A True Demonstration: Bellarmine and the Stars as Evidence Against Earth’s Motion in the Early Seventeenth Century

In 1615 Robert Cardinal Bellarmine demanded a “true demonstration” of Earth’s motion before he would cease to doubt the Copernican world system. No such demonstration was available because the geocentric Tychonic world system was a viable alternative to the heliocentric Copernican system. On the contrary, recent work concerning early observations of stars suggests that, thanks to astronomers’ misunderstanding of the images of stars seen through the telescope, the only “true demonstration” the telescope provided in Bellarmine’s day showed the earth not to circle the Sun. This had been discussed by the German astronomer Simon Marius, in his Mundus Iovialis, just prior to Bellarmine’s request for a “true demonstration.”

In the early seventeenth century, careful telescopic observations made by skilled astronomers did not support the heliocentric world system of Copernicus, but rather the geocentric world system of the great Danish astronomer Tycho Brahe. This was because early telescopic astronomers did not understand the limitations of their telescopes, which produced spurious views of stars. Since they failed to realize this, they concluded that the stars were not sufficiently distant to be compatible with the Copernican system.¹

In April 1615, in a letter offering his opinion on the Copernican world system, Robert Cardinal Bellarmine wrote:

I say that if there were a true demonstration that the sun is at the center of the world and the earth in the third heaven, and that the sun does not circle the earth but the earth circles the sun, then one would have to proceed with great care in explaining the Scriptures that appear contrary, and say rather that we do not understand them then that what is demonstrated is false. But I will not believe that there is such a demonstration, until it is shown me. Nor is it the same to demonstrate that by supposing the sun to be at the center and the earth in heaven one can save the appearances, and to demonstrate that in truth the sun is at the center and the earth in heaven; for I believe the first demonstration may be available, but I have very great doubts about the second, and in the case of doubt one must not abandon the Holy Scripture as interpreted by the Holy Fathers.²

Bellarmine had been drawn into a growing controversy concerning Galileo Galilei and the Copernican world system. The discoveries Galileo had made with the telescope in the years prior to 1615 had certainly undermined traditional Aristotelian and Ptolemaic ideas about the heavens. The Sun’s spots and the Moon’s Earth-like features, such as mountains and plains, were inconsistent with their being made of an unchanging and unearthly “quintessence.” Jupiter’s satellites revealed that the Earth was not the center of all heavenly motion. Venus’s phases showed that body to circle the Sun. That the Milky Way was composed of stars and that far more stars existed than could be seen with the unaided eye illustrated the limitations of existing knowledge.

In reaction to Galileo’s success, certain opponents of Galileo and Copernicus had begun to turn to biblical rather than scientific arguments to make their case. In December 1613 the Grand Duchess Dowager Christina and some associates had challenged one of Galileo’s friends and followers, the Benedictine Fr. Benedetto Castelli, on the issue of the Earth’s movement.³ Castelli mentioned this to Galileo in a letter dated December 14. Castelli noted to Galileo that those who challenged him “admitted as true all the celestial novelties you have discovered,” but they challenged the conclusion that the Earth moves, “especially since Holy Scripture [is] clearly contrary to this claim.”⁴

Galileo replied to Castelli with a substantial letter dated December 21 in which he refuted the biblical objection to the Copernican system. This letter was not published, but copies were circulated.⁵ Moreover, someone produced altered copies of the letter so as to cast an unfavorable light on Galileo. For example, Galileo wrote to Castelli that “in the Scripture one finds many propositions...