Galileo between Jesuits: The Fault is in the Stars

TL;DR

This paper explores 17th-century Jesuit debates on the structure of the universe, highlighting how observations supported a universe with large, dim stars and how this understanding influenced responses to Copernican theory.

Contribution

It uncovers the historical context of Jesuit astronomers' views on star sizes and their impact on the acceptance of Copernicanism in the 17th century.

Findings

Jesuit astronomers understood star size implications for Copernican universe

17th-century observations supported a universe with large, dim stars

The universe then was radically different from modern understanding

Abstract

In the middle of the seventeenth century, Andr\'e Tacquet, S.J. briefly discussed a scientific argument regarding the structure of a Copernican universe, and commented on Galileo Galilei's discussion of that same argument -- Galileo's discussion in turn being a commentary on a version of the argument by Christoph Scheiner, S.J. The argument was based on observations of the sizes of stars. This exchange involving Galileo and two Jesuits illustrates how through much of the seventeenth century, science -- meaning observations measurements, and calculations -- supported a view of the Copernican universe in which stars were not other suns, but were dim bodies, far larger than the sun. Johannes Kepler emphasized this, especially in arguing against Giordano Bruno. Jesuit astronomers like Tacquet and Scheiner understood this. Those who might have listened to Jesuit astronomers would likewise have understood this -- Robert Bellarmine, for example, whose role in the debate over Copernicanism is well known. To many, such a universe was, in the words of Galileo's Dialogue character Sagredo, "beyond belief," and no modern view of a universe of many distant suns would be scientifically supportable until after Tacquet's death in 1660. The Copernican universe of the seventeenth century looked radically different from the universe as modern astronomers understand it, and recognizing this fact allows for interesting questions to be asked regarding the actions of those, such as Bellarmine, who were responding to the work of Copernicus.