The question of annual stellar parallax is usually viewed as having been a"win-win situation" for seventeenth-century astronomers who subscribed to theCopernican view of universe in which the Earth orbits the Sun and the Sun isone of many suns (the fixed stars) scattered throughout space. Detectingparallax would be solid evidence for the Earth's motion, but failure to detectparallax could be explained by the stars lying at great distances. Recent workpertaining to Galileo's observations of double stars illustrates Galileo'sskill as an observer. It also indicates that, given the knowledge of optics ofthe time, Galileo could expect his measurements to be accurate enough that theywould have revealed stellar parallax had it existed. Thus parallax was not a"win-win situation" after all. It could be solid evidence against the Earth'smotion, evidence which fortunately did not dissuade Galileo from the Copernicanview.
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