Galileo is famous as an “astronomer,” from his association with the telescope and largely derived from his notorious troubles with the Church. However, mapping stars and constructing calculations was not the primary interest of this diversely talented scholar. We should think of his focus as creating a new physics, based on a unified theory of matter and motion, contradicting the traditional Aristotelian physics. He rejected distinction of terrestrial (the four elements) and celestial (the ætherial fifth element) realms, along with their distinct natural motions. He produced a way of using mathematical proportions and relations to describe the properties of motion, and matter acting in lawlike ways. He accomplished this by using simple machines, measurements, and experiments to explore those physical relationships. His campaign against the Scholastic categories and explanations was not accomplished simply, quickly, or consistently. That is, questions he opened in the 1580s-90s sometimes did not come to fruition until the 1630s, and even then, he could not resolve all of the problems he raised. Nonetheless, we can see him throughout his long career thinking deeply about the nature of matter and of motion, and attempting to find a new, unified theory. His goals do relate to astronomy – he was an early Copernican, and he knew that he needed physical principles to explain how things move on a moving earth. One way to view him is as using Copernican astronomy as a way to critique Aristotle, but also as an open problem that a new physics would solve.

Galileo wrote early treatises on “motion” (1590) and on mechanics (1600), disputing Scholastic explanations and attempting to build a system built on weight and balance (equilibrium). He was hampered by not being able to measure all the properties he needed, and by not yet seeing how to bring all phenomena into one system as “dynamics” (action over time) instead of static relationships. He did develop novel experiments with balances, specific gravities, inclined planes, falling bodies, acceleration, and pendulums. These led to articulation of some particular rules or relationships, such as the law of pendulums and ideas about acceleration (1603-1609). Not fully able to resolve these phenomena into a cohesive theory, Galileo left most of it unpublished until his summary work of 1638.