Tools
Featured
Topics
Products

Moon Glossary: Lunar Terms and Definitions

A | B | C | D | E | F | G | H | I | L | M | N | O | P | Q | R | S | T | U | W | X, Y, Z


A

Albedo — This term is a unit-less measure that refers to the how much an object diffusely reflects light from the sun.

Angular Diameter — The measurement of the diameter of a distant object from the perspective of the angle resulting between the observer and the object's outer edges. Also known as the 'visual diameter'.

Annular Eclipse — Refers to an solar eclipse where the Moon is between the Sun and the Earth, but with the Moon appearing smaller than the Sun to the observer, leaving a bright ring around the circumference of the Moon.

Anomalistic Month — The length of time that the Moon takes to pass between perigee (or apogee) passages — that is, the 27.55455 day period during which the Moon moves from its closest (or farthest) point to the Earth, all the way around and back again.

Anorthositic Rock — A calcium-rich rock type found in abundance on the Moon.

Aphelion — The farthest point from the Sun in a planetary body's orbit.

Apogee — The farthest point from the Earth in the Moon's orbit.

Apogean Tide — The name for low tide when the Moon is at it's farthest point from the Earth.

Apolune — The farthest point from the Moon in an object's orbit around it.


B

Basalt — Volcanic rock formed by rapidly cooling lava. Found on the moon and the earth.

Breccia — A type of rock that is composed of a matrix of different materials, minerals and fragments of other rocks. Found on the moon and the earth.


C

Celestial equator — The projection of the Earth's equator into space. It could be considered an invisible belt that surrounds the Earth in the minds of astronomers.

Celestial Mechanics — The specialized areas of astrology that describes the gravitational effects of heavenly bodies as well as their motion.

Colongitude — Also called selenographic colongitude, it is the longitude of the Moon's morning terminator.

Conjunction — The term applied when two planets are in close proximity to each other in the sky, from the perspective of an Earth-bound observer.

Crater Wall — The cliff-like wall formed by the impact of a meteor with a celestial body such as a planet or moon.

Crescent Moon — The famous image of the Moon frequently used in the media, consisting of only a thin crescent slice of the Moon being visible from Earth. This phase of the moon occurs just after the New Moon phase, which is also known as Dark of the Moon. There is also a Crescent Moon phase just prior to the next New Moon as well.


D

Dark of the Moon — So named because during this phase, the Moon is not visible in the sky, Dark of the Moon is also known as New Moon.

Declination — Declination is the position of a celestial body, such as the Moon, in the equatorial coordinate system. Declination is measured by degrees in relation to the celestial equator.

Diurnal — In astronomy, diurnal generally refers to the motion of an object in a 24 hour period. An example would be Moon-rise. These activities repeat every 24 hours. The diurnal arc describes the amount of time a celestial object takes to transition from fully risen to fully set.

DST — Daylight Savings Time.


E

Earthshine — The light of the sun that is reflected back into space by the Earth, and which can illuminate other objects such as the Moon.

Eclipse — Any interference between the light from the Sun and the object being illuminated. The Moon frequently moves between the Earth and the Sun, blocking out the Sun's rays.

Ecliptic — A term applied to the invisible path in the sky that the Sun moves through during the year, in relation to Earth and the other planets.

Elongation — The angle found between a planet and the Sun, from the perspective of the Earth.

Ephemeris — An astronomical text which contains the position of celestial bodies in the sky as seen from Earth at specific times. These positions are given in coordinates that astronomers can then use to locate and view these bodies.

Equatorial Tide — A tide with a period of 328 hours (approximately every two weeks) that occurs when the Moon is positioned above the equator.


F

Far Side — The side of the Moon which faces away from the Earth and which is not visible via planetary observation.

First Quarter Moon — During this phase of the Moon fifty percent of it is illuminated and visible from the Earth. It occurs after the Crescent phase and before the Waxing Gibbous phase.

Full Moon — During this phase of the Moon it is entirely illuminated and visible from the Earth. The Moon is opposite the Sun in this phase, with the Earth in between.


G

Gibbous Moon — There are two Gibbous phases of the Moon, with the first representing the growing moon between the First Quarter and the Full Moon and the second when it describes the Moon growing smaller as it shrinks down from the Full Moon to the Last Quarter. These phases are referred to as the Waxing and Waning Gibbous, respectively.

Gravity — Gravity is the attractive force which governs the motion of the celestial bodies. Gravity controls the orbits of all planets in our solar system as well as our solar system's relative motion to the Universe. It also plays a significant role in the distribution of mass throughout the Universe.

Grazing Occultation — A phenomenon in which the varied topography of the Moon's surface causes stellar objects such as stars to disappear and reappear at the northern or sudden limit of the Moon intermittently. This occurs due to the motion of the Moon itself across the path between a certain patch of the heavens and the Earth.

