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astronomy
the scientific study of the universe; It includes the observation and interpretation of celestial bodies and phenomena.
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geocentric
describes the concept of an Earth-centered universe
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heliocentric
describes the view that the sun is at the center of the solar system
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retrograde motion
the apparent westward motion of the planets with respect to the stars
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astronomical unit (AU)
average distance from Earth to the sun; 1.5 × 108, or 150 million kilometers
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rotation
the spinning of a body, such as Earth, about its axis
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revolution
the motion of one body about another, as Earth about the sun
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precession
a slow motion of Earth’s axis that traces out a cone over a period of 26,000 years
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perihelion
the point in the orbit of a planet where it is closest to the sun
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aphelion
the place in the orbit of a planet where the planet is farthest from the sun
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