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accelerate
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to change velocity; to speed up, slow down, or change direction.
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apparent magnitude
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a measure of how bright a star looks in the sky; the larger the number, the dimmer the star appears to us
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astrology
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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
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celestial equator
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a great circle on the celestial sphere 90° from the celestial poles; where the celestial sphere intersects the plane of Earth’s equator
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celestial poles
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points about which the celestial sphere appears to rotate; intersections of the celestial sphere with Earth’s polar axis
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celestial sphere
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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
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circumpolar zone
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those portions of the celestial sphere near the celestial poles that are either always above or always below the horizon
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cosmology
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the study of the organization and evolution of the universe
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ecliptic
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the apparent annual path of the Sun on the celestial sphere
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epicycle
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the circular orbit of a body in the Ptolemaic system, the center of which revolves about another circle (the deferent)
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geocentric
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centered on Earth
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heliocentric
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centered on the Sun
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horizon (astronomical)
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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
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horoscope
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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
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parallax
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the apparent displacement of a nearby star that results from the motion of Earth around the Sun
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planet
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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
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precession (of Earth)
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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
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retrograde motion
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the apparent westward motion of a planet on the celestial sphere or with respect to the stars
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year
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the period of revolution of Earth around the Sun
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zenith
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the point on the celestial sphere opposite the direction of gravity; point directly above the observer
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zodiac
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a belt around the sky about 18° wide centered on the ecliptic