Dioptrics

...and while he shakes the walls of the fiery sphere, he is himself harmed by the excessive force of this battering ram original: arietis. Kepler uses a metaphor: Pena's argument is like a siege engine that is so powerful or poorly handled that it damages the person using it.. He believes it is essential to the strength of his argument to admit absolutely no refractions of the stars. Consequently, he does not hesitate to dismiss the reliability of the astronomers' observations that Vitello Witelo (c. 1230–1285) was a medieval physicist whose work on optics was the standard text until Kepler wrote this volume. cites. Vitello had claimed that light rays are refracted, and that this is noticeable in the Moon, whose latitude often appears different from what the Tables of Motions Mathematical tables used by astronomers to predict where a planet or moon should be located at a specific time. would allow. Pena counters that the cause is not refraction, but parallax parallax: the apparent shift in an object's position when viewed from different locations. Because the Moon is close to Earth, its position looks different depending on where on Earth you are standing., a phenomenon well known to astronomers.

Good heavens, what a strange and muddled situation! For both men hold a true doctrine, yet both prove it improperly, and in the course of proving it, they fall into errors regarding related matters. Vitello says truly that the bending of starlight occurs because of the density of the air. He is also correct—though by chance—that this can be detected in the Moon. However, he was greatly mistaken in assuming that the Moon’s un-refracted position The "true" position of the Moon if there were no atmosphere to bend the light. could be accurately determined using the calculations available in his time. He set those calculations as a rule for evaluating observations and detecting refractions, and in this, he was vehemently deceived.

Thus, I would not easily say that anyone before Tycho Brahe Tycho Brahe (1546–1601) was a Danish astronomer whose precise observations provided the data Kepler needed to discover the laws of planetary motion. successfully detected the refraction of the Moon—not only because the old calculations were uncertain, but also because previous observers were negligent. Brahe, however, detected refractions not only via the Moon (which is harder due to its rapid and variable motion) but primarily through the fixed stars. Yet even with the Moon alone, refraction could be easily detected, even if the calculations of its movement aren't perfectly certain.

So much for Vitello’s mistake. Now let us examine Pena’s critique. He is correct in maintaining that no refraction occurs because of a "sphere of fire"; however, he is wrong to add that no refraction occurs at all, not even because of the air. Finally, he clumsily counters Vitello’s argument (which, as said, was already weak and flawed) by attributing to parallax what Vitello attributed to refraction. Yet astronomers know that these two phenomena have opposite effects. Refraction makes the Moon appear higher, while parallax makes it appear lower. Pena failed to weigh this fact. But as I said, there is no harm, even if Pena [refutes] Vitello's...