Moon Definitions and Termonology

What do all the moon terms mean?

Here's an alphabetical glossary of moon terminologies

A - B - C - D - E - F - G - H - I - J - K - L
M - N - O - P - Q - R - S - T - U - VW - XYZ






A


  • Albedo - An albedo is determined by measuring the the light shining on an object and
    the ratio of light that is reflected back.  If something reflects 100%
    of the light it has an albedo of 1. The moon's albedo is 0.12.
  • Angular Diameter - The diameter of an object in the distance as measured by the
    angle formed from a point representing an observer and the
    outer edges of the object.
  • Annular Eclipse - An eclipse of the Sun when the Moon is farthest
    away in its orbit around Earth. At this point, its apparent
    diameter is not large enough to completely obscure the sun.
    During an annular eclipse, a ring of light is left uncovered
    around the dark circle produced by the Moon.
  • Anomalistic Month- The period of time it takes the Moon to go from one point
    of apogee (or perigee) to the next: 27.55455 days.
  • Anorthositic Rock- One of the types of rocks found on the Moon at higher elevations.
  • Aphelion- The point in a planet's orbit around the Sun when it is farthest
    from the Sun (opposite of perihelion).
  • Apogee- The point in the Moon's orbit when it is farthest from the
    Earth (opposite of perigee). 
  • Apogean Tide The low tide of the month that occurs when the Moon is at
    tide apogee (farthest from Earth).
  • Apolune- The point in the orbit of an object around the Moon (such
    as a spacecraft) when it is farthest from the Moons surface.
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B


  • Basalt - A type of igneous rock crated from lava and found on the luna surface in low areas, also found on earth.
  • Breccia - A composite rock found on the moon and formed from small pieces of different materials, also found on earth.
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C


  • Celestial equator - An imaginary extension of the Earth's equator into the sky.
    The celestial equator is 90 degrees from each of Earth's celestial poles.
  • Celestial Mechanics - The branch of astronomy dealing with the motions and
    gravitational effects of celestial objects.
  • Colongitude - The longitude on the surface of the Moon marked by the terminator
    the edge of the area illuminated by the Sun.
  • Conjunction - The position of two celestial bodies when they are in line
    with one another as seen by an observer on Earth. The new
    moon is also referred to as moon in conjunction with the
    Sun (opposite of opposition).
  • Crater Wall - The circular wall formed by the impact of a meteorite on the
    lunar surface.
  • Crescent Moon - A phase of the Moon just before and after the new
    moon, when only a thin curved section is lighted by the Sun.
    The last crescent moon before the new moon is sometimes
    called the old crescent moon, and the first crescent moon
    is sometimes called a young crescent moon.
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D


  • Dark of the Moon - Another name given to the new moon.
  • Declination - The angle measured between the celestial equator and an object in the sky.
  • Diurnal - A reference to a period of one day.
  • DST - Daylight Savings Time.
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E


  • Earthshine - Reflected light from the earth. Earthshine can illuminate a young crescent
    moon so that the whole face can be faintly seen.
  • Eclipse - The blocking of light from the sun when the earth comes between the sun and
    moon or the moon between the sun and earth.
  • Ecliptic - The imaginary plane formed by the earth's orbit around the sun or the plane formed
    by the apparent motion of the sun through the sky.
  • Elongation - The angle of a planet away from the Sun or Moon from the Earth as viewed from Earth.
  • Ephemeris - A publication with information on how to locate the moon in the sky at a
    particular time.
  • Equatorial Tide - A tide produced semi-monthly by the positions of the Moon over the equator.
  • Escape Velocity - The speed needed to "break free" from
    the gravitational field and get into space
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F


  • Far Side - The side of the Moon facing away from the Earth.
  • First Quarter Moon- The phase of the Moon when it is 90 degrees away from a line between the Sun and the Earth, measured east-
    ward from the Sun, as seen from the north. The angle of illumination creates a half circle picture of the Moons surface,
    with the lighted half being on the right side.
  • Full Moon - The phase of the Moon when it is on the opposite side of the
    Earth from the Sun and receives sunlight across its entire
    face, forming a circle of light. At this point, the Moon is in
    opposition to the Sun.
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G


  • Gibbous Moon - The phase of the Moon when it is getting larger after
    the first quarter moon phase (waxing gibbous) or smaller
    after the full moon but before the last quarter moon (waning
    gibbous).
  • GMT - Greenwich Mean Time (see below)
  • Gravity - One of the fundamental forces of nature, defined as the constant force of attraction between all objects in the universe.
    The gravitational force is inversely proportional to the
    square of the distance between the objects and proportional
    to the masses.
  • Grazing Occultation - An occultation by the Moon of a planet or star where
    occultation the path of the planet or star only intercepts the north or
    south limb of the Moon.
  • Greenwich Mean Time - Time as measured from the 0 degrees longitude position
    of the Greenwich Observatory in England, also known
    as Universal Time (UT) or .
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H


