Dioptrics

...clears up the [confusion] of tangled reasonings. For indeed, this argument, bound by no such evident necessity, bears witness only to probability. Therefore, Pena, distrusting this doubtful argument, timidly let go of the Earth's mobility as Copernicus Nicolaus Copernicus (1473–1543), who proposed the heliocentric model where the Earth revolves around the Sun. teaches it; on the other hand, with a mere wink of his "Optical Eye," he conceived a confidence in some other investigated, very slow motion of the Earth. If this were granted, he thinks it follows that the fixed stars would seem to take on an unequal motion—a motion of the fixed stars to which the differing consensus of the centuries bears witness.

But oh Pena, this is not to commend the excellence of Optics—to strain its powers on impossible things. Bucephalus The famous horse of Alexander the Great. was entirely noble, even if he could not imitate the wings of Pegasus The mythical winged horse.. And if someone who claimed to have seen Bucephalus flying were proven wrong, the glory of Bucephalus would not therefore fall. Too much, oh Pena, has this reasoning of yours departed from optical principles; too many things stand between your optical assumption and that which you conclude from it.

First, you were not touched by that concern regarding the truth of the Observations, which we send away today from that profound antiquity Kepler is skeptical of the accuracy of ancient astronomical data compared to modern measurements.. Next, you allege the motion of the fixed stars as a thing seen by the eyes. And yet, it is far too distant from the sight of the eyes: only by a composition of three of the most subtle reasonings into one—and that not a very strict one—does an Astronomer finally dare to pronounce in which place of the Zodiac in any given century a certain fixed star stands.

Finally, what you call the motion of a fixed star from the point of the equinox is, on the contrary, most truly the recession of the equinoctial point from the fixed star Today known as the "precession of the equinoxes," caused by the Earth's axial wobble.; where the equinoctial point wanders very far from Pena's conception. For what else is the equinoctial point than the imaginary intersection of two imaginary circles? One of these is understood to be continued by the sun through the orbit of the Earth Tellus This refers to the ecliptic, the apparent path of the Sun against the background stars. all the way into the highest ether; the other is likewise understood to be continued from the center of the Earth Terra through the terrestrial equinoctial The celestial equator. all the way beneath the fixed stars—and that not in every position of the Earth, but only then, when the Earth is in the equinoctial points.

But this matter is taught in astronomy, and in my Commentaries on Mars Original: "meis de Marte Commentarijs." This refers to Kepler's landmark work, Astronomia Nova (1609).. In vain, therefore, from such uncertainties, the ill-informed Pena attributed some new motion to the Earth, and a very slow one at that, by which motion it might be exiled from the center of the world: