Kepler Works Out The Planets And Their Orbits

Kepler was asked personally by Tycho Brahe, the eminent and famous Danish astronomer, to carry forward Tycho’s exceptional measurement and calculation work on the positions of the stars and planets.

In due course, as I will discuss in this article, Kepler used Brahe’s meticulous data to develop laws of planetary motion which significantly moved forward the entire science of astronomy.

Johannes Kepler was born in Germany in 1571. He seems to have had a fairly unhappy childhood in which he suffered illness that left him physically weak. He also experienced an unpleasant breakup of his parents’ marriage.

Fortunately for the young Kepler, it became clear that he possessed an extrordinary intellect.

Consequently, it was decided he should pursue a career in the Church. This offered the main opportunity at that time, for someone without connections, to follow an intellectual path for employment.

Kepler worked hard on his studies and obtained admission to the University of Tubingen when he was 17 years old. Here, he divided his studies between divinity, in preparation for his planned Church career, and astronomy. He was presumably good at both subjects and found it hard to decide which path to follow.

It seems it was his friends who could see better that he, that his talents should be directed towards astronomy. In 1594, he accepted a prestigious professor position at Gratz University.

As an astronomy professor at that time, he was also required to do what we now call astrology.

Paart of his job was to interpret star and planetary patterns in the sky, in order to make predictions about life and politics here on Earth.

Consequently, his work contains a strange mixture of “hard science”, together with fanciful absurdities. However, it seems that it was the astrological work that first brought Kepler into contact with Tycho Brahe and Gallileo. It was therefore very helpful to Kepler.

Brahe trusted Kepler with all his years of measurement data on the stars and planets, and charged Kepler with carrying forward the quest for understanding.

So Kepler had a wonderful repository of observational data in front him, especially of the planet Mars, courtesy of Tycho.

He also was one of the first important astronomers who had access to a telescope. So he set about making sense of the motion of the five planets known at that time (Mercury, Venus, Mars, Jupiter and Saturn).

Kepler’s big breakthrough was realising that planetary orbits just could not be properly represented by a circle. Astronomers had been wrong for nearly two thousand years, by believing in the perfection and unchallengability of the circular orbits!

He was able to build on the Copernican Model, with the Sun at the centre of the planets’ orbits. Tycho Brahe had never accepted this model as correct, but Kepler was able to use Tycho’s data to prove that it provided the best fit to the observations.

He worked out that the planetary orbits are in fact elliptical. Each planet becomes closer to, and then further away from the Sun, during the course of each revolution.

Kepler did an enormous number of calculations to derive a relationship between the distance of a planet from the Sun, and the time taken to complete one orbit. He was overjoyed to find a simple relationship, as expressed in his Three Fundamental Laws Of Planetary Motion.

His work culminated with these laws and a new set of planetary tables (“The Rudolphine Tables”), which enable people to work out where the planets would be on a given day, more accurately than ever before.