De sphaera mundi

Sacrobosco's De sphaera mundi was the most successful of several competing thirteenth-century textbooks on this topic.

It is divided into nine parts: the "first moved" (primum mobile), the sphere of the fixed stars (the firmament), and the seven planets, Saturn, Jupiter, Mars, the sun, Venus, Mercury and the moon.

There are two movements: one of the heavens from east to west on its axis through the Arctic and Antarctic poles, the other of the inferior spheres at 23° in the opposite direction on their own axes.

[2]: 465 Though principally about the universe, De sphaera 1230 A.D. contains a clear description of the Earth as a sphere which agrees with widespread opinion in Europe during the higher Middle Ages, in contrast to statements of some 19th- and 20th-century historians that medieval scholars thought the Earth was flat.

[3]: 19, 26–27  As evidence for the Earth being a sphere, in Chapter One he cites the observation that stars rise and set sooner for those in the east ("Orientals"), and lunar eclipses happen earlier; that stars near the North Pole are visible to those further north and those in the south can see different ones; that at sea one can see further by climbing up the mast; and that water seeks its natural shape which is round, as a drop.

A volvelle from a sixteenth-century edition of Sacrobosco 's De Sphaera
Picture from a 1550 edition of De sphaera , showing how the curvature of the Earth makes the mast of an approaching ship appear first