Moon Glossary: Lunar Terms and Definitions

Utah Moon

Taken with a Canon T1i 500D and a 8” f3.9 Newtonian at Prime Focus.

Lou Eastman

Our Moon glossary includes definitions of lunar terms that are commonly used on Almanac.com. You can find our full astronomical glossary in The Old Farmer’s Almanac.

Apogee (Apo.)

The point in the Moon’s orbit that is farthest from Earth.

Conjunction

The time at which two or more celestial bodies appear closest in the sky. Inferior (Inf.): Mercury or Venus is between the Sun and Earth. Supe-rior (Sup.): The Sun is between a planet and Earth. The best times for viewing the closely aligned bodies are given in Sky Watch.

Eclipse, Lunar

The full Moon enters the shadow of Earth, which cuts off all or part of the sunlight reflected off the Moon. 
See our Eclipses page for this year’s dates.

  • Total: The Moon passes completely through the umbra (central dark part) of Earth’s shadow.
  • Partial: Only part of the Moon passes through the umbra.
  • Penumbral: The Moon passes through only the penumbra (area of partial darkness surrounding the umbra).

Eclipse, Solar

Earth enters the shadow of the new Moon, which cuts off all or part of the Sun’s light. 

  • Total: Earth passes through the umbra (central dark part) of the Moon’s shadow, resulting in totality for observers within a narrow band on Earth.
  • Annular: The Moon appears silhouetted against the Sun, with a ring of sunlight showing around it.
  • Partial: The Moon blocks only part of the Sun.

Epact

A number from 1 to 30 that indicates the Moon’s age on January 1 at Greenwich, England; used in calculations for determining the date of Easter.

Golden Number

A number in the 19-year cycle of the Moon, used in calculations for determining the date of Easter. (Approximately every 19 years, the Moon’s phases occur on the same dates.) Add 1 to any given year and divide by 19; the remainder is the Golden Number. If there is no remainder, use 19.

Illumination

The Moon appears to be a circular disk which, at any specific time, is illuminated to some degree by the Sun’s light. When we refer to the percent illumination, we are referring to the percent of that circular disk which is lit up. 

So, when it’ s a New Moon, the percent illuminated is 0; at First and Last Quarters it is 50%; and at Full Moon it is 100%. During the crescent phases the percent illuminated is between 0 and 50% and during gibbous phases it is between 50% and 100%.

See more about Moon Phases below.

Meridian (Passing the Meridian)

A meridian is a location’s longitude (a line that runs from the North Pole to the South Pole of Earth). When we use the word meridian, it’s usually referring to the moment that the Moon crosses or passes a location’s meridian; this marks the exact moment when the Moon reaches the highest position in the sky.

Moon on Equator

The Moon is on the celestial equator.

Moon Rides High/Runs Low

The Moon is highest above or farthest below the celestial equator.

Moonrise and Moonset

When the Moon rises above or sets below the horizon. Specifically, it’s the moment that the upper edge of the Moon’s disk touches the horizon. Find your local Moon rise and set times.

Moon’s Phases

The changing appearance of the Moon, caused by the different angles at which it is illuminated by the Sun. See our Moon Phase Calendar for local dates and times.

  • First Quarter: Right half of the Moon is illuminated.
  • Full: The Sun and the Moon are in op- position; the entire disk of the Moon is illuminated.
  • Last Quarter: Left half of the Moon is illuminated.
  • New: The Sun and the Moon are in conjunction; the Moon is darkened because it lines up between Earth and the Sun

Moon’s Place, Astronomical

The position of the Moon within the constellations on the celestial sphere.

Moon’s Place, Astrological

The position of the Moon within the tropical zodiac, whose twelve 30° segments (signs) along the ecliptic were named more than 2,000 years ago after constellations within each area. Because of precession and other factors, the zodiac signs no longer match actual constellation positions.
You can purchase the Astrology Moon’s Place Calendar here.

Opposition

The Moon or a planet appears on the opposite side of the sky from the Sun (elongation 180°).

Perigee (Perig.)

The point in the Moon’s orbit that is closest to Earth.

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