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"The great circle of the horizon (the limiting line) divides the upper part of the sphere from the lower. It is the circle which the view of those who stand under an open sky see coming to an end and separating the part of the heavens that can be seen from that which cannot be seen. The limit of the horizon line varies from location to location, the pole being the point directly overhead every horizon." (Cosmographiae introductio, chap. III - Hessler 2008)