Book on the Universe

The oblique orb only declines from the equator and the shortest days from the mediocre ones by 23 degrees, indeed. Therefore, the two smallest circles near the poles are distant from the pole of the world or the equator just as much as the oblique orb is distant from it, so that they are nothing other than a sign of the revolution of the pole of the oblique orb itself. The meridian circle is the greatest, divided into 360 degrees, passing through the poles of the world and through the middle of the heaven, placeable everywhere at the middle space between sunrise and sunset; when the Sun reaches it, it is noon by day, and midnight by night. It is divided into four times 90 degrees from the Horizon, which is also called the boundary, because it separates the seen part of the heaven from the unseen. This also divides the equator into four