Greenwich Mean Time — Time derived from the annual average movement of the sun over the zero degrees longitude position of the Greenwich Observatory in England. The mean is used in order to compensate for the Sun's non-uniform motion. It has since been replaced in astronomical circles by Universal Time.


H

Half Moon — Term that is used to describe the First Quarter Moon and the Last Quarter Moon.


I

Intercalation — The source of leap years, or the addition of an extra day or other period of time in order to reconcile the solar year with that of the calendar we use. This is necessary because the solar year contains approximately 365.25 days, making it necessary to add a full extra day to the calendar every four years. In the past, much longer periods of time were used during intercalation. Intercalary days or months can also be added to the lunar calendar.


L

Lacus — Areas on the Moons surface that have the topography of lakes are prefaced with this Latin term for lake.

Last Quarter — The phase of the Moon between the Waning Gibbous and the Waning Crescent, where fifty percent of the Moon is still visible before the Waning Crescent phase wipes it from the sky.

Latitude — Coordinate system used on a planetary body to give the location of a point in relation to its equator. There are ninety degrees of latitude north and south of the equatorial line.

Librations — The gentle rocking motion of the Moon as it orbits the Earth that allows observation of the side that normally faces away from our planet. In total, through this irregular motion fifty-nine percent of the Moon can be seen.

Limb — The outermost edge of a planetary body or celestial object.

Longitude — Coordinate system used on a planetary body to give the location of a point in relation to a reference meridian. Meridians are lines drawn by cartographers and astronomers that pass through the northern point on the horizon, meeting at the celestial pole. As there is no natural starting meridian, the base or center meridian is chosen arbitrarily on each planet or moon. On Earth's Moon, Sinus Medii represents the zero degree point for longitude, with ninety degrees of longitude available to the east and west.

Lunar Day — There are two definitions of this term. The first refers to the period of time it takes for the Moon to spin completely on its axis in terms of its position to the sun. The second is the amount of time it takes for the Moon to complete a single orbit around the Earth. Due to the eccentric orbit, a Lunar Day varies in length. Lunar Eclipse — This event occurs when the Full Moon moves through the shadow cast by the Earth as it passes between the Sun and the Moon.

Lunar Interval — The difference in time between a phase of the Moon or tide occurring at the Greenwich meridian and a local meridian. This interval represents the passage of the Moon during this time.

Lunar Rays — Lines scored into the crust of a celestial body caused by mass ejected on impact from a meter. Found on both the Moon and Mars.

Lunitidal Interval — The period of time that occurs between the Moon passing over a point on Earth and the next high tide for that point.


M

Magnitude — The brightness of a celestial body. A lower magnitude number indicates a brighter object.

Mare — Name given to plains composed of basalt on the Moon. From the Latin word for 'sea', so-called due to their large landmass. These basalt deposits were left by the eruptions of now-extinct volcanoes.

Mascon — A region in the crust of a celestial body which is denser than average and as such acts to create a local gravitational anomaly. On the moon, there are several basins which demonstrate these properties.

Meridian — Meridians are lines drawn by cartographers and astronomers that pass through the northern point on the horizon of a planetary body or moon, meeting at the celestial pole.

Meteoroid — A meteoroid is any body drifting in space that has a large enough mass to be detected but which is significantly smaller than an asteroid. The exact limit at which a meteoroid is reclassified as an asteroid is up for debate, but generally, any object smaller than 50 meters in diameter is considered a meteoroid.

Moon — The natural satellite in orbit around the planet Earth. Also the classification for any natural satellite found in orbit around any other planet.

Moon Rise — Similar to sunrise, it is the first appearance that the Moon makes over the Earth's horizon, and as such it is relative to the geographical position of the observer.

Moon Set — The opposite of Moon Rise, it is the when the Moon disappears behind the Earth's horizon, relative to the observer.


N

Nadir — The point with a negative ninety degree inclination in relation to the observer, or the point directly beneath their feet.

Neap Tide — When the Moon is at its First Quarter or Last Quarter, its forces are partially cancelled out by the Sun. This leads to a lower high tide than normal.

Near Side — The side of the Moon that is visible from the Earth.

New Moon — If the Moon is on the same side of the Earth as the Sun, then the face of the Moon that can be seen from the Earth is no longer illuminated by the Sun's rays — as only the opposing side is facing the Sun. As a result, it is invisible in the Earth's sky.

Nodes — Also called Lunar Nodes, these are the points where the orbit of the moon intersects the path of the Sun in terms of how it appears to move against the stars. This solar path is called the ecliptic.

