The Copernican Revolution established the fact that planets revolve around the sun. But the orbits of the planets were still believed to be circular since even Copernicus thought that the circle is a “perfect” curve. Two astronomers, Tycho Brahe and Johann Kepler, brought about the next revolution.

No two men could have been more different. Tycho Brahe (1546-1601) was the son of the governor of the Helsingborg castle in Denmark and was brought up by his uncle who died when Tycho was still very young, leaving him with a vast inheritance. Tycho had his heart set on astronomy. He travelled all over Europe acquiring the necessary astronomical instruments, set up a small observatory at Scania in 1571 and became the most accurate observational astronomer before the days of the telescope.

Kepler (1571-1630), on the other hand, was everything Tycho was not. He was born in Germany to a good-for-nothing mercenary soldier and a quarrelsome mother who, in her later years, almost got burnt at the stake as a witch. He was sickly and depressed, but managed a good education only because his superior intelligence was recognised by the local duke who gave him a scholarship. Kepler was training to be a Lutheran minister after his formal education at Tubingen University, but by accident became a mathematics teacher in a Lutheran school at Graz in Austria.

As an astronomy student, Kepler was strongly influenced by Copernican concepts. Though his early attempts to fix planetary orbits were not very successful, it brought him in contact with Tycho. In 1597, as religious disputes became intense in his hometown, Kepler accepted a position in Tycho’s observatory in Prague. With the death of Tycho in 1601, Kepler inherited the vast amount of astronomical observations which Tycho had recorded. In particular, Tycho had also observed the motions of the planets, with unprecedented accuracy and this is what Kepler used.

Kepler started by looking for simple rules describing the motion of the planets — especially that of Mars. The failure of simple models finally forced Kepler to realise that planets do not move in circular orbits but in ellipses with the sun at one focus. This is now known as Kepler’s first law. An ellipse can be thought of as a circle with two centres. If the tip of the pencil moves keeping a constant distance from a point, the resulting curve is a circle; if the pencil moves keeping the sum of the distances from two points a constant, you end up tracing an ellipse. Kepler’s first law says the planets move along such a curve with the sun located at one of the two centres. The extraordinary accuracy of Tycho’s observations was instrumental in eliminating several other simpler models; some models differed from that of elliptical orbits only by a fraction of a degree in their predicted positions which would have been ignored in Ptolemy’s days.

What are more remarkable are two other laws that Kepler formulated. These helped to bring more order to the reigning chaos. Based again on Tycho’s meticulous observations, Kepler could conclude that the line joining a planet and the sun sweeps out equal areas in equal intervals of time. And finally, he could spell out a clear relation between the period of revolution of a planet and its distance from the sun.

This third law served as a significant step in a new direction. By relating the orbital properties of various planets to their distances from the sun, it suggested that the sun is the cause of planetary motion. This idea was very much present in Kepler’s writings, but it took the genius of Isaac Newton (1643-1727) to form a workable law out of this suggestion. Newton showed that the three laws of Kepler will arise directly from one simple postulate: the sun attracts the planets with a force that falls inversely as the square of the distance.

Newton did more and that is where true scientific progress lies. Instead of just saying that the sun exerts a gravitational force on planets, he boldly generalised the idea as a universal law of attraction between any two bodies. So the force that makes an apple fall to the earth is the same force that keeps the moon orbiting the earth! This was the first unification in the history of physics — a unification of terrestrial gravity with celestial phenomena, bringing the latter into the firm grasp of the laws of physics.

The work of these three geniuses — Tycho, Kepler and Newton — is a classic example of how science progresses. The Scientific Method of understanding natural phenomena involves the following steps. First, you need clear, well-defined and accurate observations. Before Tycho and Kepler, we only had broad philosophical ramblings regarding planetary motion. Tycho took the first step of providing accurate observations. The second step involves making sense out of the observations and stating them in terms of certain rules. Kepler took this second step of encoding Tycho’s observations in the form of three simple laws. The real breakthrough is at next level in which you provide an explanation of these rules in terms of a more fundamental law, which gives you fresher insights. This is what Newton did. The key difference between Newton’s law of gravitation and Kepler’s laws (both of which explained planetary motion) is that Newton’s law of gravity allows us to understand a much wider class of phenomena than just planetary motion. This is the key feature of a truly scientific explanation — it gives more than what you put into it. Newton made new predictions about the world all of which, of course, could be verified.

At last, the heavens are in order and are coming into the grip of science.

T. Padmanabhan is an astrophysicist with the Inter-University Centre for Astronomy and Astrophysics, Pune.