Glossary of Lunar Terminology albedo A measure of the reflectivity of the Moon's gabbro A coarse crystalline rock, often found in the visible surface. The Moon's albedo averages 0.07, which lunar highlands, containing plagioclase and pyroxene. means that its surface reflects, on average, 7% of the Anorthositic gabbros contain 65-78% calcium feldspar. light falling on it. gardening The process by which the Moon's surface is anorthosite A coarse-grained rock, largely composed of mixed with deeper layers, mainly as a result of meteor­ calcium feldspar, common on the Moon. itic bombardment. basalt A type of fine-grained volcanic rock containing ghost crater (ruined crater) The faint outline that remains the minerals pyroxene and plagioclase (calcium of a lunar crater that has been largely erased by some feldspar). Mare basalts are rich in iron and titanium, later action, usually lava flooding. while highland basalts are high in aluminum. glacis A gently sloping bank; an old term for the outer breccia A rock composed of a matrix oflarger, angular slope of a crater's walls. stony fragments and a finer, binding component. graben A sunken area between faults. caldera A type of volcanic crater formed primarily by a highlands The Moon's lighter-colored regions, which sinking of its floor rather than by the ejection of lava. are higher than their surroundings and thus not central peak A mountainous landform at or near the covered by dark lavas. Most highland features are the center of certain lunar craters, possibly formed by an rims or central peaks of impact sites. upwelling of material after an impact event. impact site A crater or other feature on the Moon cleft see rille. formed by the impact of a meteorite, asteroid, or comet co-longitude see selenographic co-longitude. with the lunar surface. Craters include everything from concentric crater A crater consisting of multiple rings, huge multi-ringed impact basins like Mare Orientale or one inside the other, and so looks something like a Mare Nectaris to minute crater pits. bulls-eye. Hesiodus and Taruntius are good examples KREEP A mineral containing potassium (K), rare earth of concentric craters. elements (REE), and phosphorus (P). crater A physical depression, of approximately circular lib ration The apparent rocking motion of the Moon, outline, on the Moon's surface. either in longitude or latitude. The eccentricity of the crater cone The hill, often steep, that a volcanic crater Moon's orbit causes libration in longitude. The inclin­ may build about itself. ation of its orbit to the celestial equator causes libration craterlet Any lunar crater that is less than approxim­ in latitude. The fact that we observe the Moon from the ately 8 km (5 miles) in diameter. surface of an extended and rotating globe (the Earth) crater pit A very small craterlet. itself causes a minor effect, the diurnal libration. In dome A low, rounded elevation on the Moon, possibly practice, the three types of libration combine to make of volcanic origin. Some may be as much as 16 km (10 occasionally visible those regions near the limb that miles) in diameter, but only 15-60 m (50-200 ft) high. would otherwise be permanently out of view. earthshine The dim illumination of that part of the limb The "edge" of the Moon's disk as it appears from Moon not lit by the direct light of the Sun. It is most the Earth. easily seen around the time of the crescent Moon, and lunar transient phenomenon (LTP) A short-lived and is caused by sunlight reflected from the Earth. unusual apparent change in the appearance of a lunar ejecta hypothesis The hypothesis that lunar rays, and feature; also known as a transient lunar phenomenon certain other phenomena, are formed by solid bodies (TLP). ejected from a lunar crater during impact. lunation The interval between one new moon and the next fault A fracture of the lunar surface, along which there (or one full moon and the next). It is equal to 29 days, has been slippage, either vertical or horizontal; a sign 12 hours, 44 minutes. Also called the synodic month. of tectonic activity. mare (pl. maria) A relatively smooth area of the lunar feldspar An alumino-silicate material. surface, Latin for "sea." Maria are composed of solid­ foreshortening An optical effect that distorts the appar­ ified lavas which are darker in hue than the rougher ent dimensions of features near the lunar limb. highland areas. It is the dark maria that cause the Features near the limb appear crowded together in a famous "man in the Moon" effect, seen near the time of radial direction. full moon. Glossary of Lunar Terminology 161 moonquake A seismic disturbance in the Moon's rampart An old term for a crater's walls. interior. ray system Bright streaks crossing the lunar