Kepler: Greater than Copernicus

Copernicus is generally credited with having first described the “true” arrangement of the solar system at a time when Science was being reborn after the Dark Ages. So revolutionary was his book De Revolutionibus that he withheld it for more than 10 years before finally publishing a small number of copies in 1543. As he feared, the book was subsequently placed on the Catholic church’s list of prohibited books and Galileo was warned not to repeat Copernicus’ ideas.

Nevertheless, Copernicus was correct in claiming that the Earth orbits around the Sun, and not vice versa as Ptolemy had stated some 1400 years earlier. Despite their different stances, both men held firmly to Plato’s previous suggestion that all celestial objects must move in perfect circles to comply with the then-popular belief in geometrical perfection.

One of the fundamental failings of Ptolemy’s geocentric model of “the Universe” (as it was regarded then) was the fact that the planets were sometimes seen to move backwards across the sky, in what is known as retrograde motion. Ptolemy explained this behaviour by suggesting the planets move in their own small circles, called epicycles, while simultaneously circling the Earth.

By switching from a geocentric (Earth-centred) model to one that is heliocentric (Sun-centred) Copernicus is credited with having removed the need for epicycles. In fact, his model still didn’t quite match observations unless epicycles were retained, albeit in a more minor way.

According to George Forbes’ excellent History of Astronomy, first published in 1909; “Nobody seemed to dare to depart from this fetish of uniform angular motion and circular orbits until the insight, boldness and independence of Johann Kepler opened up a new world of thought and of intellectual delight.”

Kepler initially assisted Danish astronomer Tycho Brahe, who documented the positions of hundreds of stars and recorded a supernova event in 1572, before publishing his own ideas in 1609. Brahe believed in a static Earth but Kepler disagreed. To his initial disappointment, when Kepler analysed Brahe’s data in detail he found there were still observational discrepancies even after the Sun had been placed at the centre of the Solar System.

Various refinements were suggested by others but Kepler solved this problem by abandoning the long-standing fixation with perfect circles, and allowing orbits to be elliptical instead. The Sun was no longer at a unique central point in the Solar System but was instead simply at one of the two focal points of the ellipse that defined each of the planets’ orbits. This in turn meant that planets must change their distance from the Sun as they orbit around it – something that neither Ptolemy nor Copernicus envisaged.

Kepler went on to explain the motions of the planets using a force that was directed towards the Sun. He also stated that the force is stronger for greater masses and that bodies falling towards the surface of the Earth are attracted to its centre, not to the surface itself.

Forbes recounts Kepler’s words (originally in Latin) as follows: “Bodies are attracted to the Earth’s centre not because it is the centre of the Universe but because it is the centre of the attracting particles of the Earth.”

Given all of this, history has clearly attributed to Kepler much less credit than he deserves, for not only did he achieve more than Copernicus (and Ptolemy before him) but he also laid the ground for Newton’s laws of gravitation, which followed some 60 years later.