**The German astronomer Johannes Kepler **(books by this author) **discovered the third law of planetary motion on this date in 1618**. Before Kepler, Copernicus had come up with three basic theories: one, that planets have a circular orbit; two, that the sun is the center of the orbit; and three, that a planet’s speed is constant. Kepler’s three laws fine-tuned the theories of Copernicus.

By 1609, Kepler had come up with two laws based on his study of the work of 16th-century Danish astronomer Tycho Brahe. The first is sometimes known as the Law of Ellipses; it states that planets follow an elliptical path around the sun — not circular, as Copernicus had thought. And the sun is not in the center of the ellipse, but at a focal point.

Kepler’s second law is the Law of Equal Areas. Unlike Copernicus’s theory, he determined that the speed at which a planet moves through space is not constant: a planet moves faster when it’s nearer the sun, and it slows down when it’s in the farthest reaches of its orbit, due to the effect of the sun’s gravitational pull. But Kepler figured out that if he drew an imaginary line from the center of a planet to the center of the sun, it would sweep across equivalent total areas in equivalent periods of time.

Kepler’s third law, which he discovered on this date, is known as the Law of Harmonies. While his first and second laws describe the motion of individual planets, the Law of Harmonies compares the motion of different planets. Kepler discovered that the square of a planet’s orbital period is directly proportional to the cube of its average distance from the sun, and the ratio is almost exactly the same for every planet in our solar system.