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summer solstice

June 21; longest period of daylight in the year; represents the start of summer

winter solstice

December 21; shortest day of the year; represents the start of winter

equinox

day and night are of equal length

geocentric model

proposed by Aristotle; Earth-centered model of the solar system

heliocentric model

proposed by Copernicus; Sun-centered model of the solar system

Kepler

discovered that orbits of the planets were ellipses and not circles

celestial sphere

imaginary "sphere of sky" surrounding Earth

ecliptic

apparent path of the Sun through the sky during the year

celestial equator

imaginary line around the celestial sphere directly above Earth's equator

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