Key Terms

accelerate

to change velocity; to speed up, slow down, or change direction.

apparent magnitude

a measure of how bright a star looks in the sky; the larger the number, the dimmer the star appears to us

astrology

the pseudoscience that deals with the supposed influences on human destiny of the configurations and locations in the sky of the Sun, Moon, and planets

celestial equator

a great circle on the celestial sphere 90° from the celestial poles; where the celestial sphere intersects the plane of Earth’s equator

celestial poles

points about which the celestial sphere appears to rotate; intersections of the celestial sphere with Earth’s polar axis

celestial sphere

the apparent sphere of the sky; a sphere of large radius centered on the observer; directions of objects in the sky can be denoted by their position on the celestial sphere

circumpolar zone

those portions of the celestial sphere near the celestial poles that are either always above or always below the horizon

cosmology

the study of the organization and evolution of the universe

ecliptic

the apparent annual path of the Sun on the celestial sphere

epicycle

the circular orbit of a body in the Ptolemaic system, the center of which revolves about another circle (the deferent)

geocentric

centered on Earth

heliocentric

centered on the Sun

horizon (astronomical)

a great circle on the celestial sphere 90° from the zenith; more popularly, the circle around us where the dome of the sky meets Earth

horoscope

a chart used by astrologers that shows the positions along the zodiac and in the sky of the Sun, Moon, and planets at some given instant and as seen from a particular place on Earth—usually corresponding to the time and place of a person’s birth

parallax

the apparent displacement of a nearby star that results from the motion of Earth around the Sun

planet

today, any of the larger objects revolving about the Sun or any similar objects that orbit other stars; in ancient times, any object that moved regularly among the fixed stars

precession (of Earth)

the slow, conical motion of Earth’s axis of rotation caused principally by the gravitational pull of the Moon and Sun on Earth’s equatorial bulge

retrograde motion

the apparent westward motion of a planet on the celestial sphere or with respect to the stars

year

the period of revolution of Earth around the Sun

zenith

the point on the celestial sphere opposite the direction of gravity; point directly above the observer

zodiac

a belt around the sky about 18° wide centered on the ecliptic