June 21; longest period of daylight in the year; represents the start of summer
December 21; shortest day of the year; represents the start of winter
day and night are of equal length
proposed by Aristotle; Earth-centered model of the solar system
proposed by Copernicus; Sun-centered model of the solar system
discovered that orbits of the planets were ellipses and not circles
imaginary "sphere of sky" surrounding Earth
apparent path of the Sun through the sky during the year
imaginary line around the celestial sphere directly above Earth's equator
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