Cosmography

FIRST PART
On the Circles of the Sphere.
Chapter III.

What a Sphere is.

An ornamental drop cap 'S' featuring a seated figure, possibly a scholar or deity, holding a globe or celestial sphere, surrounded by decorative foliage and architectural elements.A sphere is a certain solid contained by a single surface, in the middle of which is a point from which all lines drawn to the circumference are equal.

What the axis of the Sphere is.

The axis of the sphere (according to Diadochus) is the name given to its diameter, around which it is turned. The poles of the world (which are also called the hinges and vertices) are the outermost points terminating the axis. One of these is called the northern, the other the southern. The northern, which is also called the Arctic, Boreal, and Aquilonar, is always visible from our place of habitation. The southern, however, which is called the Meridional and Antarctic, always lies hidden beneath the horizon as far as our hemisphere is concerned.

On the six major circles of the Sphere.

The Horizon (which we also call the Finitor) is the circle which separates the visible part of the world from the invisible—that is, the lower hemisphere from the upper.

The Meridian is the circle which is drawn through the poles of the world and the vertical point; when the sun falls upon this circle above the horizon, it creates midday, but below the horizon, it creates midnight.

The Equinoctial is a major circle dividing the sphere into two equal parts; when the sun traverses it (which happens twice in a year), the days become equal to the night throughout the entire world.

The Zodiac circle, which is called the Oblique circle by the Philosopher, is that which contains the twelve signs, reaching on one side to the circle of Cancer and on the other to Capricorn, and cutting the equinoctial through the middle, as it is also cut by the same—namely, at the beginnings of Aries and Libra. The zodiac is also understood, on account of the actual passage of the wandering stars, to have a width of 16 degrees, which the ecliptic divides through the middle, leaving 8 degrees of width on either side. However, we must understand all the other circles to exist by reason alone, without any width or depth, just like a line; for they can by no means be discerned in the heavens by the senses.