VI.

Kepler and Newton

From the scientific viewpoint, the Copernican theory was only a rearrangement of the planetary orbits. The ancient Greek theory that planets move in perfect circles at fixed speeds was retained in the Copernican system. Precise new observations, however, showed that this could not be the case. From 1580 to 1597 Danish astronomer Tycho Brahe observed the Sun, Moon, and planets from his island observatory near Copenhagen, Denmark, and later in Germany. Based on the data compiled by Brahe, his German assistant, Johannes Kepler, showed that the planets revolve around the Sun, not in circular orbits with uniform motion, but in elliptical orbits at varying speeds. He also discovered that their relative distances from the Sun can be calculated from the observed periods of revolution.

The English physicist Sir Isaac Newton was the genius who developed the mathematical equations that describe the motions of the planets. He had to invent new forms of mathematics, including calculus, to help him solve this problem. What Newton showed was that the most natural state of motion is a straight line. Since planets move along curved (elliptical) paths, some force must be acting on them. Newton called this force gravity. He showed that the force of gravity between two objects must be directly proportional to their mass and inversely proportional to the square of the distance between them. Newton was able to prove mathematically that if gravity behaved in this way, then the only orbits permitted were exactly those described by Kepler. In Newton’s day, gravity had been associated with the Earth alone; if you drop something, it falls to the ground. Newton’s great insight showed that this force is universal. It acts everywhere, including on the planets.