The Sphere of the World

...is of the same nature as its parts: but the parts of water—as happens in droplets and in the dewdrops on plants—naturally seek a round shape: therefore, so does the whole of which they are parts.

That the Earth is the Center of the World.

That the Earth is situated in the middle of the firmament original: "firmamentum"; in medieval cosmology, this is the outermost sphere of the universe containing the fixed stars is shown in this way. To those existing on the surface of the Earth, the stars appear to be of the same size whether they are in the middle of the sky, or near their rising, or near their setting; and this is because the Earth is equally distant from them. For if the Earth were closer to the firmament in one part than in another, someone standing on the part of the Earth’s surface that was closer to the firmament would not see exactly half of the heavens.

However, this is refuted by Ptolemy Claudius Ptolemy (c. 100–170 AD), the Greco-Roman mathematician and astronomer whose geocentric model dominated scientific thought for over a millennium and all the philosophers, who say that wherever a person may be, six signs of the zodiac rise for them and six set, and half of the sky is always visible to them, while the other half remains hidden.

This is also a sign that the Earth is like a center and a point with respect to the firmament; because if the Earth were of any significant size relative to the firmament, it would not be possible for half the sky to be seen. Likewise, if one imagines a flat surface passing through the center of the Earth and dividing it into two equal parts—and consequently dividing the firmament itself—an eye placed at the center of the Earth would see half of the firmament. The same eye, existing on the surface of the Earth, would see that same half.

From these things, it is gathered that the distance from the surface to the center of the Earth is negligible original: "insensibilis", literally "insensible" or "unnoticeable" to the senses, and consequently the size of the whole Earth is negligible in comparison to the firmament. Alfraganus Ahmad ibn Muhammad ibn Kathir al-Farghani, a 9th-century Persian astronomer whose "Elements of Astronomy" was a primary source for medieval scholars also says that the smallest of the fixed stars visible to the eye is larger than the entire Earth; yet that star is like a point compared to the firmament. Therefore, this is even more true of the Earth, since it is even smaller than such a star.

Concerning the Immobility of the Earth.

That the Earth is held immovably in the center of all things, even though it is exceedingly heavy, is suggested by its weight. For every heavy thing naturally tends toward the center. The center is indeed a point in the middle of the firmament; therefore, since the Earth is exceedingly heavy, it naturally tends toward that point. Likewise, whatever moves from the center toward the circumference of the sky is said to "ascend"; if the Earth moved away from the center, it would therefore be "ascending," which is considered an impossibility.