[Prev Page][Contents][Next Page]

The Scientific Revolution

Kepler's Laws

Kepler was the first to have a law named after him since the ancient Greeks, a tribute to Johannes himself but also a mark of the times, that people believed they were accomplishing feats as great as the ancients had managed. The most significant, for our narrative, was Kepler's description of how a body might move in an elliptical orbit.

The ellipse was the key to making Copernicus' system work. Kepler had inherited the careful observations of Brahe, and these allowed him to work out the mathematics. What he discovered was that a body moved faster as it moved through one end of the ellipse than it did when moving along the other end.

So long as astronomers had the planets moving in perfect circles, their speed was assumed to be constant, and so the tables of predictions were never quite right. Kepler allowed the planets to move in ellipses, and at last the planets began to show up on time.

Kepler needed Copernicus to put the sun in the center of the universe (still no one imagined our Sun to be just another star). He needed Galileo to demonstrate both that Copernicus was right and that the bodies in the heavens were not perfect (and so their motion likewise might not be perfect). And he needed Brahe's precise observations to provide the raw data.

One issue yet remained: if the planets were ordinary bodies moving through the sky, subject to the same laws that applied to moving bodies here on Earth, what kept the planets up there? Everything else falls to the ground. Why not them?

That was the puzzle solved by Sir Isaac Newton.