THE PROLOGUE OF

IOANNIS DE SACRO-

BOSCO

to his Sphere.

A decorative woodcut headpiece features symmetrical foliate scrolls and central circular patterns.

We distinguish the treatise on the sphere into four chapters. In the first, we intend to speak of the composition of the sphere, what a sphere is, what its center is, what the axis of the sphere is, what the pole of the world is, how many spheres there are, and what the form of the world is. In the second, of the circles from which the material sphere is composed, and that super-celestial one, which is understood to be composed through that which is imagined by it. In the third, of the rising and setting of the signs, of the diversity of days and nights, and of the division of climates. In the fourth, of the circles and motions of the planets, and of the causes of eclipses.

Chapter One.

The sphere, therefore, is described by Euclid thus. 2 A sphere is the movement of the circumference of a semicircle, which, with the diameter fixed, is carried around until it returns to its own place. That is: A sphere is such a round and solid object which is described by the arc of a semicircle carried around. A sphere is also described by Theodosius thus: 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