Absolute altitude, The height of any point on the Lunar surface in comparison to the "reference sphere," a perfect sphere of 3476 kilometers in diameter which represents the mean height of average terrain on the Moon.
Albedo, A measure of reflectiveness or reflective power, or, specifically, the light that is reflected by the surface of a body such as the Moon. The term "low albedo" generally refers to dark features; "high albedo" generally refers to lighter-colored features.
ALSEP, Acronym for the Apollo Lunar Surface Experiments Package. The precise makeup of the instruments for each ALSEP varied from mission to mission.
Anorthosite, Granular igneous rock usually comprised of soda-lime feldspar.
Apollo, Name given to the United States manned Lunar program, which included the first successful landing of a human crew on the Moon (Apollo 11, 20 July 1969). Other highlights of the program included the first manned Lunar orbit (Apollo 8, December 1968); the first use of an electric vehicle, the "Lunar Rover," to explore the Moon's surface (Apollo 15, July 1971); and the first scientist-astronaut to visit the Moon (Harrison H. Schmitt, Apollo 17, December 1972). The program began tragically with the deaths of Apollo 1 astronauts Virgil I. Grissom, Edward H. White II and Roger B. Chaffee in a launch pad fire during a preflight test in January 1967.
Basin, A large impact crater, usually with a diameter in excess of 100 kilometers. Most basins have been modified by degradation of the original basin relief through downslope movement of debris and flooding of the basin interior by lavas.
Breccia, Coarse-grained rock composed of angular fragments of pre-existing rock.
Caldera, A large volcanic depression at the summit of a volcano, caused by collapse or explosion.
Catena pl. catenae, Chain of craters.
Cavus pl. cavi, Hollows or irregular steep-sided depressions, usually in arrays or clusters.
Chasma pl. chasmata, A deep, elongated, steep-sided depression.
Colles, Small hills or knobs.
Corona pl. coronae, Ovoid-shaped feature.
Crater pl. craters, A typically bowl-shaped or saucer-shaped pit or circular depression, generally of considerable size and with steep inner slopes, formed on a surface or in the ground by the explosive release of chemical or kinetic energy; e.g., an "impact crater" or an "explosion crater".
Diurnal, Having a daily cycle, or recurring every day.
Dorsum pl. dorsa, Ridge.
Ejecta, The material thrown out of an impact crater by the shock pressures generated during the impact event. Ejecta generally covers the surface around an impact crater to a distance of at least one crater diameter, with individual streamers of material extending well beyond this distance. The ejecta blanket of a crater becomes less visible with increasing age of the crater. (See also rays)
EVA, Acronym for Extra-Vehicular Activity; any activity that takes an astronaut outside the spacecraft during the mission.
Facula pl. faculae, Bright spot.
Far side, The surface of the Moon that is not generally visible from Earth due to its unique orbital pattern, which keeps the Lunar "face" turned toward Earth. Often incorrectly referred to as the "dark side of the Moon," in reality at least one-half of the Lunar surface is receiving sunlight at all times.
Farrum pl. farra, Pancake-like structure, or a row of such structures.
Flexus, A very low curvilinear ridge with a scalloped pattern.
Fluctus, Flow terrain.
Fossa pl. fossae, Long, narrow, shallow depression.
Full moon, Lunar phase during which the entire visible surface is under illumination. (See also new moon.)
Gibbous moon, The phase of the Moon during which more than half, but less than all, the visible hemisphere of the Moon is illuminated by sunlight.
Highlands, The densely cratered portions of the Moon that are typically at higher elevations than the mare plains; often referred to as "terrae." The highlands contain a significant proportion of anorthosite, an igneous rock made up almost entirely of plagioclase feldspar.
KREEP, Elemental composite materials used by scientists as a chemical tracer, consisting of potassium (K), Rare Earth Elements and phosphorous (P).
Labyrinthus pl. labyrinthi, Complex of intersecting valleys.
Lacus, "Lake"; small plain.
Large ringed feature, Unusual ringed features on the lunar surface that cannot be classified under another descriptor.
Lava, Volcanic rock extruded by the eruption of molten material. An extensive segment of the lunar surface, specifically in the mare regions, is comprised primarily of basalt resulting from lava flows.
Limb, The outer edge of a Lunar or other planetary disk.
Linea pl. lineae, A dark or bright elongate marking; may be curved or straight.
LRV, Acronym for Lunar Roving Vehicle or Rover, four-wheeled vehicle used for transportation during manned exploration of the Moon during final Apollo missions.
Luna, Accepted common name for Earth's Moon; derived from the Latin word for "light."
