Moon, name given to the only natural satellite of Earth. The Moon is the second brightest object in Earth’s sky, after the Sun, and has accordingly been an object of wonder and speculation for people since earliest times. The natural satellites of the other planets in the solar system are also sometimes referred to as moons.

Telescopes have revealed a wealth of lunar detail since their invention in the 17th century, and spacecraft have contributed further knowledge since the 1950s. Earth’s Moon is now known to be a slightly egg-shaped ball composed mostly of rock and metal. It has no liquid water, virtually no atmosphere, and is lifeless. The Moon shines by reflecting the light of the Sun. Although the Moon appears bright to the eye, it reflects on average only 7 percent of the light that falls on it. This reflectivity, called albedo, of 0.07 is similar to that of coal dust.

The diameter of the Moon is about 3,480 km (about 2,160 mi), or about one-fourth that of Earth. The Moon’s mass is only 1.2 percent of Earth’s mass. The average density of the Moon is only three-fifths that of Earth, and gravity at the lunar surface is only one-sixth as strong as gravity at sea level on Earth. The Moon moves in an elliptical (oval-shaped) orbit around Earth at an average distance of 384,403 km (238,857 mi) and at an average speed of 3,700 km/h (2,300 mph). It completes one revolution in 27 days 7 hours 43 minutes. For the Moon to go from one phase to the next similar phase—as seen from Earth—requires 29 days 12 hours 44 minutes. This period is called a lunar month. The Moon rotates once on its axis in the same period of time that it circles Earth, accounting for the fact that virtually the same portion of the Moon (the “near side”) is always turned toward Earth.


The Moon shows progressively different phases as it moves along its orbit around Earth. Half the Moon is always in sunlight, just as half of Earth has day while the other half has night. Thus, there is no permanent “dark side of the Moon,” which is sometimes confused with the Moon’s far side—the side that always faces away from Earth. 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 near side is completely in shadow. About a week after a new moon, the Moon is in first quarter, resembling a luminous half-circle; another week later, the full moon shows its fully lighted near side; 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 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 as it progresses from full to new, and to be waxing as it proceeds from new to full.

At any one time, an observer on Earth 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 edges because the viewing angle from Earth changes slightly as the Moon moves through its elliptical orbit. This slight relative motion is called libration.


Ancient observers of the Moon believed that the dark regions on its face were oceans, giving rise to their name maria (Latin for “seas”). This term is still used today although these regions are now known to be completely dry. The brighter regions were held to be continents. Modern observation and exploration of the Moon has yielded far more comprehensive and specific knowledge.

The Moon has no movement of wind or water to alter its surface, yet it was geologically active in the past and is still not totally unchanging. Craters cover the surface, and meteors continue to create new craters. Billions of years ago volcanic eruptions sculpted large areas of the surface. Volcanic features such as maria, domes (low, rounded, circular hills), and rilles (channels or grooves) are still discernable. Scientists have also recently discovered evidence of ice in permanently shadowed areas of the surface.

A Craters

The Moon’s surface is covered with craters overlain by a layer of soil called regolith. Nearly all the craters were formed by explosive impacts of high-velocity meteorites. Billions of years of this meteorite bombardment ground up the Moon’s surface rocks to produce the finely divided rock fragments that compose the regolith. Craters range in size from microscopic to the South Pole-Aitken Basin, which measures over 2,500 km (1560 mi) in diameter and would nearly span the continental United States. The highest mountains on the Moon, in the Leibnitz and Doerfel ranges near the south pole, make up the rim crest of the South Pole-Aitken Basin and have peaks up to 6,100 m (20,000 ft) in height, comparable to the Himalayas on Earth. At full moon long bright streaks that radiate from certain craters can be seen. These streaks are called ray systems. Ray systems are created when bright material ejected from the craters by meteorites splashes out onto the darker surrounding surface.

The biggest of the Moon’s craters were created by the impacts of large remnants from the formation of the planets billions of years ago when the young solar system still contained many such remnants. Astronomers, however, have directly observed meteorites forming small craters on the Moon’s surface. Seismometers operating on the lunar surface have also recorded signals indicating between 70 and 150 meteorite impacts per year, with projectile masses from 100 g to 1,000 kg (4 oz to 2,200 lb). Hence the Moon is still being bombarded by meteorites, although neither as often nor as violently as in the distant past.

