The Sphere of Johannes de Sacrobosco

SPHAERAE IOAN.

That the Earth is also round is clear in this way. Signs and stars do not rise and set equally for all people existing everywhere; rather, they rise and set sooner for those who are toward the east. The cause of why they rise and set sooner and later for some is the bulge of the Earth; which is well demonstrated by what happens in the heights. For one and the same eclipse of the Moon 16 number*, which appears to us in the first hour of the night, appears to the easterners around the third hour of the night. From this, it is certain that for them it was night earlier, and the sun set for them earlier than for us. The cause of this thing is only the bulge of the Earth.

A circular diagram represents the Earth or the celestial sphere. It features a central horizontal line and a vertical line intersecting at the center, where a small face (representing the Earth or an observer) is depicted. The outer circumference is marked with various letters and tick marks. Darkened segments appear along the perimeter, representing the shadow or the progression of an eclipse.