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1.If the earth moves around the sun, then we ought to observe a stellar parallax. 2.There is no observable stellar parallax. 3.Therefore, the earth does not move around the sun. The argument is valid (i.e., modus tollens), the first premise seems quite reasonable and the second premise was in fact true—no one before, during or for almost three centuries after Copernicus had Figure 8.9

216 common sense logic
ever observed any stellar parallax. Yet, Copernicus had anticipated this objection and thought he had a good response. He said that we cannot observe a stellar parallax because the stars were so incredibly far away that our instruments were simply not sensitive enough.29 Copernicus’s response undoubtedly strikes a responsive note for contemporary Westerns who are fully accustomed to thinking in terms of thousands, and even millions, of light-years. (A light year equals the distance traveled by a beam of light in one year or approximately 5.8 trillion miles.)30But what does that prove? To Copernicus’s contemporaries, who were accustomed to thinking of a finite universe with the sphere of the stars only one sphere removed from the orbit of Saturn, such a response must surely have seemed as ad hoc as our imaginary flat-earthers’ assumption that light is bent by strong gravitational fields (see figure 8.8 above). There was another problem with a sun-centered theory of the universe. What kept such massive bodies like the earth and the planets revolving eternally around the sun? Copernicus had no answer. When it came to explaining the physics (as opposed to the mathematics) of a moving earth, Copernicus’s theory was less com- prehensive. On the older earth-centered theory, the earth didn’t have to move and the other planets were composed of a totally different kind of material, aether, whose natural state was to revolve in perfect circles. Historian Stephen Mason summed the situation well. “The Copernican system had not been widely accepted during the six- teenth...
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