The Sphere of the World

This is clearly shown by those things that occur in the heights of the heavens. For one and the same lunar eclipse original: "eclipsis lune" which appears to us in the first hour of the night appears to those in the East around the third hour of the night. From this, it is certain that night came to them earlier, and the sun set for them earlier than for us. The cause of this is nothing other than the curvature of the earth original: "tumor terre"; literally a "swelling," used here to describe the physical bulge or convex shape of a sphere.

That the earth also has a curvature from the north original: "septentrione" to the south original: "austrum" and vice-versa is shown in this way. For those living toward the north, certain stars are of perpetual appearance original: "sempiternę apparitionis"; these are circumpolar stars that never dip below the horizon from the viewer's perspective, namely those that are close to the arctic pole. Others, however, are of perpetual occultation stars that never rise above the horizon for a specific location, such as those that are close to the antarctic pole.

If, therefore, someone were to travel from the north toward the south, they could travel so far that the stars which were previously of perpetual appearance to them would now begin to set. And the further they approached the south, the more they would move toward setting. Furthermore, that same person would be able to see stars which had previously been of perpetual occultation to them. The reverse would happen to someone traveling from the south toward the north. The cause of this thing is the curvature of the earth.

Also, if the earth were flat from east to west, the stars would rise as quickly for those in the West as for those in the East, which is clearly false. Likewise, if the earth were flat from north to south and vice-versa, the stars which were of perpetual appearance to someone would always appear to them no matter where they traveled, which is also false. But the reason the earth appears flat to human sight is because of its immense size.

¶ That water is round.

That water also has a curvature and approaches a round shape is shown thus. Let a mark be placed on the seashore and let a ship leave the port; let it move so far away that an eye located at the foot of the mast cannot see the mark. However, while the ship remains in that position, the eye of the same person, if placed at the top of the mast, will clearly see that mark. Yet the eye of the one at the foot of the mast ought to see the mark better than the one at the top, as is shown by drawing lines from each to the mark. There is no other cause for this than the curvature of the water original: "tumor aquę". For we must exclude all other impediments, such as clouds and rising vapors. Also, since water is a homogeneous body a substance that is the same throughout; medieval science held that such elements naturally sought a spherical shape, the whole...