts, a volcanic rock type widely distributed on earth, but with certain important differences. Evidence indicates that the lunar highlands, or continents, may be formed of a less dense plutonic igneous rock called anorthosite, which consists almost entirely of the mineral plagioclase. Other important lunar sample types include glasses, breccias (complex assemblages of rock fragments cemented together by heat or pressure, or both), and the soils or regolith (finely divided rock fragments produced by many millions of years of meteoritic bombardment).

The moon’s magnetic field is not as strong or widespread as that of the earth. Some lunar rocks are weakly magnetic, indicating that they solidified in a somewhat stronger magnetic field. Magnetic and other measurements indicate an internal temperature of the moon as high as 1600° C (2912° F), which is above the melting point of most lunar rocks. Evidence from seismic recordings suggests that some regions near the lunar center may be liquid.

Seismometers operating on the lunar surface have also recorded signals of between 70 and 150 meteorite impacts per year, with masses from 100 g to 1000 kg (0.22 to 2200 lb). Hence the moon is still being bombarded by meteoroids, although not as often as in the past, and this may be a problem for engineers who design permanent bases for the lunar surface. The surface is covered by a layer of rubble, which may be several kilometers deep in the maria and of as yet unknown depth in the highlands. The rubble zone is believed to have been formed by the impacts of meteoroids.

Origin of the Moon

Before the modern age of space exploration, scientists had three major theories for the origin of the moon: fission from the earth; formation in earth orbit; and formation far from earth. Then, in 1975, having studied moon rocks and close-up pictures of the moon, scientists proposed what has come to be regarded as the most probable of the theories of formation, planetesimal impact.

Formation by fission from the earth

The modern version of this theory proposes that the moon was spun off from the earth when the earth was young and rotating rapidly on its axis. This idea gained support partly because the density of the moon is the same as that of the rocks just below the crust, or upper mantle, of the earth. A major difficulty with this theory is that the angular momentum of the earth, in order to achieve rotational instability, would have to have been much greater than the angular momentum of the present earth-moon system.

Formation in orbit near the earth

This theory proposes that the earth and moon, and all other bodies of the solar system, condensed independently out of the huge cloud of cold gases and solid particles that constituted the primordial solar nebula. Much of this material finally collected at the center to form the sun.

Formation far from earth

Planetesimal impact

First published in 1975, this theory proposes that early in the earth’s history, well over 4 billion years ago, the earth was struck by a large body called a planetesimal, about the size of Mars. The catastrophic impact blasted portions of the earth and the planetesimal into earth orbit, where debris from the impact eventually coalesced to form the moon. This theory, after years of research on moon rocks in the 1970s and ’80s, has became the most widely accepted one for the moon’s origin. The major problem with the theory is that it would seem to require that the earth melted throughout, following the impact, whereas the earth’s geochemistry does not indicate such a radical melting.