The Terrestrial Equator is an imaginary circle around the Earth equidistant from the Poles, separating Northern from Southern hemisphere. The Celestial Equator is an extension of the Terrestrial Equator and is equidistant from the Poles of the heavens. The Ecliptic is the path of the Earth's revolution around the Sun, and is inclined to the Celestial Equator at an angle of 23° 27', called "0bliquity of Ecliptic". Direction of the Earth's axis never varies. Tropics are circles on celestial sphere 23° 27' on each side of Equator.
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