Luna Society - The Moon - Lunar republic


The Full Moon Dictionary
and Glossary of Lunar Terms

Many features of the Moon are described by scientific terms with their roots in ancient languages, including Latin and Greek. The following glossary of Lunar terms comprises a basic dictionary of the Moon and its study.

Words in boldface are defined within the particular entry. Italicized words are defined elsewhere in the glossary.

A | B | C | D | E | F | G | H | I | J | K | L | M
N | O | P | Q | R | S | T | U | V | WXYZ


The Apollo 11 spacecraft launches from Kennedy Space Center on 16 July 1969.

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.

Apogee, The point in the Moon's orbit where it is furthest from Earth. At its apogee, the Moon is 406,700 kilometers from Earth. (See also perigee.)

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.

Return to top of page


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.

Return to top of page


Most craters fit a standard description: sloping exterior, bowl-shaped interior and central cone formation.

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".

Return to top of page


Diurnal, Having a daily cycle, or recurring every day.

Dorsum pl. dorsa,  Ridge.

Return to top of page


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.

Return to top of page


The Moon's far side from Luna 3

The first, crude photograph of the Moon's far side, sent back by Luna 3 in October 1959.

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.)

Return to top of page

Waxing gibbous Moon

A waxing (increasing), gibbous (more than half, but not yet full) 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.

Return to top of page


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.

Return to top of page


KREEP, Elemental composite materials used by scientists as a chemical tracer, consisting of potassium (K), Rare Earth Elements and phosphorous (P).

Return to top of page


Labes, Landslide.

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.

Spherical Luna 1, the first man-made satellite to reach the Moon.

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.

Return to top of page


The Mare Nectaris ("Sea Of Nectar"), visible in the lower left corner of the photo. The large, circular formation to the right of center is the remains of the ancient crater Fracastorius.

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.

Return to top of page


New moon, lunar phase during which the entire visible surface is in darkness. (See also "full moon.")

Return to top of page


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.

Return to top of page


At apogee, the Moon appears slightly smaller (left) to the human eye, while at perigee it appears slightly larger (right), owing to the elliptical shape of its orbit around Earth.

Palus pl. paludes, "Swamp"; small plain.

Patera pl. paterae, An irregular crater, or a complex one with scalloped edges.

Perigee, The point in the Moon’s orbit where it is closest to Earth. At its perigee, the Moon is 356,400 kilometers from Earth. (See also apogee.)

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.

Return to top of page


Rays emanating from Copernicus crater

Rays, shown as light-colored streaks emanating from the crater Copernicus (center of photo) in this example, are the result of materials ejected from meteor impacts.

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.

Rupes, Scarp.

Return to top of page


Selene and Endymion, from the Glyptotek Copenhagen

Selene, Greek goddess of the Moon (right), and her beloved, King Endymion of Elis.

"From her immortal head a radiance is shown from heaven and embraces Earth; and great is the beauty that arises from her shining light."

— Homer

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.)

Return to top of page

The imaginary line separating the lighted and shaded areas on the Moon or other planetary bodies is known as the terminator.


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.

Return to top of page


Undae,  Dunes.

Return to top of page

The Vallis Alpes is a 180-kilometer long cleft, or valley, in the Moon's floor near Mare Frigoris and the Montes Alpes (Lunar Alps).


Vallis pl. valles, Valley.

Vastitas pl. vastitates, Extensive plain.

Return to top of page


Waning moon, Period during which illumination of the visible lunar surface decreases (following the most recent full moon) until it reaches complete darkness ("new moon").

Waxing moon, Period during which illumination of the visible lunar surface increases (following the most recent new moon) until it reaches complete illumination ("full moon").

Return to top of page

The Moon - Luna Society International - Lunar Republic

[Return To Luna Society Front Page]

The Lunar Republic™, The Full Moon Atlas™, The Lunar Consulate™, The Lunar Registry™ and the phrases
 "The Official Website Of The Moon"™ "Welcome To The Moon!"™ and "Rekindling The Dream"™
are international trademarks of Lunar Republic, S.A. and Luna Society International.
Contents copyright © 1999-2012 by the Lunar Republic Society. All rights reserved.
We encourage you to read our privacy statement and terms of use.
Information current through

Flag of the Lunar Republic

NEW YORK NY 10001-7945 USA