Lessons in Astronomy

[...to the forme]r has been devised for explaining, which they call the material Sphere; to the latter, that which they call the Theory of the Planets: hence it follows that two parts of Astronomy are usually distinguished, of which one is named Spherical Doctrine, the other Theoretical.

Therefore, we also shall treat of both in summary, since it is worthwhile first of all to understand the matter according to the commonly received opinion, which supposes such a System of the World as was described just above: for that reason, we shall carry this out in two Books; in the First of which we shall treat those things which pertain to the Spherical part, and in the Second, those which pertain to the Theoretical.

Furthermore, because two other Systems have now become very famous, one that of Copernicus, the other that of Brahe, both of which have gained noble supporters: for this reason, we shall subjoin a Third Book to these as a kind of appendix, so that it may also be possible to know what those systems are and how they are defended by their followers.