TO THE READER CONCERNING THE HYPO-

THESES OF THIS WORK.

I do not doubt that certain learned men, now that the reputation of this work’s novel hypotheses has spread—which posits that the earth is mobile and the Sun, indeed, stands immobile in the center of the universe—will be vehemently offended; and they will think that the liberal arts, which were rightly established long ago, ought not to be disturbed. But if they wish to weigh the matter exactly, they will find that the author of this work has committed nothing that deserves to be criticized. For it is the duty of the astronomer to compile the history of celestial motions through diligent and skillful observation. Then, he must conceive and devise the causes of these same motions, or hypotheses, since he can in no way attain the true ones, by which those same motions can be correctly calculated from geometric principles, both for the future and the past. This artist has brilliantly achieved both of these things. For it is not necessary that these hypotheses be true, or even probable; rather, it is sufficient if they provide a calculation consistent with observations. Unless, perhaps, someone is so ignorant of geometry and optics that he considers the epicycle of Venus to be probable, or believes it is the cause of Venus sometimes preceding and sometimes following the Sun by forty degrees or more. For who does not see that, if this is posited, it necessarily follows that the diameter of the star in the perigee the point in an orbit closest to the earth appears more than four times larger, and the body itself more than sixteen times larger than in the apogee the point in an orbit farthest from the earth, which is, however, contradicted by the experience of all time? There are other things in this discipline no less absurd, which it is not necessary to examine at present. For it is sufficiently clear that this art is completely and simply ignorant of the causes of the unequal apparent motions. And if it devises any causes by imagining them, as it certainly devises many, it does not do so in order to persuade anyone that they are so, but only that they might correctly establish the calculation. When, however, diverse hypotheses for one and the same motion sometimes offer themselves (as in the motion of the Sun, eccentricity and the epicycle), the astronomer will grasp that one which is most easily understood. The philosopher, perhaps, will require