surface, multi-ring(ed) impact basin A very large impact site, radiating from certain craters. They are most apparent such as Mare Orientale, that displays multiple concen­ around the time of full moon. tric shock rings, forming a "bulls-eye" pattern of high­ regolith The surface layer of the Moon, composed of a lands surrounding a central depressed region. Impact dark, fine, dusty material. sites that are roughly 300 km (200 miles) across or rille (rima, cleft) A long and narrow, linear or sinuous larger were created by meteoroids with sufficient depression in the lunar surface. energy to make multiple rings. ringed plain see walled plain. occultation The passage of the Moon in front of a back­ ruined crater see ghost crater. ground star, or rarely a planet, as seen from the Earth. scarp A cliff or line of cliffs on the Moon (see also olivine An iron magnesium silicate mineral commonly wrinkle ridge). fbund on the Moon. seeing The quality of the steadiness and sharpness of a outgassing The escape of gas from a lunar formation, telescopic image, as affected by atmospheric and probably of volcanic origin. A possible explanation for thermal conditions. manyLTPs. selenographic co-longitude The lunar longitude of the oxygen isotopic abundance In the currently accepted sunrise terminator. Co-longitude is more accurate than theory of the Solar System's formation, the various phase for indicating the position of the Sun relative to chemical elements tended to condense from the the Moon because it has been corrected for the Moon's primeval solar nebula according to their atomic librations. weights, the heavier ones condensing closer to the Sun, selenography The study of the location and dimensions the lighter ones farther away. That is why the planets of the Moon's physical features (by analogy with "geo­ closest to the Sun tend to contain the heavier elements, graphy"); lunar mapping. and you can tell how far from the Sun a planet (or piece selenology A now seldom-used term for the study of the of planet) formed just by looking at its chemical com­ Moon's composition and physical nature. Today, position. Isotopes are variants of the same chemical "lunar geology" is used instead. element, with different numbers of atomic particles in sinus Latin for "bay," a lava-filled impact site located the atomic nucleus. The ratio of two different isotopes at the edge of any of the lunar maria. A well known of oxygen is known as the oxygen isotopic abundance, example is Sinus Iridum, on the "shore" of Mare and this quantity is very nearly the same for lunar and Imbrium. terrestrial material. Sun angle The angle at which the Sun appears with ref­ palus Latin for "marsh." A lunar area that looked to erence to a feature under observation. classical observers like a terrestrial marsh. An example terminator The great circle on the Moon that is the is Palus Putredinis. boundary between day and night. Lunar features are phase The fraction of the Moon's Earth-facing hemi­ best seen when they are on or near the terminator, as sphere illuminated by the Sun. the Sun's low angle creates shadows that emphasize plagioclase A calcium feldspar mineral commonly found vertical relief. on the Moon. terraced crater A crater with inner walls that have plate tectonics The theory that the Earth's crust consists terraces - concentric ledges. Terraced craters usually of huge plates that move away from ridges and either show sunken floors and a well-defined central peak. drop downwards or slide laterally where they may walled plain (ringed plain) A lunar feature consisting of collide with other plates, often building mountain a mare-like floor surrounded by a sharp-crested rim; ranges in the process. There is little or no evidence of an old name for a large crater. plate tectonics on the Moon. wrinkle ridge An elevated, usually sinuous feature pyroxene The ortho-form is an iron magnesium silicate common on the lunar maria, but of uncertain origin. mineral commonly found on the Moon. The dino-form Where they extend to the highlands they are known as also contains calcium. scarps. Further Reading Alter, D, Lunar atlas, North American Aviation, Downey, Hartmann, WK, Radial structures surrounding lunar CA, 1964; reprinted by Dover, New York, 1968. basins. I. The Imbrium system. Communications of the Alter, D, Pictorial guide to the Moon, 2nd edn. Thomas Lunar and Planetary Laboratory, 2(24):1-15, 1963. Crowell, New York, 1973 [1st edn 1967]. Hartmann, WK, Radial structures surrounding lunar Baldwin, RB, The face of the Moon. University of Chicago basins. II. Orientale and other systems; Conclusions. Press, Chicago, 1949. Communications of the Lunar and Planetary Baldwin, RB, The measure of the Moon. University of Laboratory, 2(36):175-91,1964.