B Volcanic Features

Maria, domes, rilles, and a few craters display indisputable characteristics of volcanic origin. Maria are plains of dark-colored rock that cover approximately 40 percent of the Moon’s visible hemisphere. The maria formed when molten rock erupted onto the surface and solidified between 3.16 billion and 3.96 billion years ago. This rock resembles terrestrial basalt, a volcanic rock type widely distributed on Earth, but the rock that formed the maria has a higher iron content and contains unusually large amounts of titanium. The largest of the maria is Oceanus Procellarum, an oval-shaped plain on the near side of the Moon 2,500 km by 1,500 km wide. Photographs of the side of the Moon not visible from Earth have revealed a startling fact: The far side generally lacks the maria that are so conspicuous a feature of the visible side. This probably reflects the fact that the Moon’s crust is thicker on the far side than on the near side, and therefore the lavas that form the maria were more easily erupted through the thinner crust. Rilles are of two basic types: sinuous and straight. Sinuous rilles are meandering channels that are probably lava drainage channels or collapsed lava tubes formed by large lava flows. Straight rilles are large shallow troughs caused by movement of the Moon’s crust; they may be up to a thousand kilometers long and several kilometers wide. Domes are small rounded features that range from 8 to 16 km (5 to 10 mi) in diameter and from 60 to 90 m (200 to 300 ft) in height. Domes, thought to be small inactive volcanoes, often contain a small rimless pit on their tops.

Magnetic and other measurements indicate a current temperature at the Moon’s core as high as 1600°C (2900°F), above the melting point of most lunar rocks. Evidence from seismic recordings suggests that some regions near the lunar center may be liquid. However, no evidence of recent volcanic activity has been observed.

C Ice

Temperatures on most of the Moon’s surface are too extreme for water or ice to exist, ranging from a maximum of 127°C (261°F) at lunar noon to a minimum of -173°C (-279°F) just before lunar dawn. Temperatures in permanently shadowed areas near the lunar poles, however, may consistently be as low as -220°C (-364°F). In 1996 a team working with data gathered by the Clementine spacecraft announced that frozen water may exist in one of these shadowed areas near the Moon’s south pole. Clementine was a joint venture by the Department of Defense and the National Aeronautics and Space Administration (NASA). The spacecraft’s radar showed what may be an 8,000 sq km (3,000 sq mi) area covered with a mixture of dirt and ice crystals. Clementine was launched in 1994 and gathered data for four months.

NASA launched the Lunar Prospector spacecraft toward the Moon in 1998. Prospector returned data confirming the Clementine discovery and suggesting that a significant amount of water exists in the dark areas near the lunar poles in the form of ice crystals mixed with soil. Estimates of the amount of water on the Moon vary widely, from 10 million to 6 billion metric tons.

In 1999, at the end of the Lunar Prospector’s mission, scientists programmed the spacecraft to crash at a specific spot likely to contain water, hoping that the debris that rose with the impact would contain detectable water vapor. Although no water was detected after the crash, scientists could not conclude that no water existed on the Moon. They acknowledged several other possible explanations for the result: The spacecraft might have missed its target area, the telescopes used to observe the crash might have been aimed incorrectly, or the magnitude of the impact created by the Lunar Prospector spacecraft may have been insufficient to generate a large cloud of water vapor.


Measuring the ages of lunar rocks has revealed that the Moon is about 4.6 billion years old, or about the same age as Earth and probably the rest of the solar system. Before the modern age of space exploration, scientists had three major models for the origin of the Moon. The fission from Earth model proposed that the young, molten Earth rotated so fast that it flung off some material that became the Moon. The formation in Earth orbit model claimed that the Moon formed independently, but close enough to Earth to orbit the planet. The formation far from Earth model proposed that the Moon formed independently in orbit around the Sun but was subsequently captured by Earth’s gravity when it passed close to the planet. None of these three models, however, is entirely consistent with current knowledge of the Moon. 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: a giant, planetary impact.

The giant impact model proposes that early in Earth’s history, well over 4 billion years ago, Earth was struck by a large planet-sized body. Early estimates for the size of this object were comparable to the size of Mars, but a computer simulation by American scientists in 1997 suggested that the body would have to have been at least 2.5 to 3 times the size of Mars. The catastrophic impact blasted portions of Earth and the impacting body into Earth orbit, where debris from the impact eventually coalesced to form the Moon. After years of research on lunar rocks during the 1970s and 1980s, this model became the most widely accepted one for the Moon’s origin. The giant impact model seems to account for all of the available evidence: the similarity in composition between Earth and Moon indicated by analysis of lunar samples, the near-complete global melting of the Moon (and possibly Earth) in the distant past , and the simple fact that the other models are all inadequate to one degree or another. Research continues on the ramifications of such a violent lunar origin to the early history of Earth and the other planets.


