Moon terminology: 72 lunar terms explained.

The angle of an object above the observer's horizon, measured from 0 degrees at the horizon to 90 degrees at the zenith.

The point in the Moon's orbit where it is farthest from Earth.

A compass direction measured clockwise around the horizon, usually from 0 degrees at north through 90 degrees at east.

A popular name for the reddish Moon seen during a total lunar eclipse, when sunlight filtered through Earth's atmosphere reaches the lunar surface.

A calendar nickname for an extra full Moon. It commonly means the second full Moon in one calendar month, though a seasonal definition also exists.

A phase in which less than half of the Moon's visible disk is illuminated. A crescent may be waxing or waning.

The moment a celestial object crosses the observer's meridian and usually reaches its highest altitude for that passage.

An informal and misleading phrase often used for the lunar far side. Both the near side and far side receive sunlight over a lunar month.

A celestial coordinate comparable to latitude on Earth. It measures angular distance north or south of the celestial equator.

The faint glow on the Moon's unlit portion caused by sunlight reflecting from Earth to the Moon and back toward the observer.

An event in which one celestial body moves into another body's shadow or blocks the observer's view of another body.

A measure of how much of an eclipsed body's diameter is covered by a shadow at a specified time, commonly at greatest eclipse.

A period of several weeks when the Sun is close enough to a lunar node for solar or lunar eclipses to occur.

The Sun's apparent yearly path across Earth's sky. The Moon and planets remain near this path because their orbits have relatively small tilts.

Either of two yearly moments when the Sun crosses Earth's celestial equator and day and night are roughly equal in length.

The lunar hemisphere that faces away from Earth most of the time. It is not permanently dark.

The phase near one quarter of the way through the lunar cycle, when half of the Moon's visible disk appears illuminated and the lit portion is growing.

The phase when the Moon is opposite the Sun in Earth's sky and its Earth-facing hemisphere appears almost completely illuminated.

A phase in which more than half, but not all, of the Moon's visible disk is illuminated. A gibbous Moon may be waxing or waning.

The instant when the alignment of an eclipse is at its maximum, such as when the Moon passes closest to the axis of Earth's shadow during a lunar eclipse.

The traditional name for the full Moon nearest the Northern Hemisphere's September equinox.

The apparent boundary where sky meets land or sea. Local terrain and buildings can create a practical horizon above the astronomical horizon.

The traditional name for the full Moon that follows the Harvest Moon.

The fraction of the Moon's visible disk that appears lit by direct sunlight, commonly expressed as a percentage.

A very large impact depression. Many lunar basins were later flooded by basaltic lava and became maria.

A depression formed when an asteroid, meteoroid, or comet strikes the Moon's surface.

The Moon's apparent nodding and rolling motion, which lets observers on Earth see slightly more than half of its surface over time.

The apparent outer edge of the Moon's disk as seen by an observer.

The interval from one local lunar noon to the next, lasting about 29.5 Earth days because the Moon rotates while orbiting Earth.

An eclipse that occurs when the full Moon passes through some part of Earth's shadow.

The Moon's bright, elevated, heavily cratered terrain. The highlands are generally older than the darker maria.

A general term for a cycle based on the Moon. Its exact length depends on the reference point, such as phase, stars, or orbital apsides.

Either point where the Moon's tilted orbit crosses the ecliptic. Eclipses require the Moon to be near a node.

The changing apparent shape of the Moon's sunlit portion as the Moon moves around Earth.

Also called: Moon phase

A dark plain of ancient basaltic lava on the Moon. Mare is singular; maria is plural.

Also called: Lunar sea, Maria

An imaginary great circle passing through the north point, zenith, and south point of an observer's sky.

Also called: Meridian passing, Upper transit, Culmination

An informal name for a full or new Moon occurring near apogee, when the Moon is relatively far from Earth and appears slightly smaller.

The perception that the Moon looks larger near the horizon than when it is high in the sky, even though its angular size changes very little.

The moment the Moon's upper edge appears above the observer's horizon, with standard calculations accounting for atmospheric refraction.

The moment the Moon's upper edge disappears below the observer's horizon, with standard calculations accounting for atmospheric refraction.

The lunar hemisphere that faces Earth most of the time because the Moon rotates synchronously with its orbit.

The phase when the Moon is near the Sun in Earth's sky and its illuminated hemisphere faces mostly away from Earth.

An event in which a nearer celestial body passes in front of and hides a more distant object.

An alignment in which an object appears opposite the Sun in Earth's sky. A full Moon occurs near lunar opposition.

The tilt of an orbit relative to a reference plane. The Moon's orbit is tilted to the ecliptic, which is why eclipses do not happen every month.

A lunar eclipse in which part, but not all, of the Moon enters Earth's umbra.

The lighter outer part of a shadow where the light source is only partially blocked.

A lunar eclipse in which the Moon passes through Earth's penumbra but does not enter the umbra.

The point in the Moon's orbit where it is closest to Earth.

The angle between the Sun and observer as seen from the Moon. It determines how much of the visible lunar disk is illuminated.

Bright streaks of material thrown outward from a relatively young impact crater.

The bending of light through Earth's atmosphere, which makes objects near the horizon appear slightly higher than their geometric positions.

The loose layer of dust, broken rock, glass, and debris covering much of the Moon's solid bedrock.

Also called: Lunar soil

A celestial coordinate comparable to longitude on Earth, measured eastward along the celestial equator.

A long, narrow channel or crack on the lunar surface, often formed by ancient volcanism or crustal fracturing.

A period of about 18 years and 11 days after which eclipses with similar geometry recur.

The roughly 27.3-day interval the Moon takes to complete one orbit relative to the background stars.

An eclipse that occurs when the Moon passes between the Sun and Earth and casts its shadow on Earth.

A tide with a larger-than-average range that occurs near new and full Moon, when the Sun, Earth, and Moon are nearly aligned.

A traditional name commonly used in North America for the full Moon in June. The name does not describe the Moon's color.

An informal name for a full or new Moon occurring near perigee, when the Moon appears somewhat larger than average.

Rotation in which a body's spin period matches its orbital period, causing the same hemisphere to face the object it orbits.

Also called: Tidal locking

The roughly 29.5-day interval from one occurrence of a lunar phase to the next occurrence of the same phase.

The moving boundary between daylight and darkness on the Moon. Low-angle light near it makes surface relief easier to see.

The long-term gravitational process that synchronized the Moon's rotation with its orbit, leaving nearly the same side facing Earth.

A lunar eclipse in which the entire Moon passes inside Earth's umbra, often giving the Moon a copper or red appearance.

The interval during a total eclipse when the eclipsed disk is completely within the relevant shadow or completely covers the light source.

A passage across a reference line or another body's disk. In daily observing, meridian transit means crossing the local meridian.

The darkest central part of a shadow, where the light source is completely blocked.

The part of the lunar cycle when the illuminated fraction visible from Earth is decreasing.

The part of the lunar cycle when the illuminated fraction visible from Earth is increasing.

The point in the sky directly above an observer, corresponding to an altitude of 90 degrees.