"The circle that is the horizon is determined by the zenith that is its upper pole and is a point equally distant from all points on it. The circle of the horizon divides the hemispheres of the earth into east and west, but for those who live beneath the equinoctial, it goes through the two poles of the world. The zenith point of every possible horizon is always a fourth of a circle, 90°, and the circumference of the horizon is four times the distance between the zenith and the horizon." (Cosmographiae introductio, chap. IV - Hessler 2008)