Nodical Month — A period of 27.212220 days representing the movement of the Moon starting and finishing at a specific Lunar Node.


O

Occultation — The act of one celestial body obscuring another as a result of moving between the observer and the object being observed. The most well known occultations are the lunar and solar eclipses.

Old Crescent Moon — Phase of the Moon that occurs between the Last Quarter and the New Moon. In this phase, the Moon is visible only as a very thin crescent.

Opposition — The term used to describe two celestial bodies as being opposite each other in the sky, relative to the observer. An example is when the Moon is opposite the Sun, with the observer on the Earth in the middle.

Orbital Eccentricity — Objects orbit in an elliptical fashion; eccentricity can be thought of as the amount by which the orbit of a celestial body deviates from a circular shape.


P

Palus — A Latin term meaning 'swamp' that is used to describe topographical features on the moon which resemble dark plains or swamps.

Parallax — The apparent motion of an observed object against the background caused by the movement of the observer. An example of this is the motion of the stars as seen from observatories on Earth.

Partial Eclipse — When a celestial body gets between another object and a light source, it casts 2 shadows. The umbra blocks all of the light from the light source (usually a sun), and the penumbra blocks only a portion of that light. A partial eclipse occurs when an object finds itself in the penumbra.

Penumbra — The name given to the shadow cast by a celestial object that only blocks a portion of the light.

Perigee — The closest point to the Earth in the Moon's orbit.

Perigean Tide — The tide that occurs when the moon is at its closest point to the earth during its orbit. These tides are higher than normal.

Perihelion — The closest point to the Sun in a planetary body's orbit.

Perilune — The closest point to the Moon in an object's orbit around it.

Phases — When applied to the Moon, the phases refer to the different illumination that it undergoes during it's orbit around the Earth and the Sun. The most commonly known phases of the Moon include Full Moon and the Quarter Moon.


Q

Quadrature — Term that describes two celestial bodies appearing to be ninety degrees apart, from the perspective of the observer. An example is when the Moon appears to be at a right angle to the Sun, as viewed from Earth.


R

Radius — Half the diameter of any sphere or circle.

Regression of nodes — The westward movement of the nodal points of the Moon's orbit where it intercepts the orbit of the sun, also known as the elliptic.

Revolution — A way to describe the movement of one celestial body as it orbits another. A complete revolution is a complete orbit.

Rille — Grooves in the Moon's surface which resemble canals or canyons.

Rotation — The motion of a sphere which is spinning around its own axis. An example would be a basketball spinning on the tip of a finger.


S

Saber's Beads — Detached points of light seen along the limb of very young and old lunar crescents. The striking resemblance to 2nd and 3rd contacts during a total solar eclipse was first noted by American astronomer Stephen Saber.

Saros Cycle — This cycle predicts the occurrence of eclipses, as after a period of 18 years and 11.3 days the Earth, Moon and Sun all line up in the formation necessary to generate this celestial event.

Satellite — Any object that orbits another celestial body.

Selenography — The study of the topography and features of the Moon's surface.

Sidereal Month — Period of 27.32166 days which represents the Moon's movement through space relative to a start point amongst the stars.

Sinus — Latin term that describes topographical features of the moon that resemble bays.

Solar Eclipse — Term that describes when the Sun is obscured by the Moon from the perspective of Earth.

Spring Tide — When the conjunction or opposition of the Sun, the Earth, and the Moon occurs, approximately at the times of the Full Moon and the New Moon, the Sun acts to augment the tidal forces of the Moon, creating a higher than normal tide.


T

Tektites — Objects made from natural glass that are created from the impact of meteorites. They can be found on the Earth and the Moon.

Terminator — The line which delimits night (shadowed portion) and day (sunlit portion) on a celestial body. The Moon's phases illustrate this.

Tides — The rising and falling levels of the ocean in comparison with the bodies of land on Earth. Tides are the result of the gravitational forces of the Sun and the Moon acting upon the planet.

Transit — The movement of a celestial body across another, from the viewpoint of an observer.

Tropical Month — The period of time, (27.321582 days ), that it takes for the Moon to return to a specific celestial longitude (its position in reference to the stars).


U

Umbra — The name given to the shadow cast by a celestial object that entirely blocks out illumination.


W

Waning Moon — The term used to describe the period of the Moon as it moves from a Full Moon to a New Moon, decreasing in visibility with respect to an Earth-bound observer.

Waxing Moon — The term used to describe the period of the Moon as it moves from a New Moon to a Full Moon, increasing in visibility with respect to an Earth-bound observer.


X, Y, Z

Young Crescent Moon — Name for the initially visible crescent of the Moon immediately after the New Moon.

Zenith — The point with a ninety degree inclination in relation to the observer, or the point directly above them.