Dioptrics

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...it would have been better to retain the Earth’s motion, which the most excellent masters Referring to astronomers like Copernicus. introduced; this motion certainly carries the Earth around the center of the planetary world at such an interval as is thought to be the semi-diameter of the Sun’s sphere This refers to the Earth's orbital radius around the Sun, or 1 Astronomical Unit..

I cannot pass over, however, without also eliminating this error of Pena’s from his own preface, where he accuses Copernicus’s critique of the Ptolemaic Claudius Ptolemy (c. 100–170 AD), the proponent of the geocentric model whose lunar model had significant mathematical flaws regarding distance. lunar hypothesis of being false. For by this accusation, the reputation of such a great artist is greatly harmed among the unlearned. Copernicus refuted Ptolemy, whose assumptions show the Moon when bisected: the first or last quarter moon phase, when only half the disk is illuminated to be nearly twice as close to the Earth as when it is full. Copernicus took an Optical argument, and indeed the best one: that it ought to appear nearly twice as wide in body when bisected as when full; whereas experience testifies to a diameter that is constant and varies by only a few minutes. Here Pena, abusing the subtlety of the optical axiom cited by Copernicus (which is repeated in these propositions at Number 67), dismisses the argument irrelevantly. For what then, if the most apparent diameters of the moon are not precisely in the inverse proportion of their distances? If they are nevertheless nearly so, did Copernicus therefore say nothing? A general of an army says he cannot capture a city in which there are ten thousand garrisoned soldiers, except with fifty thousand. What then if he lacks just one from this number, will he therefore hesitate over the capture?

But I return to the number of dogmas which Pena most truly proves from Optics; of which this is the fourth: that it is most correctly argued from optics that there is no sphere of fire above us. If this foundation is undermined, it cannot be obscure to any Philosopher of this time how great a ruin of Aristotelian Meteorology Aristotle’s Meteorologica explained atmospheric and celestial phenomena based on the arrangement of the four elements; the "sphere of fire" was thought to exist between the air and the moon. follows. For if there were fire beneath the heaven—whether visible or invisible—a very large refraction of rays would occur. For fire seeks higher places because it is of a thinner substance than air. Just as an inflated bladder emerges from the depths of the water, pushed upward by the weight of the water, so also the fiery substance finds the cause of its ascent from its own thinness, for it is driven by the thicker body of the surrounding air.