Moon-Related Words & Phrases: A Glossary

Apogee (ap-uh-jee). This is the most distant point (ap-) on an elliptical orbit around Earth (-gee). The equivalent point for Earth’s orbit around the Sun is called aphelion (ap-he-lee-un or af-he-lee-un).

Blood Moon. This is a recently coined term for a full moon during a total lunar eclipse. When the Moon is inside Earth’s shadow, with the Sun completely blocked as seen everywhere on the lunar surface, the Sun’s rays bend as they go through Earth’s atmosphere. The atmosphere acts like a lens and bends the rays far enough that Earth’s sunrises and sunsets all illuminate the Moon. Each lunar eclipse is different. The color ranges from bright orange to brick to the complete disappearance of the Moon in the sky (temporarily).

Crescent. This is the phase of the Moon when the Earth-facing hemisphere is illuminated over less than 50 percent of its area. We view a crescent between new moon and first quarter (half moon) in the evening sky and between third quarter (also a half moon, facing the other direction) and new moon in the morning sky.

Dark Moon. This is a recently coined term for the new moon each month.

Dark side of the Moon. Dark Side of the Moon is not just an album by the English rock band Pink Floyd—it’s also an incorrect description used by some when referring to the hemisphere of the Moon invisible from Earth. “Far side of the Moon” or “lunar far side” are correct phrases to use. “Dark side of the Moon” could refer to the Moon’s night side, but this is not common usage.

Earthshine. Also called earthlight. Most easily observed when the Moon is a relatively narrow crescent, sunlight reflecting from Earth’s day side is the earthshine illuminating the Moon’s night side. Scientists have used the brightness of earthshine to gauge the cloudiness of Earth’s dayside visible from the Moon.

Full Moon. A full moon occurs in a syzygy wherein the order of alignment in space is Sun-Earth-Moon. The Moon is exactly 180 degrees opposite the Sun, as measured in celestial longitude, though its angular separation from the Sun may be less than 180 degrees because of its tilted orbit. The Sun is almost fully illuminating the hemisphere facing Earth; a telescope can show the slight “phase defect” around the “limb” (disk edge) of a full moon. The phase defect is best seen when the Moon’s orbit carries if off the ecliptic circle (the Sun’s apparent “orbit” in the sky, over the course of a year) and we see a little around to the night side, near either the Moon’s north pole or its south pole.

We never get to see a brightly illuminated, perfectly full moon. That’s because Earth blocks the sunlight falling where the Moon would have to be to be seen perfectly full. Earth’s shadow is eclipsing the Moon when it is in that position. A full moon rises around local sunset, is highest around midnight, and sets around local sunrise.

(Note: Celestial longitude is measured along the ecliptic, a circle in the sky defined by the apparent position of the Sun’s center as it changes through the year due to Earth’s orbital motion. The angle measurement is not made using Right Ascension, which is defined by projecting Earth’s meridian lines into the sky.)

Gibbous. This is the phase of the Moon seen when the Earth-facing hemisphere is illuminated over more than 50 percent of its area.

Micromoon. This is a recently coined term to describe the full moon near apogee. Apogean full moon is more descriptive of “micromoon” events but is, perhaps, more limited in time by its specification of apogee.

Near side of the Moon. This phrase describes the hemisphere of the Moon facing Earth.

New Moon. A new moon occurs in a syzygy wherein the order of alignment in space is Sun-Moon-Earth. The Moon is exactly aligned with the Sun, as measured in celestial longitude, though it may actually pass a few degrees above or below the Sun because of its tilted orbit. The Moon is completely shadowing its own Earth-facing hemisphere, while its farside is (almost) fully illuminated. A thin crescent is not visible at new moon because the roughness of the Moon’s surface prevents us from seeing any illuminated surfaces. The silhouette of the new moon can be seen during a solar eclipse but the eclipse may actually not be occurring at exactly when the Sun and Moon have the same celestial longitude. Much more often, the Moon’s orbit carries if off of the ecliptic circle so there is no eclipse and we simply cannot see the Moon for roughly 24 hours (and usually longer) centered on the moment of new moon.

(Note: Celestial longitude is measured along the ecliptic, a circle in the sky defined by the apparent position of the Sun’s center as it changes through the year due to Earth’s orbital motion. The angle measurement is not made using Right Ascension, which is defined by projecting Earth’s meridian lines into the sky.)

Perigee (pear-ih-jee). This is the closest point (peri-) on an elliptical orbit around Earth (-gee). The equivalent for Earth’s orbit around the Sun is perihelion (pear-ih-hee-lee-un).

Phase. “Phase” is commonly used as a stand-in generic reference for the apparent shape and position of the Moon or its age (= number of days since the most recent new moon). Astronomers often use it as “phase angle,” a numerical value that indicates the angular separation in the sky of the Moon (or another object) from the Sun as seen by the observer. New moon has a phase angle of about 0°, first quarter has a phase angle of about 90°, full moon has a phase angle of about 180°, and last quarter has a phase angle of about 270°.

Quarter Moon. Usually used with the modifier “first” or one of either “third” or “last.”

A first quarter moon has traveled one-quarter of the way around its orbit circling Earth, starting from the new moon position (explained above). From Earth’s surface we see 50% (half) of the Moon illuminated by the Sun, half in darkness, in the shade of its own sunward hemisphere. A first quarter moon rises around local noon, is highest at sunset, and sets around local midnight.

“Second quarter” is almost never used; it would refer to the full moon, explained above.

“Third quarter” and “last quarter” mean the same thing and are used interchangeably. The Moon has traveled three-quarters of the way around its orbit. We see the other half of the Moon illuminated by the Sun, half in the darkness of self-shade. A third quarter moon rises around local midnight, is highest at sunrise, and sets around local noon.

Supermoon. This term was coined by an astrologer in 1979 to describe periods when the new moon or the full moon occur near perigee. The definition reads “…a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee). In short, Earth, Moon and sun are all in a line, with Moon in its nearest approach to Earth.” Not surprisingly, only full moons near perigee receive attention since new moons in this configuration are invisible (except during a solar eclipse). Perigean syzygies (singular: syzygy, an alignment more completely defined below) including perigean new moon and perigean full moon, are more descriptive of “supermoon” events but are, perhaps, more limited in time by their specification of perigee. The media use “supermoon” only for perigean full moon.

Syzygy (sih-zih-jee). Alignments of the Sun, Earth, and Moon (or Sun, Moon, and Earth). Ocean tides are near maximum during syzygean perigees (new or full moon). The highest tides occur during syzygean perigees when the Earth is at perihelion.

Waning. “Waning” describes the decreasing amount of illumination of the Moon’s face after full moon as it moves to new moon.

Waxing. “Waxing” describes the increasing amount of illumination of the Moon’s face before full moon, starting from new moon until full moon.