Luna, Name given to a pioneering series of Soviet automatic interplanetary stations successfully launched in the direction of the Moon. The Luna program, which began with the launch of Luna 1 in January 1959 (the first man-made probe to reach the Moon), also sent back the first crude photographs of the far side (Luna 3, October 1959). The program was discontinued following Luna 24 in August 1976.
Lunan, Of or relating to Luna or its inhabitants.
Lunar eclipse, Period in which the Earth is positioned so as to obscure the Moon from sunlight.
Lunation, The period of time it takes the Moon to complete one set of phases (the "synodic month"), specifically from New Moon to New Moon, averaging 29 days, 12 hours, 44 minutes and 2.9 seconds. Lunations are numbered sequentially, beginning with Lunation 1 which commenced on 16 January 1923. Lunation 1000 will commence on 25 October 2003.
Macula pl. maculae, Dark spot; may be irregular.
Mare (pronounced "mahr-ay") pl. maria (pronounced "ma-ree-ah"), "Sea"; a large circular plain on the Moon; specifically, the low albedo plains covering the floors of several large basins and spreading over adjacent areas. The mare material is comprised primarily of basaltic lava flows, in contrast to the anorthosites in the highlands.
Mascon, Concentrations of mass on the lunar surface (from mass concentrations).
Massif, A massive topographic and structural feature, commonly formed of rocks more rigid than those of its surroundings. These rocks may be protruding bodies of basement rocks, consolidated during earlier orogenies.
Mensa pl. mensae, A flat-topped prominence with cliff-like edges.
Mons pl. montes, Mountain.
New moon, lunar phase during which the entire visible surface is in darkness. (See also "full moon.")
Oceanus, A very large dark area on the Moon.
Orogeny pl. orogenies, The process of forming mountains, usually as a result of the folding of the surface of a region.
Palus pl. paludes, "Swamp"; small plain.
Patera pl. paterae, An irregular crater, or a complex one with scalloped edges.
Phase angle, The angle between the incident sunlight and the viewing direction when looking at an illuminated surface. Low phase angles result in relatively few shadows being cast by the surface relief.
Planitia pl. planitiae, Low plain.
Planum pl. plana, Plateau or high plain.
Promontorium pl. promontoria, A high point of land; headland.
Ray, A streamer of ejecta associated with an impact crater. Rays are most often of higher albedo than their surroundings. The albedo contrast may result from either disruption of the local surface by the ejecta or by emplacement of ejecta on the surroundings, or both.
Regio pl. regiones, A large area marked by reflectivity or color distinctions from adjacent areas; a broad geographic region.
Regolith, A residual mixture of fine dust and rocky debris, usually produced by meteor impacts, covering the lunar surface.
Rille, One of the several trenchlike or cracklike valleys, up to several hundred kilometers long and one to two kilometers wide, commonly occurring on the Moon's surface. Rilles may be extremely irregular with meandering courses ("sinuous rilles"), or they may be relatively straight ("normal rilles"); they have relatively steep walls and usually flat bottoms. Rilles are essentially youthful features and apparently represent fracture systems originating in brittle material.
Rima pl. rimae, Fissure.
Scarp, A change in topography along a linear to arcuate cliff. The cliff may be the result of one or more processes including tectonic, volcanic, impact-related, or degradational processes. The term "rupes" is generally used in lunar geography when referring to this type of feature.
Scopulus, Lobate or irregular scarp.
Secondary craters, Craters produced by the impact of debris thrown out by a large impact event. Many secondary craters occur in clusters or lines where groups of ejecta blocks impacted almost simultaneously.
Selene (pronounced "suh-lee-nee"), The Greek goddess of the Moon.
Selenology, The scientific study of the history of the Moon, as recorded in rocks, minerals and other materials found on the lunar surface; from Selene, Greek goddess of the Moon.
Sinus, "Bay"; small plain.
Sulcus pl. sulci, Subparallel furrows and ridges.
Synodic month or synodic period, The period of time it takes for one body to orbit around its primary, such as the Moon around Earth. The Lunar synodic month is measured as the time it takes to complete one set of phases, specifically from New Moon to New Moon. (See also lunation.)
Terminator, The line separating the illuminated and dark areas of a planetary body; the dividing line between day and night as observed from a distance.
Terra pl. terrae, Extensive land mass; often used as descriptor for lunar highlands.
Tessera pl. tesserae, Tile-like, polygonal terrain.
Tholus pl. tholi, Small domical mountain or hill.
Vallis pl. valles, Valley.
Vastitas pl. vastitates, Extensive plain.
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