Shining a light on Harriot and Galileo: On the mechanics of reflection and projectile motion

Abstract

Decades before Newton’s Principia ushered in the age of modern science, Aristotelian physics faced a serious challenge against its weakest point, in the quest to construct a new theory of projectile motion. Yet how were such new principles of motion conceived, without reference to an established theory of mechanics? This paper explores the conceptual space between the rejection of Aristotle’s physics and the appearance of Newton’s physics in which people such as Harriot and Galileo sought new ways to understand mechanical phenomena. This paper seeks to show that both Harriot and Galileo looked to the (early seventeenth-century) zeitgeist of accepting the law of reflection as a law of oblique projection-rebound for all material bodies, for their foundational (non-Aristotelian) conceptual resource. The kinematics and dynamics of light’s compound, oblique motion had been developed over centuries within optics, and constituted the ‘ideal’ resource for novel analyses of oblique projectile motion. Whereas Harriot’s inclusion of air resistance restricted the generalizability of optical principles to his simplest case, the upright parabola, Galileo’s idealized analysis of projectile motion in a vacuum represented all cases of projectile motion as upright parabolas. He was thus able to generalize these optical principles to projectile motion per se.