Chapter XVII - On the Diameters of the rest of the Planets, of the Proportion of the Celestial Spheres, and of the Parallax of the Sun

Summary

I shall here say something which may tend to throw light upon the dimensions of the stars, and upon the horizontal parallax of the Sun, a matter of the greatest importance, and one which has been the subject of much fruitless speculation; but I will not speak dogmatically, nor, as I may say, “ex cathedrâ,” but rather for the sake of promoting discussion, and with the view of examining other men's opinions.

John Kepler, the prince of astronomers, speaking of the relative proportion of the planets (Astr. Cop. page 484), thinks it “quite agreeable to nature that the order of their magnitudes and of their spheres should be the same; that is to say, that of the six primary planets, Mercury should be the least, and Saturn the largest, inasmuch as the former moves in the smallest, and the latter in the largest orbit.”

“But as the dimensions of their bodies may be regarded as threefold, either according to their diameters, their superficies, or their bulk,” he is doubtful which should be preferred. He thinks the first proportion “to be beyond question contrary to original reasons, as well as to the observations made on the diameters by means of the Belgian telescope.” He advocates the second, because the original reasons are preferable; whilst Remus Quietanus, a man well versed in practical observations, defends the third; and with him Kepler at length agrees, retaining this proportion in the Rudolphian tables.