  • Half Moon - The phase of the Moon also known as the quarter moon, first
    quarter moon, or last quarter moon.
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I


  • Inclination - The angle between a reference plane and another plane
    or axis of direction.
  • Intercalation - A method of synchronizing a lunar calendar with a solar year
    by adding extra days or months. Extra days are known as
    intercalary days and extra months are known as intercalary.
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L


  • Lacus - Latin for lake. An area on the surface of the Moon resembling a lake.
  • Last Quarter - The phase of the Moon when it is 90 degrees away moon from a line between
    the Sun and the Earth, measured westward from the Sun, as seen from the north.
    The angle of illumination creates a half circle of the Moons surface, with the
    lighted half being on the left side. Also referred to as the third quarter moon.
  • Latitude - Lines of measurement around a planet or the Moon, parallel to its equator.
    Measured in degrees, with the equator being 0 degrees and the poles 90 degrees north or south.
  • Librations - The irregular motions of the Moon in its elliptical orbit around Earth that allow slightly more
    than half of the Moons surface to be visible over a period of time.
  • Limb - The visible edge of a planet or moon The visible edge of a planet or moon.
  • Longitude - Lines of measurement at right angles to the equator of a planet or the Moon.
    Measured in degrees of angle from a designated line of 0 degrees. On the Moon,
    0 degrees longitude is at the center of the visible face, in the Sinus Medii.
  • Lunar Day - The period of time between two successive transits of the Moon over the same meridian.
    The mean lunar day is 24-84 hours (1.035 times the mean solar day). Not the same thing
    as a day on the Moon, which corresponds to a synodic month.
  • Lunar Eclipse - An eclipse created by the Earth coming between the Sun and the Moon.
    Lunar eclipses always happen during the full moon phase.
  • Lunar Interval - The elapsed time between the transit of the Moon over the Greenwich meridian and a local meridian.
  • Lunar Month - the time it takes for the Moon to complete one
    cycle of phases.: a period of 29.531 days.
  • Lunar Rays - Visible streaks on the surface of the Moon which radiate away from some craters.
  • Lunartidal Interval - The length of time between the transit of the Moon and the following high or low tide.
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M


  • Magnitude - A numerical value indicating the brightness of an object in space.
  • Mare - Latin for sea (plural: maria). An area on the surface of the Moon
    (or Mars) that is low, dark, and formed from ancient lava flows.
  • Mascon - An area of the Moons surface formed from dense, thick lunar material
    and having strong local gravitational effects.
  • Mass - The mass of a body is a measure of how much matter it
    contains. It is different to weight as weight depends on what gravity the
    mass is being measured in. The moon's mass is 8 x 1019 tons, or
    7.347 x 1022 kg.
  • Mass Ratio - How much of one mass will fit in another. The
    Earth and Moon have a mass ratio of 0.01230 (or 1/81.301)
  • Mean - A mathematical average of a set of numbers or measurements,
    with the mean equalling the sum of the numbers divided by the
    number of units. The mean distance of the Moon from earth, for example, is
    the average of all the different distances.
  • Meridian - An imaginary line that passes directly north and south
    through an observer or specified location on Earth. A plane
    extended from this line into space passes through the zenith
    (point above the observer).
  • Meteoroid - A small body drifting through space. If a meteoroid is pulled
    into a planet's or moons gravitational field, it is called a
    meteor. if the meteor survives a trip through the atmosphere
    and lands, it is called a meteorite. If meteorites are not burned
    up in the process of entering an atmosphere (or if there is no atmosphere),
    they may strike the surface and if large enough create a craters.
  • Moon - The natural satellite of the Earth or the natural satellite of any planet.
  • Moon Rise - The point in time when the upper limb of the Moon is even with the Earth's
    horizon as the Moon sets in the west.
  • Moon Set - The point in time when the upper limb of the Moon is even with the Earth's
    horizon as the Moon rises in the east.
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N


  • Nadir - An imaginary point directly under an observer on the surface
    of the Earth, extending through the Earth and into the sky.
  • Neap Tide - The lowest high tide of the lunar month, occurring near the first and last
    quarter moon phases.
  • Near Side - The side of the moon facing earth.
  • Nodes - The imaginary points at which the orbital path of the Moon
    or other celestial body crosses the ecliptic.
  • Nodical Month - A lunar cycle measured by the Moon moving from one node and back again.
    A period of 27.21222 days.
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O


  • Occulation - The movement of one celestial object behind another. Such as a star
    behind the moon.
  • Old Crescent Moon - Another name for the thin crescent of the Moon that
    is still illuminated by the Sun before the Moon goes
    completly dark at the new moon phase.
  • Opposition - A specific point in time when a moon or planet is 180
    degrees away from the Sun, on the other side of the Earth.
    The Moon is full when it is in opposition (opposite of con,
    junction).
  • Orbital Eccentricity - The degree to which an elliptical orbit is elongated.
    Measured by the distance between the foci divided by the
    major axis.
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P


