Introductory Astronomical Writings of the Ancients

circles [one] must understand as lacking breadth, theoretically observable through the position of the stars, the observation of the dioptra itself, and our own conception. For the only circle in the cosmos that is sensible is that of the Milky Way. The rest are theoretically observable.

Only five parallel circles are inscribed on the sphere, not because these alone are parallel in the cosmos—for the sun, day by day, as far as sense is concerned, revolves in a circle parallel to the equator according to the rotation that occurs in the cosmos, so that between the tropical circles, one hundred and eighty-two parallel circles twice over are drawn by the sun; for there are just as many days between the turnings. Furthermore, all the stars are carried on parallel circles day by day. Yet not all these are inscribed together on the sphere, because they contribute much to other inquiries in astrology. For it is impossible for the sphere to be rightly constructed without all the parallel circles, nor can the magnitudes of the nights and the days be discovered with precision without the aforementioned circles. Nevertheless, toward the first introduction of astrology, as they contribute no result, they are not inscribed on the sphere. But the five parallel circles, because they provide certain defined results for the first