Consider, for an instant, what is involved in the theory of revolving material crystal shells. The stars are at an immense distance, all fixed to a crystal surface, which revolves once in twenty-four hours. The sun is situated on the surface of another shell, but it can not be in one fixed spot on the surface, for we see it rise and set at different points of our horizon at different times of the year. What kind of a crystal shell is it upon which the sun can glide so far and no farther? No wonder that certain medieval writers felt the necessity of imagining two shells for each luminary between which the motion took place with freedom, beyond which there was no passage. What sort of shells are those that correspond to the planets, each of which moves at various rates in varied directions—sometimes eastward, sometimes westward, sometimes north, sometimes south? The details of a scheme like this are literally unthinkable. It must be accepted, if at all, by faith—by a faith founded in phrases.
The ancient astronomers did not, in general, seek knowledge for its own sake. They were either concerned about some practical matter, as the length of the year, the prediction of the seasons and the like; or else sought acquaintance with some aspect of divine or partly divine matter, such as formed the planets and the stars. The science of the middle ages has been summarized in a sentence: 'It was all divination, clairvoyance, unsubjected to our modern exact formulas, seeking in an instant of vision to concentrate a thousand experiences' (Pater). A few of the ancients, Archimedes and Aristarchus, for example, had what we call the modern spirit. Roger Bacon was the first to formulate it. Newton may be taken as its first thorough-going representative, for even Kepler and Galileo were deeply tinged at times with the medieval color.
The Meteorologica and the De Cœlo of Aristotle (384–322 B. C.) were the text-books of the middle ages. The doctrine of material spheres was frankly adopted in these books and in the writings derived from them. The geometric scheme of Eudoxus was transformed into a clumsy mechanism, and its complexity was further increased by the addition of other spheres, so that fifty-six in all were necessary to explain celestial motions. "The glorious philosopher, to whom nature opened her secrets most freely, proved in the second chapter of his De Cœlo, that this world, the earth, is of itself stable and fixed to all eternity. . . . Let it be enough to know, upon his great authority, that this earth is fixed and does not revolve, and that it, with the sea, is the center of the heavens. These heavens revolve round this center continuously even as we see" (Dante, Convito, iii., chap. v.). Until we remember that mechanics was an unknown science to the ancients and in the middle ages, it is almost impossible to conceive how professors could teach, or students accept, a system like Aristotle's that was, in essence, unintelligible. While Cremonini was expounding the De Cœlo
