Didascalicon, Book II: the sciences, their parts and their discipline
Chapter XV. On astronomy
Nor is it contradictory that above we attributed immovable magnitude to geometry and movable to astronomy; for this was said according to the first discovery, according to which also geometry is called the measure of the earth. Or we may say that what geometry considers in the sphere of the world, that is the dimension of the heavenly regions and circles, is immovable, in so far as it pertains to geometrical consideration. For geometry does not consider motion but space. But what astronomy contemplates is movable, that is, the courses of the stars and the intervals of times. And so we shall say universally that immovable magnitude is subject to geometry, movable to astronomy; for although both treat of the same thing, one contemplates what remains, the other what passes.