MOON

sole natural satellite of the Earth. It revolves around the planet from west to east at a mean distance of about 384,400 km (239,900 miles). The Moon is less than one-third the size of the Earth, having a diameter of only about 3,476 km (2,160 miles) at its equator. It is only 1/81.3 as massive as the Earth and has a density of roughly 3.34 grams per cubic centimetre, as opposed to 5.52 for the terrestrial body. The Moon shines by reflected sunlight, but its albedoi.e., the fraction of light received that is reflectedis only 0.073. The Moon rotates about its own axis in about 29 1/2 days, which is virtually identical to the time it takes to complete its orbit around the Earth. As a result, the Moon always presents nearly the same face to the Earth. The rate of actual rotation is uniform, but the arc through which the Moon moves from day to day varies somewhat. Accordingly, the face that the Moon turns to the Earth is subject to a corresponding variation, the lunar globe (as seen by a terrestrial observer) slightly oscillating in a period nearly equal to that of revolution. This apparent oscillation is called optical libration, and its amount is commonly between 6 and 7. The surface of the Moon has been a subject of continuous telescopic study from the time of Galileo's first observation in 1609. In his Almagestum novum (1651) the Italian Jesuit astronomer Giovanni B. Riccioli designated the dark areas on the Moon as seas, with such fanciful names as Mare Imbrium (Sea of Showers) and Mare Nectaris (Sea of Nectar). This nomenclature continues to be used even though it is now known that the Moon is completely devoid of surface water. During the centuries that followed the publication of these early works, more detailed maps and eventually photographs were produced. By 1959 the side of the Moon facing away from the Earth had been photographed by a Soviet space probe. By the late 1960s the U.S. Lunar Orbiter missions had yielded closeup photographs of the entire lunar surface, including both the visible and far sides. The dominant process affecting the Moon's surface has been impact. The surface is covered by a regolith layer consisting of fine-grained rock fragments created by micrometeorite bombardment that has pulverized the surface materials. The most striking formations on the Moon are its craters. These features, which measure up to about 200 km (320 miles) or more in diameter, are scattered over the surface in great profusion and often overlap one another. Most of the large craters were produced by meteorites hitting the lunar surface at high velocity. Many of the smaller onesthose measuring less than 1 km (0.6 mile) acrosscould have been formed by explosive volcanic activity, however. Many craters have a surrounding ring; such a structure is sometimes quite low, though a typical one may be about 1,500 m (4,920 feet) above the surrounding landscape. In many cases, there is a central peak or several peaks within a crater. The darker areas of the Moon, known as maria (see mare), have relatively few craters. They are thought to be huge lava flows that spread over an area after most of the craters had already been formed. Another notable type of topographic feature is the rille, a deep trench that may extend several hundred kilometres. Rilles tend to occur in parallel groups just within mountain ranges or hills bordering maria or larger craters. Various theories for the Moon's origin have been proposed. At the end of the 19th century the English astronomer Sir George H. Darwin advanced a hypothesis that attracted considerable attention for decades. On the basis of the mathematical theory of solar tides, Darwin suggested that the Moon had been originally part of the Earth but was broken away by tidal action and receded from the planet. In 1930 Sir Harold Jeffreys cogently demonstrated that such a process was highly improbable. Another popular theory of lunar genesis that arose during the 1950s postulated that the Moon formed elsewhere in the solar system and was then later captured by the Earth. The basic assumptions on which this idea was grounded have since been found to be physically implausible and so the theory was dismissed. The coaccretion hypothesis suggested that the Earth and Moon formed at the same time from a primordial cloud; this theory cannot account for the large angular momentum of the Earth-Moon system as it revolves around the Sun, however. Today, most investigators favour an explanation known as the giant-impact hypothesis. According to this theory, a Mars-sized body struck the proto-Earth early in the history of the solar system. As a result, a cloud of fragments was ejected into orbit around the Earth, and these later accreted into the Moon. Prior to the collision, both bodies had differentiated into a metallic core and a silicate-rich mantle, so that the ejected matter (i.e., the proto-Moon) was predominantly silicates while the metallic cores coalesced in the Earth. designated in astronomy, sole natural satellite of the Earth. For centuries, speculation and scientific investigation have been centred on the Moon; it was the first new world to be visited by humans. Although many questions remain about lunar history, it is clear that the Moon holds keys to understanding the origin of the solar system. Additional reading The finest book about lunar exploration is David Thomas (ed.), Moon: Man's Greatest Adventure, rev. ed. (1973), splendidly illustrated, with text by scholars describing the history and culture, engineering and projects, and early scientific results of the great human drive that began in ancient times and culminated in the Apollo missions. A more modest but still comprehensive and well-illustrated book is Patrick Moore and Charles A. Cross, The Moon (1981). Don E. Wilhelms, John McCauley, and Newell J. Trask, The Geologic History of the Moon (1987), is illustrated with many beautiful explanatory pictures and drawings. The rich and expanding scientific literature of the Moon is well represented in Proceedings of Lunar and Planetary Science, papers from the annual Lunar And Planetary Science Conference. Stuart Ross Taylor, Planetary Science: A Lunar Perspective (1982); and W.W. Mendell (ed.), Lunar Bases and Space Activities of the 21st Century (1985), are collections of specialized papers, although most of the material is accessible to the general reader. Some examples of relevant literature on theories of lunar origin are W.K. Hartmann, R.J. Phillips, and G.J. Taylor (eds.), Origin of the Moon (1986); and A. Vitiazev, G. Pechernikova, and V. Safronov, Terrestrial Planets: The Origin and Evolution (1990). The realization that the Moon is an enormous storehouse of resources that may be useful to humankind in the future has prompted several publications, including Gerard K. O'Neill, The High Frontier (1977, reissued 1989); a series of conference papers, Space Manufacturing (biennial); and a book commissioned by NASA to serve as a primary reference, Grant Heiken, David Vaniman, and Bevan French (eds.), Lunar Sourcebook: A User's Guide to the Moon (1991), much of it easily read by the nonspecialist, which with its compendious bibliography is an excellent summary of what is known about the Moon. James D. Burke