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"The equator (also called the belt of the equinoctial) is a great circle dividing the sphere into two parts. Any point on the circle of the equator is equally distant from both poles. It is called this because when the sun crosses it , it is the equinox throughout the world and the length of the day and the night are equal." (Cosmographiae introductio, chap. III - Hessler 2008)