The Sixteenth-Century Empirical Disproof of Ptolemaic Geocentrism: Paper Ii

The standard model of the Universe in the 16th and 17th centuries was a geocentric planetary system imbedded in a homocentric shell that held the stars. In a previous paper, we discussed the book Alae seu Scalae Mathematicae of 1573 in which Thomas Digges laid out his methodology for the measurement of small diurnal parallaxes of celestial objects. Further, we discussed that book's sequel, the essay A Perfit Description of the Caelestiall Orbes of 1576, in which he claims to disprove the theory that the Earth is the center of the Universe. Here we examine the empirical grounds upon which Digges' claim rests in order to see whether the data that he might have collected support his claim. The methodology of the present argument is spelled out. We conclude that line-of-sight instruments like cross-staffs cannot account for Digges' claims, but that the higher resolution offered by telescopes can. The context of Digges' works is briefly discussed.