  • Palus - Latin for swamp. An area on the surface of the Moon that is
    dark and resembles a swamp.
  • Parallax - The perceived movement of a distant object such as a moon,
    planet, or star due to the movement of the Earth.
  • Partial Eclipse - A lunar eclipse in which the Moon only partly enters
    the dark, umbral shadow of the Earth but is inside the secondary,
    penumbral shadow. Also refers to a solar eclipse
    when the Moon does not line up completely between the
    Earth and Sun and only partly obscures the Sun. This type
    of eclipse also produces a penumbra as well as an umbra.
  • Penumbra - The lighter part of a shadow that is formed by diffused light
    in an area around the edges of an object.
  • Perigee - The point in the Moons orbit when it is closest to the Earth
    (opposite of apogee).
  • Perigean Tide - The high tide of the month that occurs when the Moon is at
    tide perigee (closest to to Earth).
  • Perihelion - The point in a planet's orbit around the Sun when it is closest
    to the Sun (opposite of aphelion).
  • Perilune - The point in the orbit of an object (such as a spacecraft)
    around the Moon when it is closest to the Moons surface.
  • Phases - The visible changes that the Moon goes through in every
    lunar month, caused by the changing angle of illumination
    from the Sun. There are four specific phases - new moon,
    first quarter moon, full moon, and last quarter moon - and
    also non-specific phase names such as waxing moon, waning
    moon, gibbous moon, and crescent moon.
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Q


  • Quadrature - The position of the Moon or a planet when it is at right angles to the Sun.
    The Moon is in first quarter phase when it is in east quadrature to the Sun
    and last quarter phase when it is in west quadrature.
  • Quarter Moon - The phase of the Moon that can be either the first quarter
    moon or the last quarter moon. This phase occurs when the
    Moon is 90 degrees away from a line between the Sun and
    the Earth. In the northern hemisphere, the angle of illumination
    creates a half circle picture of the Moon's surface,
    with the lighted half being on the right side during first quarter
    moon and on the left side for last quarter moon.
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R


  • Radius - The linear measurement from the center of a sphere to the
    surface, or half of the diameter.
  • Regression of nodes - The backwards movement of the Moons nodes relative to the
    direction of orbit.
  • Revolution - The movement of one body around another in an orbit. Not
    to be confused with rotation.
  • Rille - A valley or small canyon on the surface of the Moon.
  • Rotation - The spinning of a body around its own axis. Not to be confused
    with revolution
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S



  • Saros Cycle - A cycle of lunar months lasting 18 years and 11.3 days, the
    time it takes the Moon, the Earth, and the Sun to return to
    the same position relative to each other.
  • Satellite - An object that is in orbit around another object in space.
  • Selenography - The science dealing with the study of the surface of the
    Moon.
  • Selenology - The science dealing with the study of the Moon. From the
    Greek goddess, Selene.
  • Sidereal Month - A lunar month measured by a return to a specific position
    marked by a certain star: a period of 27.32166 days.
  • Sinus - Latin for bay. An area on the surface of the Moon resembling
    the bay of an ocean.
  • Solar Eclipse - An eclipse caused when the Moon comes directly between
    the Earth and the Sun, temporarily blocking out Su's disk
    in the sky.
  • Spring Tide - The highest tides in a lunar month, occurring near new and
    full moons, when the Earth, Sun, and Moon are aligned.
  • Synodic month - A lunar month - the time it takes for the Moon to
    complete one cycle of phases.: a period of 29.531 days.
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T


  • Tektites - Small particles on the Moo's surface made of grasslike material and formed from the impact of meteorites.
  • Terminator - The line formed by the edge of the illuminated portion of the
    Moon.
  • Tides - The cyclical movement of bodies of water or land on the
    Earth or the Moon caused by the gravitational pull of the
    Earth, Moon, and Sun.
  • Transit - The point when the path of the Moon, the Sun, a star, or a planet takes it across the meridian.
  • Tropical Month - The time required for the Moon to move from the first point of Aries and back: a period of 27.321582 days.
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U


  • Umbra - The darker core of a shadow, usually cone shaped, and surrounded
    by a lighter penumbral shadow. Also refers to the
    darker center of sunspots.
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V W
  • Waning Moon - The period in the Moo's monthly cycle after the full
    moon and before the new moon. During this period, the
    lighted portion of the Moons surface is decreasing.
  • Waxing Moon - The period in the Moo's monthly cycle after the new
    moon and before the full moon. During this period, the lighted
    portion of the Moo's surface is increasing.
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X Y Z

  • Young Crescent Moon - Another name for the thin crescent of the Moon that is illuminated by the Sun just after the new moon.
  • Zenith - The imaginary point directly above an observer on Earth (opposite of nadir).
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