Section 10 includes a philosophically important result that has gone largely unnoticed in the literature on the Principia. Newton's argument that terrestrial gravity extends to the Moon depends crucially on Huygens's precise measurement of the strength of surface gravity. This theory-mediated measurement was based on the isochronism[36] of the cycloidal pendulum under uniform gravity directed in parallel lines toward a flat Earth. But gravity is directed toward the center of the (nearly) spheri