Dioptrics

[He] concludes as an optical scientist that starlight descending obliquely At an angle. through these vast volumes—since the Earth is situated away from the centers of certain spheres—would be refracted according to optical laws. If this were granted, all certainty of astronomical observations would be destroyed; yet, experience itself provides testimony to their reliability. This same conclusion follows from the perceptible proportion of the body of the Earth relative to the sphere of the moon. For even if we ignore eccentric orbs Planetary paths that do not share the same center as the Earth. and place the Earth at the very center of all the spheres, the surface of the Earth itself still stands at a significant distance from the center of the lunar sphere (which the Earth occupies with its own center). Consequently, the rays of the stars descending to the surface of the Earth where we live would cut through the lunar sphere at an angle, and it would happen that those refracted rays would disturb the certainty of our sight.

Before Pena has even exited the entryway of this most beautiful demonstration, he stumbles far too improvidently, removing the distinction not only between the spheres themselves, but even between the air and the ether The "ether" was historically considered the pure, thin substance of the heavens, distinct from the "air" of our lower atmosphere.. While he makes the material of the ether the same as the material of this air we breathe, he demonstrates by his very error how much it matters for one walking in the palace of Philosophy to have the "eyes of Optics" wide open. For by the very same argument that removes the distinction between the celestial spheres, the distinction is instead firmly established between this air and the ether which succeeds it just above the mountain peaks.

For even if astronomical observations are not disturbed by some complex system of refractions tangled among themselves—as the distinction and solidity of spheres would require, if they existed—they are nonetheless disturbed by a uniform pattern of refraction whenever the stars approach the horizon. These refractions can come from nowhere else but the surface of this air we breathe; so much so, indeed, that in the optical branch of astronomy I have even been able to investigate the height of that atmospheric surface from the surface of the Earth. Pena appeals to experience, bringing in Gemma Frisius Reiner Gemma Frisius (1508–1555), a Dutch mathematician and instrument maker. as an eyewitness with his astronomical staff term: baculo astronomico—A "cross-staff" used for measuring angles between celestial bodies., who denied that any refractions had been detected by him. Clearly, the wonderful diligence of the great Master Tycho Brahe Tycho Brahe (1546–1601), whose high-precision observations were essential to Kepler's work. was not yet known to Pena; Tycho, partly through a multitude of assistants, and partly through the great size and precision...