A decorative drop cap S.

On the circles of the sphere.

CHAPTER III.

WHAT IS A SPHERE?

A sphere is a certain solid, contained by one surface, in the middle of which is a point from which all lines drawn to the circumference are equal.

What is the axis of a sphere. The poles of the world (called hinges and vertices) are the extreme points terminating the axis. Northern Pole. One of these is called Northern, the other Southern. The Northern, which is also called the Arctic, Borealis, and Aquilonarius, always appears in our habitation. The Southern, which is called Meridional and Antarctic, always lies hidden below the horizon relative to our hemisphere. Southern Pole.

What is the axis of a sphere?

The axis of a sphere (according to Diodochus) is called its diameter, around which it rotates.

Handwritten marginal note in Latin, cursive humanist hand: These are the difficult parts of Theodosius, which are difficult in fact, not just in theory.

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On the six greater circles of the sphere.

What is the horizon.

The Horizon (which we also call the Finitors) is the circle that divides the seen part of the world from the unseen, that is, the lower hemisphere from the upper.

What is the meridional circle.

The Meridian circle is that which is drawn through the poles of the world and the vertical point. When the Sun falls upon it above the horizon, it makes noon; below the horizon, it makes midnight.

What is the equinoctial circle.

The Equinoctial is the greater circle, dividing the sphere into two equal parts, which when the Sun traverses (which happens twice a year), the days become equal to the night in the whole circle of the earth.

What is the zodiac circle.

The Zodiac circle, which is called Oblique by the philosopher, is that which contains twelve signs, comprehending on one side the circle of Cancer, and on the other Capricorn, and cutting the equinoctial through the middle, and it is cut by the same, namely at the beginnings of Aries and Libra. The zodiac is also understood, because of the true movement of the wandering stars, to have a width of 16 degrees, which the ecliptic divides through the middle, leaving 8 degrees of latitude on either side. We must understand all other circles by reason only, without any width or depth, like a line, for they cannot be discerned at all by the senses in the heavens.