Absolute and Relative Motion

Abstract

At the outset of Book I of his Principia, Newton defined terms such as inertia, quantity of motion and centripetal force. Before proceeding to his Laws of Motion, Newton presents the reader with a Scholium in which he carefully considers the meaning of the terms space, time, place and motion. Most importantly, he asks the reader to distinguishing between absolute and relative measures of each of the these quantities. To put this into a historical context, consider how the concept of relative motion shaped the sun-centered view of our solar system. One hundred years before the birth of Newton, Copernicus had invoked the concept of relative motion in order to make plausible his heliocentric model of the solar system. His opponents, who were committed geocentrists, claimed that Earth seems stationary and therefore must truly be stationary at the center of the universe. Not so, said Copernicus: since motion is measured relative to a particular observer, it may be that the sun and the stars are stationary and Earth itself is in motion. This naturally raises the question: are the sun and stars really stationary? More generally, is it ever possible to determine whether any body is truly at rest or in motion?

In philosophical disquisitions, we ought to abstract from our senses, and consider things themselves, distinct from what are only sensible measures of them.

—Isaac Newton