The term equinox (in Latin aequinoctium) derives from two Latin words: aequus, whose meaning is “equal,” and noctium (nox, noctis), referring to “night.” Its definition involves that of the celestial sphere (which is the infinite apparent sphere around the Earth that is imagined to have our planet as its center and that contains the sun, the moon, and all the planets and stars), the ecliptic (the plane on which the orbit of the Earth around the sun lies), and the celestial equator (the huge imaginary circle on the celestial sphere that lies in the same plane as our planet's equator): An equinox, in fact, can be explained as either of two points (equinoctial points) on the celestial sphere where an intersection between the ecliptic and ...

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