Have you ever gazed at the Moon and wondered about its position in the sky or the terms used to describe the Moon’s movements? Look no further than our Moon glossary! This guide is your one-stop shop for deciphering lunar lingo, perfect for astronomers or casual skywatchers. Let’s demystify the Moon.
Moon Glossary of Lunar Terms
Our Moon glossary includes definitions of lunar terms commonly used on Almanac.com. You can find our full astronomical glossary in The Old Farmer’s Almanac.
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.
- Superior (Sup.): The Sun is between a planet and Earth. The best times for viewing the closely aligned bodies are given on 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.
Learn about solar eclipses throughout history.
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, is used in calculations to determine 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 that is lit up.
So, when it’s a New Moon, the percent illuminated is 0; at the First and Last Quarters, it is 50%, and at the Full Moon, it is 100%. During the crescent phases, the percent illuminated is between 0 and 50%, and during the 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 usually refers 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 the 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: The right half of the Moon is illuminated. (Why is it called a Quarter Moon and not a Half Moon?)
- Full: The Sun and the Moon are in opposition; the entire disk of the Moon is illuminated. When is the next Full Moon?
- Last Quarter: The 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.
- Crescent: When the Moon appears early in its first quarter or late in its last quarter, only a small arc-shaped section is visible and illuminated by the Sun. This is the captivating crescent Moon!
- Gibbous: The gibbous Moon phase is fatter than half but smaller than full. As for its name, “gibbous” is an old-fashioned word that means humped or swollen like the back of a camel, but it’s really not used anymore for anything but the Moon’s appearance. Learn more about the gibbous Moon.
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. See what sign the Moon is located in today.
Opposition
The Moon or a planet appears on the opposite side of the sky from the Sun (elongation 180°).
The point in the Moon’s orbit that is closest to Earth.
What astrology terms do you want to learn? Leave them in the comments, and we may just add the definition to this page!
