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

Two of the publications of Thomas Digges on astronomy contain the contradiction that small planetary parallax angles can be measured with a new kind of instrument, but three years later he asserts observational proof of the heliocentric planetary model of Nicholas Copernicus without identifying the new instrument. This assertion rests on the capacity of an instrument to measure angles so small that current instruments like cross-staffs are not equal to the task. We have posited that the only instrument with the necessary optical resolution is the telescope. This paper posits further that Digges uses trigonometry and measurements of elongation and phase angles to evaluate the relative positions through time of superior planets seen from Earth. If Digges observed the planets telescopically, he could have established the relative distances of the superior planets in units of the Sun–Earth distance. We argue that Thomas Digges disproved Ptolemaic geocentrism and that his assertion to have proved Copernican heliocentrism empirically is reasonable.