Learn about the moon

The moon moves about the earth at an average distance of 384,403 km (238,857 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 (see Time). 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.

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 127C (261F) at lunar noon to a minimum of -173C (-279F) just before lunar dawn.

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.

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-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 (see Space Exploration). 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 (see Radiation Belts); 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.

It is now known, from measuring the ages of lunar rocks, 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 (see Feldspar). 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). See Geology; Igneous Rock.

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 1600C (2912F), 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. See Meteor.

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. The scientist spent 22 hours exploring the Taurus-Littrow Valley region, covering 35 km (22 mi) in a lunar roving vehicle. Intensive analysis of the data and rocks obtained by the lunar missions continues. Meanwhile, many active research programs are observing and measuring many other regions of space-the craters of Mercury, the volcanoes and canyons of Mars, the gigantic magnetic and radiation fields of Jupiter, and the enormous energies of the sun. Combined with the scientific information gathered from the moon, scientists are beginning to get a greatly improved understanding of the complex origin, structure, and evolution of the solar system, and of its relationship to life and the human race.