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Primer: Siderius Nuncius February 4, 2009

Posted by Will Thomas in EWP Primer.
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Up until 1610, Galileo Galilei (1564-1642) had made his living as a university mathematician, first at Pisa then at Padua near Venice.  At that time, mathematics was a relatively low university subject, primarily studied as a path toward an education in medicine, law, or theology and philosophy (Scholastic philosophy).  Subjects within the rubric of mathematics included the sciences of mechanics, optics, and astronomy.  The development of geometric and mathematical theories within these sciences constituted logical arguments, but were considered descriptive of the behaviors—rather than explanatory of the natures—of things.  Astronomy, for example, largely involved  the deployment of geometrical methods of predicting future positions of the sun, moon, and planets, leaving their physical qualities, habits of motion, and arrangement to the philosophers.

Galileo’s work in mathematics and mechanics was wide-ranging and ambitious, challenging philosophical assumptions such as that heavier objects fall more quickly, and making use of experimental trials.  He also became aware of Copernicus’ heliocentric theory of the universe (1543) while a mathematician.  Still, as a university mathematician, however much he felt his work bore upon philosophical forms of knowledge, he was not in a position to make any direct challenge to the work of natural philosophers.

Galileos observations of Jupiters moons

Galileo's observations of Jupiter's moons

This situation changed in 1610.  In the summer of 1609, Galileo heard reports of an invention in Holland permitting distant objects to be seen as though they were up close.  Working with his knowledge of lenses, he managed to create his own telescope, which he continued to improve over the ensuing months.  In December that year, he began making observations of celestial objects, seeing new stars, resolving the Milky Way into stars, seeing detailed surface features of the moon, and finally, in January 1610, little stars changing positions around the planet Jupiter.  Galileo understood his observations as an opportunity to advance his career in new directions.

Galileo had already been career-minded to that point, taking steps toward possible court sponsorship by becoming for a time a tutor to the heir of the Grand Duke of Tuscany, Cosimo de Medici.  Then, in 1609, the Grand Duke died, elevating Cosimo to the title.  By March 1610, Galileo’s book reporting his telescopic observations, entitled Siderius Nuncius (“The Sidereal Messenger”), was already in print.  Not only did Galileo dedicate the book to Cosimo, he used it to name Jupiter’s satellites the “Medicean Stars”.  The name was a last minute reversal from the “Cosmic Stars” which the Medici court judged a less obvious reference to Cosimo.  (Early copies of the book actually featured that name and had to have the new name pasted in over it.)

Securing a position in court was difficult, but to discover, against all expectations, new permanent features of the sky, and then to name those after the Medici was an unusual and extraordinary offering.  In short order, Galileo not only secured a position at court, he was to receive a top-tier salary, and, at his request, the additional title of court philosopher.

As a book, Siderius Nuncius quickly became an enormous success, finding an audience throughout Europe, and support from some eminent astronomers, notably Johannes Kepler.  As a contribution to natural philosophy, however, it did not represent an immediate sea change.  Galileo understood its observations of lunar mountains and satellites of Jupiter—and subsequent observations, particularly of the phases of Venus—to refute the Scholastic philosophical cosmos of perfect heavenly bodies arrayed concentrically around the earth, and to support the Copernican model.  Nevertheless, Aristotelian philosophers and Ptolemaic astronomers proved unwilling to accept its evidence, and offered heavy criticism.

Galileo’s new position in the Medici court offered him the prestige and a safe platform to continue to assault on university philosophy, and to emphasize the importance of observation versus rhetorical argumentation, going so far as to argue that Biblical exegesis should take place in view of observational knowledge in his Letter to the Grand Duchess Christina in 1615.  His new patrons expected him to engage in such debates to bring attention to their court, so long as they did not cause embarrassment or political difficulty (as, indeed, his later run-ins with the Church did).

Siderius Nuncius thus promoted a number of historical trends, significant for, but far from determining, the history of the sciences in the seventeenth century.  First, Galileo’s work served to elevate the status of mathematical sciences, at least periodically, to the level of philosophy.  By the time Newton published his Mathematical Principles of Natural Philosophy in the 1680s, mathematical astronomical, mechanical, and optical argumentation would prove to be a central tool in natural philosophical argumentation, and mathematical demonstration of observed phenomena would also become an exemplar of good philosophical practice.

Of course, Galileo’s own rather idiosyncratic brand of qualitative natural philosophical argumentation, which he deployed in his philosophical disputes, was perhaps more immediately significant for adding to a growing questioning of the Aristotelian style of argumentation.  Baconian induction, Cartesian mechanism, and other challenges would eventually cohere into a full-scale marginalization of Scholastic philosophy, though it would be a mistake to assume that any coherent program took its place.

Finally, although the application of telescopic observations to the practice of astronomy was certainly inevitable, and though it was probably the case that qualitative observation would soon become an important element of astronomy, Galileo’s central role in promoting new astronomical practices should be understood.

Stillman Drake’s Galileo at Work: His Scientific Biography (1978) remains a go-to primer on the content of Galileo’s work.  Mario Biagioli’s Galileo, Courtier: The Practice of Science in the Culture of Absolutism (1993) discusses in-depth the political jockeying and rules of etiquette that allowed Galileo to transcend his background as a mathematician and become a philosopher.


1. Mark Thompson - February 5, 2009

An excellent review of Galileo’s rapid rise to fame between 1609-1610! For a slightly different twist, please visit my journal entry at:

This will also appear as a podcast February 22 on 365 Days of Astronomy.

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