Astronomical Terms

ABERRATION OF STARLIGHT

Apparent change in the position of a star


APHELION

Point at which an object in solar orbit is farthest from the Sun


APOGEE

Point at which an orbiting object is farthest from the Earth


ARMILLARY SPHERE

Model with rings, used to show relationships among the circles on the celestial sphere


ASTROLABE

Medieval instrument consisting of a graduated vertical circle with a moveable arm, used to determine the altitude of celestial bodies


AZIMUTH

Horizontal bearing of a celestial object measured clockwise from a given direction


CELESTIAL SPHERE

Imaginary sphere around the Earth on which celestial bodies are assumed to lie, for the purpose of finding or identifying their position


CONJUNCTION

Occasion when two celestial bodies line up on the celestial sphere


DECLINATION

Angular measure of a star's position, measured in degrees north and south of the celestial equator


ECLIPTIC

Great circle on the celestial sphere representing the apparent annual path of the Sun relative to the stars


EQUINOX

Instant when the Sun lies directly overhead at the Equator


NADIR

Point in the heavens diametrically opposite the zenith, or directly under an observer


ORRERY

Mechanical model of the solar system


PERIGEE

Point at which an orbiting object is closest to the Earth


PERIHELION

Point at which an object in solar orbit is closest to the Sun


RED SHIFT

Lengthening of the wavelength of light from a receding celestial body


SOLSTICE

Farthest point north or south of the Equator that the Sun reaches each year


SYZYGY

Point in a celestial body's orbit at which it is either in opposition to or in conjunction with the Sun


ZENITH

Point in the heavens directly above an observer
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