Copernicus puts the Sun at the centre

The story

A still Earth and a turning sky

For almost two thousand years Europeans had believed that the Earth stood still at the centre of the universe, and that the Sun, the Moon, the planets and the stars moved around it. It seemed an obvious idea: you only have to look at the sky to see the Sun rise and set, and nobody feels the ground move. The most widely used model was that of the Greek astronomer Ptolemy, from the 2nd century AD, who used a complicated combination of circles to predict the positions of the planets with fair accuracy.

Ptolemy's system, however, was very complicated and needed constant adjustments. Some planets, such as Mars, sometimes seem to move backwards in the sky, and to explain this Ptolemy had imagined small circles, called epicycles, turning on top of the big circles.

A canon who studied the stars

Nicolaus Copernicus was born in 1473 in Toruń, in Poland. His father died when he was about ten, and he was brought up by an uncle on his mother's side, who later became a bishop. He studied at the University of Kraków, then in Italy, at Bologna, Padua and Ferrara, where he worked on law and medicine and learned Greek. In Italy he also came across the astronomy taught in the universities. Back in Poland he became a canon, that is, a member of the council of the cathedral of Frombork, and he lived there for the rest of his life, also taking care of administration and medicine.

In his free hours he watched the sky from a tower and did calculations. He realised that everything became simpler if you imagined that the Sun stood still at the centre, and that the Earth was a planet like the others, travelling around the Sun once a year and spinning on its own axis once a day. In this way the backward movements of Mars and Jupiter could be explained without epicycles: they were an effect of perspective, caused by the Earth, which is faster, overtaking the slower planets.

A book that waited thirty years

Before 1514 Copernicus wrote out by hand a short summary of his idea, the “Commentariolus”, and let a few friends read it. The complete book, however, he wrote with great caution and for a long time he did not publish it, perhaps for fear of criticism, or because he wanted to perfect it. What convinced him was a young maths professor from Feldkirch, in what is now Austria, Georg Joachim Rheticus, who joined him at Frombork in 1539 and helped him put the text in order and have it printed in Nuremberg.

“On the Revolutions of the Heavenly Spheres” came out in 1543. It is said that Copernicus received a copy when he was already close to death, on 24 May 1543, but we do not know whether the story is true. In the printed version an unsigned preface was added, written by a Lutheran theologian, Andreas Osiander, which presented the theory as a mere tool for calculation and not as a description of reality. Copernicus, on the other hand, believed it was true.

An idea that others had already had

Copernicus was not the first to think of a system with the Sun at the centre: a Greek astronomer, Aristarchus of Samos, had already suggested it in the 3rd century BC. His idea, however, had been rejected, and Copernicus was the first to develop it with precise calculations. His system had its limits, though: it kept the orbits perfectly circular, and it still needed small epicycles to match the observed data. For this reason it was not at first more accurate than Ptolemy's.

Copernicus was also an economist: he wrote a treatise on the value of money, in which he explained that bad money drives out good, a rule that today is called Gresham's law.