Gemma Frisius on Copernicus and Ptolemy

In 1555 Gemma Frisius wrote Epistola for the Ephemerides Novae of Johannes Stadius. In Grant McColley, 'An Early Friend of the Copernican Theory: Gemma Frisius', Isis 26 (2) (1937), 322-325, a facsimile reproduction of part of Frisius' Epistola is presented. Part of McColley's article is shown below.

Gemma Frisius is an unreserved and intelligent advocate of the heliocentric conception. He attacks the ancient astronomy because of its errors in the motion of Mercury, and its mistaken calculations of the vernal equinox. The hypothesis of Ptolemy, which appears at first sight more probable than that of Copernicus, contains such absurd suppositions as that the stars move unequally in their circles. The older astronomer assumes that the three superior planets, when rising at sunset, or in opposition to the sun, are always in the perigee of their epicycles.

In defending Copernicus, Gemma praises first his method of daily observations and geometrical demonstrations. The hypothesis of the motion of the earth and of the sun resting in the centre of the world is contrary to opinion or expectation, but it is drawn from natural phenomena, and provides a reasonable conception which is correspondent to the motions of the stars It also gives a better knowledge of the distances of the planets than does the theory of Ptolemy. Gemma Frisius then concludes the astronomical comment of his epistle with a hint of the opposition Copernicus had faced, and a further word of praise for this learned man who would not change his hypothesis because of antagonistic minds, but instead established the things sufficient for true finding and interpretation of phenomena.

Here is a translation of a relevant part of Frisius' Epistola:-

I omit mentioning the intolerable errors in the motion of Mercury. ... Although Ptolemy's hypotheses are on the surface more plausible than Copernicus's, nevertheless they involve some absurd things, both when stars are taken to move with unequal motions in their circles and when they don't have any evident causes for the motions. For Ptolemy assumes that when the three superior planets are at opposition (on the opposite side of the celestial sphere from the sun), they are at the perigee of their epicycle and he accepts this as a fact. Copernicus's hypotheses teach the same as necessary and they demonstrate it by giving reasons.

Last Updated November 2020