Historical Excursus 1: Aristotelian Physics

The Copernican heliocentric proposal, while today attracting more attention in summary descriptions of the Scientific Revolution in astronomy, begged crucial questions having to do with the physics of celestial motion. One law of ancient and medieval physics was that the cosmos could only have one center point; another, that some elements naturally ascended (fire), others descended (earth); yet a third, that celestial bodies were spherical and “perfect.” Simple heliocentrism violated the second, without violating the first or third.

With his telescope, Galileo observed and reported that the moon had craters and was thus not “perfect.” In observing Jupiter’s moons, Galileo had evidence against the first of the ancient laws. A scientifically satisfactory explanation ultimately had to wait until Newton’s seventeenth-century work on gravity and centrifugal force.

Folks today often misappreciate the sixteenth and seventeenth-century debates by celebrating the heliocentrists in general, and Galileo in particular, for (a) at last relying on observation and (b) abandoning the Bible as an authority in explaining the workings of the natural world. Neither is quite right: Medieval geocentrism was drawn from ancient geocentrists like Aristotle, Hipparchus, and Ptolemy, none of whom was Christian. Moreover, medieval astronomers were prohibited by the rules of their disciplines at the medieval universities from drawing from the Bible for their arguments (that task belonged to the theologians alone). Furthermore, the work of ancient and medieval astronomers (be they polytheist, monotheist, or not-especially-theist-at-all) was highly observational: what was proving increasingly inadequate were the mathematical equations into which the collected observational data pertaining to the movement of those celestial bodies was inserted.

Galileo Presents Telescope to Urania and Attendants (1655)


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