The Moon has no global magnetic field as does Earth. Some lunar rocks are weakly magnetic, indicating that they solidified in the presence of a magnetic field. The Moon has small, local magnetic fields that seem to be strongest in areas that are on opposite hemispheres from large basins. The origin of these local magnetic fields is unknown. Some scientists speculate that the magnetic fields were induced by the extreme shock pressures associated with the large asteroid collisions that created the basins. Others believe that the Moon originally had a global magnetic field generated by the movement of liquid metal in the core as on Earth. This global field shut down for some reason and only remnants of it exist in certain places on the lunar surface, preserved in material ejected by the asteroid collisions. The “fossil” magnetism found in some lunar rocks supports the former global field model, whereas the regional distribution of the magnetic surface anomalies tends to support the local field model. Regions of strong magnetic fields repel the charged particles that stream from the Sun in the solar wind. Scientists believe that interaction with the solar wind darkens the Moon, and that some lighter swirl-shaped regions of the Moon are protected by local magnetic fields.


Throughout the 19th and 20th centuries, visual exploration with powerful telescopes yielded fairly comprehensive knowledge of the geography 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 Luna 3 spacecraft. These photographs showed that the far side of the Moon is similar to the near side except for the absence of large maria. Craters are now known to cover the entire Moon. In 1964 and 1966 photographs from U.S. spacecraft—Ranger 7 through 9 and Lunar Orbiter 1 through 5—further supported these conclusions. The entire Moon has about 3 trillion craters larger than 1 m (3 ft) in diameter.

The successful landings of the robotic U.S. Surveyor series spacecraft and the USSR Luna series in the 1960s, and then 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 lunar surface a reality (see Space Exploration). The Apollo astronauts collected rocks, took thousands of photographs, and set up instruments on the Moon that radioed information back to Earth even after the astronauts departed. These instruments measured temperature and gas pressure at the lunar surface; heat flow from the Moon’s interior; molecules and ions of hot gases, called the solar wind, that stream out from the atmosphere of the Sun; the Moon’s magnetic field and gravity; seismic vibrations of the lunar surface caused by landslides, meteorite impacts, and so-called moonquakes; and the precise distance between Earth and the Moon.

All six manned landings on the Moon—Apollo 11, 12, 14, 15, 16, and 17—returned samples of rock and soil to Earth. These samples weighed a total of 384 kg (847 lb). The astronauts explored increasingly wider areas on the Moon with each successive flight, culminating with the 35 km (22 mi) explored using a lunar roving vehicle by the Apollo 17 crew. This final mission included the only geologist ever to walk on the Moon, Harrison (Jack) Schmitt. Analysis of the data and rocks obtained by the lunar missions continues.

In 1994, the joint Defense Department/NASA spacecraft Clementine orbited the Moon for 71 days, mapping the color and precise altitude of the lunar surface. From Clementine data, astronomers obtained their first global look at the topography and mineralogy of the Moon, finding that the Moon’s crust is indeed made of a low-iron, low-density rock called anorthosite and mapping the large, ancient basins that make up the structural framework of the Moon. Clementine also discovered ice on the Moon in the permanently dark areas near the south pole.

NASA sent a spacecraft of its own, an orbiter called Lunar Prospector, to the Moon in 1998. Lunar Prospector orbited around the Moon’s north and south poles and returned data until July 1999. The spacecraft mapped the gravitational field of the Moon, determined the distribution of radioactive elements in its crust, and confirmed the presence of ice at the lunar poles. Scientists used the spacecraft right up to its final moments. They ended Prospector’s mission by programming it to crash into the Moon’s surface and then observed the cloud of debris that rose from the impact.

Although much has been learned about the Moon in the past 30 years, much still remains mysterious. Understanding the Moon and its history is important for two reasons. First, the Moon is a natural laboratory to study the geological processes—meteorite impacts, volcanism, and large-scale movements of the crust—that have shaped all of the rocky planets. Second, the Moon’s ancient surface retains a record of events in this part of the solar system that has been erased from the much more active, dynamic surface of Earth. The impact record, which has been almost entirely erased on Earth, is especially clear on the Moon, and may contain important clues to the history of life on Earth. Thus, the Moon serves as a touchstone, allowing us to better comprehend the complex stories of all the planets in our solar system.

Reviewed By:Marius Lukosius