ASTRONOMICAL GLOSSARY

APHELION (APH.): The point in a planet's orbit that is farthest from the Sun.

APOGEE (APO.): The point in the Moon's orbit that is farthest from Earth.

CELESTIAL EQUATOR (EQ.): The imaginary circle around the celestial sphere that can be thought of as the plane of Earth's equator projected out onto the sphere.

CELESTIAL SPHERE: An imaginary sphere projected into space that represents the entire sky, with an observer on Earth at its center. All celestial bodies other than Earth are imagined as being on its inside surface.

CIRCUMPOLAR: Always visible above the horizon, such as a circumpolar star.

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. Actual dates for conjunctions are given on the Right-Hand Calendar Pages, 121–147; the best times for viewing the closely aligned bodies are given in Sky Watch on the LeftHand Calendar Pages, 120–146.

DECLINATION: The celestial latitude of an object in the sky, measured in degrees north or south of the celestial equator; comparable to latitude on Earth. This Al ma nac gives the Sun's declination at noon.

ECLIPSE, LUNAR: The full Moon enters the shadow of Earth, which cuts off all or part of the sunlight reflected off the Moon. 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). See page 102 for more information about eclipses.

ECLIPSE, SOLAR: Earth enters the shadow of the new Moon, which cuts offff 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.

ECLIPTIC: The apparent annual path of the Sun around the celestial sphere. The plane of the ecliptic is tipped 23½° from the celestial equator.

ELONGATION: The difference in degrees between the celestial longitudes of a planet and the Sun. Greatest Elongation (Gr. Elong.): The greatest apparent distance of a planet from the Sun, as seen from Earth.

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

EQUINOX: When the Sun crosses the celestial equator. This event occurs two times each year: Vernal is around March 20 and Autumnal is around September 22.

EVENING STAR: A planet that is above the western horizon at sunset and less than 180° east of the Sun in right ascension.

GOLDEN NUMBER: A number in the 19-year Metonic cycle of the Moon, used in determining the date of Easter. See page 149 for this year's Golden Number.

MAGNITUDE: A measure of a celestial object's brightness. Apparent magnitude measures the brightness of an object as seen from Earth. Objects with an apparent magnitude of 6 or less are observable to the naked eye. The lower the magnitude, the greater the brightness; an object with a magnitude of –1, e.g., is brighter than one with a magnitude of +1.

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