Introductory Astronomical Writings of the Ancients

...are so called because certain parts of them become unobservable. For the other circles are seen in their entirety during the rotation of the world. But regarding the colure circles, certain parts are unobservable—those which, from the antarctic [pole], are intercepted beneath the horizon. These circles are drawn through the tropical points, and they divide the circle through the middle of the zodiac into 30 equal parts.

Concerning the zodiacal circle.

The circle of the 12 zodiacal signs is an oblique circle. It is composed of three parallel circles, of which two are said to define the breadth of the zodiacal circle, while the one through the middle of the zodiacal signs is so called. This [middle circle] touches two equal and parallel circles: the summer tropic at the first degree of Cancer, and the winter tropic at the first degree of Capricorn. The breadth of the zodiacal circle is 12 degrees. The zodiacal circle is called oblique because it intersects the parallel circles.

Concerning the horizon.

The horizon is the circle that defines for us the visible and invisible part of the world, and it bisects the entire sphere of the world, such that one hemisphere is intercepted above the earth, and one hemisphere beneath the earth. There are two horizons: one is the sensible, and the other is that which is grasped by reason. The sensible horizon, therefore, is that which is described by our sight at the limit of vision, such that its diameter does not exceed two thousand stadia. The [horizon] grasped by reason...