Folio 7 of the Books on Cosmography.

Chapter Three: On the Circles of the Sphere

What is a Sphere.

¶ A sphere sphere: a perfectly round three-dimensional shape, like a ball is a certain solid contained within a single surface, in the middle of which is a point from which all lines drawn to the circumference are equal. original: "Sphera est solidum quoddam vna superficie contentū..." This is the classic Euclidean definition of a sphere.

What is the Axis of a Sphere.

¶ The axis of a sphere (according to the author Diodochus Proclus Diadochus, a 5th-century philosopher whose work on the sphere was a standard textbook in the Renaissance.) is the name given to its diameter, around which it revolves. The poles of the world (which are also called the "hinges" or "summits") are the extreme points marking the ends of the axis. One of these is called the North original: "Septentrionalis," referring to the seven stars of the Big Dipper. and the other is called the South. The Northern pole—which is also called the Arctic, Boreal, or Aquilonarian—is always visible in our inhabited region. The Southern pole, however—which is called the Meridional or Antarctic—always remains hidden below the horizon in our hemisphere.

On the Six Major Circles of the Sphere.

¶ The Horizon (which we also call the "Finisher") is the circle that divides the visible part of the world from the part that is not seen. That is, it separates the lower hemisphere from the upper one.
¶ The Meridian circle is that which is drawn through the poles of the world and the vertical point vertical point: the zenith, or the point directly above the observer's head. When the sun reaches this circle above the horizon, it creates noon; when it is below the horizon, it creates midnight.
¶ The Equinoctial Equinoctial: now commonly called the Equator is the greatest circle, dividing the sphere into two equal parts. When the sun travels through it (which happens twice a year), days become equal to nights across the entire world.
¶ The Zodiac circle—which the Philosopher Aristotle is often referred to simply as "The Philosopher" in Renaissance texts. calls "Oblique"—is the one that contains the twelve signs. It encompasses the Circle of Cancer on one side and Capricorn on the other. It cuts through the middle of the Equinoctial, and is likewise cut by it, specifically at the beginnings of Aries and Libra. Because of the actual movement of the wandering stars original: "errantium syderum," referring to the planets, the Zodiac is understood to have a width of 16 degrees. The ecliptic line divides this width through the middle, leaving 8 degrees of width on either side. We must understand all other circles as having only mathematical existence, without any actual width or depth—much like a line—for they cannot be perceived by the senses in the heavens.

These are the Names and Characters of the Signs of the Zodiac: