1687

Newton’s Laws of Gravity and Motion

Isaac Newton (1643–1727)

The scientific revolution begun by Aristarchus, who proposed removing the Earth from its cosmological position of centrality, was carried on for 2,000 years by scientific rebels such as Aryabhata, al-Bīrūnī, Nīlakantha, Copernicus, Tycho, Kepler, and Galileo. This revolution culminated—decisively—in the work of the Englishman Isaac Newton. Newton was a mathematician, physicist, astronomer, philosopher, and theologian, and is widely regarded as one of the most influential scientists in all of human history.

Newton was both an experimentalist and a theorist, and he excelled in both realms. He developed new concepts and tools in optics, including the first astronomical telescope to use mirrors instead of lenses, a design that bears his name. In the theoretical realm—using basic principles of then-modern physics and essentially inventing the new mathematical field of calculus as he went along—Newton discovered that Kepler’s Laws of Planetary Motion were the natural consequence of a force that exists between any two masses and that decreases as the square of the distance between them (known as 1/r² behavior). He called this force gravitas (Latin for “weight”). We now call it gravity, and the 1/r² behavior is known as Newton’s law of universal gravity.

Newton built on that foundation to derive his three famous laws of motion: (1) bodies at rest or in motion remain at rest or in motion unless acted on by an external force; (2) a body of mass (m) subjected to a force (F) will accelerate (a) at a rate according to F = ma; and (3) the mutual forces of action and reaction between two bodies are equal and opposite. Newton published these transformational theories in 1687 in a book called Philosophiae Naturalis Principia Mathematica, known and revered today simply as the Principia. Newton’s laws of gravity and motion destroyed any remaining shreds of geocentrism and were the definitive solutions to planetary orbits for more than 200 years, until Albert Einstein showed them to be a subset of an even bigger theory known as general relativity. In one of the most famously quoted examples of scientific humility, Newton once wrote, “If I have seen further, it is by standing on the shoulders of giants.” Indeed.

A replica of one of Newton’s reflecting telescopes, built in 1672.

A vintage engraving of Isaac Newton from 1856.