Note 649 — Chapter VI · COSMOGONY, COSMOLOGY, AND COSMOGRAPHY

Translation of Al-Farghānī’s Astronomy by John of Seville (or Johannes Hispanensis, or John of Luna), Nuremberg edit., diff. iv, fol. 4ro. In the De sphaera, loc. cit., John of Holywood stated that Ptolemy and all philosophers had declared that six signs and the middle of the heaven (medietas caeli)—by which he probably meant the celestial equator—were visible from any place whatsoever to which a man might go on the surface of the earth. If the earth were not at the center of the universe it would be impossible, he argued, to see the medietas caeli from those parts of the earth nearest the firmament: “aliquis existens in illa parte superficiei terrae quae magis accederet ad firmamentum non videret caeli medietatem.”

Fig. 10—Diagram to illustrate John of Holywood’s reasoning that the earth is in the center of the universe.

Figure 10 illustrates what appears to have been John’s line of reasoning as well as the flaws in it. With the earth in position I, not in the center of the universe, the celestial equator (E-E′) is invisible from all points between x and y through N (the north pole), x and y being points at which tangents E-x and E′-y touch the earth’s surface. If the earth is in the center of the universe and the sphere of the universe is incomparably great in relation to the size of the earth—something which John believed to be true (see above, p. 155)—the area between x′ and y′ will be reduced to a very small area around N′. John seems to have assumed that the universe is large enough to make this area negligible. Such an area must exist, nevertheless, with all but an infinitely great celestial sphere. But if the universe were of infinite dimensions, John’s entire argument based on the invisibility of E-E′ from an earth not in the center would fall to the ground, for all points may be deemed the center of an infinite universe. See also below, p. 426, note 118.

Ptolemy’s Almagest, I, 4, contains an argument aimed to demonstrate why the earth must be at the center of the universe. John of Holywood’s reasoning is a confused attempt to condense the argument of Ptolemy into a short space.