Orazio Grassi’s lectures on the Sphere

In 2012, the History of Science Collections of the University of Oklahoma acquired a manuscript containing notes from a series of lectures given by Orazio Grassi (1583-1654) at the Collegio Romano in 1623. Grassi lectured on a number of mathematical subjects, including on astronomy. Like many professors throughout Europe, his astronomy lectures were based on Sacrobosco’s Sphere. The first part of the University of Oklahoma manuscript, the Tractatus de sphaera, gives us a fascinating account of Grassi’s lectures on the Sphere of Sacrobosco. In 2016, OU History of Science graduate student Kraig Bartel wrote his MA thesis on Grassi’s Tractatus de sphaera. I was fortunate enough to be on Mr. Bartel’s MA committee, and this blog post draws on his thesis.

The Collegio Romano (Roman College) was a relatively new institution, compared to the many European universities that were founded in the Middle Ages. The Collegio was established in 1551 by Ignatius Loyola, founder of the Jesuit order. The Jesuit educational program was intended to be a bulwark against Protestantism and a tool of evangelization, producing men with theological training and the rhetorical and persuasive skills to convert both Protestants and non-Christians to Catholicism. But they also placed strong emphasis on mathematical subjects, including astronomy. Orazio Grassi began studying at the Collegio Romano in 1600, and became a professor of mathematics and astronomy there in 1616.

Grassi and his fellow Jesuits have an undeservedly poor reputation in popular histories of science, primarily because they had the temerity to question the work Galileo, defender of Copernicus and subsequently the patron saint of modern science. Their critiques of Galileo have been caricatured as blind adherence to religious dogma, stubborn refusal to accept any new ideas or discoveries, and just plain stupidity.

But the Jesuits who debated with Galileo were actually intelligent, sophisticated and curious. They were skilled mathematicians and extremely knowledgeable about recent work in astronomy. Many of the questions they asked about Galileo’s telescopic discoveries were sensible, and they were absolutely right that none of Galileo’s discoveries proved that Copernicus’ heliocentric model was correct (much as Galileo might try to claim this was the case).

In 1618, Orazio Grassi got into a dispute with Galileo about the nature of comets. Galileo took the traditional, Aristotelian position that comets were meteorological phenomena, that is, that they occurred in the earth’s atmosphere. Grassi took the non-traditional, innovative position that comets were celestial phenomena. Grassi’s arguments were backed by the observational evidence of the great Danish astronomer Tycho Brahe (1546-1601). In the course of this argument, Galileo uttered a dictum that is widely quoted and taken as emblematic of Galileo’s thoroughly modern rejection of traditional authority:  

“The universe cannot be read until we have learnt the language and become familiar with the characters in which it is written. It is written in mathematical language.”

What is conveniently ignored by Galileo’s hagiographers is the fact that Grassi was right about comets – they are in fact farther away from the earth than the moon – and Galileo was wrong. Galileo was not always willing to look closely at evidence that challenged his pet theories. (Let’s not even get started on Galileo’s wacky explanation of tides.)

Grassi’s lectures on the Sphere contain discussions of some of Galileo’s telescopic discoveries, including his discovery that the surface of the moon is rough and craggy rather than smooth. Whoever made these notes included a little sketch of the moon as seen through a telescope. Far from being resistant to new discoveries and ideas, Jesuits like Grassi were eager to discuss them, and to introduce their students to them. Reading Grassi’s commentary on Sacrobosco’s Sphere reveals a curious and critical man with a sharp intellect and a lively interest in new discoveries and new ideas. In the pages of his Tractatus de sphaera, he appears every bit as much a “hero” of modern science as Galileo.

Oratio Grassi, Tractatus de sphaera (1623), manuscript

Further reading

Kraig Bartel, “Orazio Grassi and a 1623 Treatise on the Sphere: Astronomy and Physico-Mathematics at the Collegio Romano in the Early Seventeenth Century” (MA thesis, Department of the History of Science, University of Oklahoma, 2016).

Stillman Drake and C. D. O’Malley, The Controversy on the Comets of 1618: Galileo Galilei, Horatio Grassi, Mario Guiducci, Johann Kepler (Philadelphia: University of Pennsylvania Press, 1960).

Renée J. Raphael, “Reading Galileo’s Discorsi in the Early Modern University.” Renaissance Quarterly 68, no. 2 (2015): 558-96.

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