Copernicus' celestial spheres


Since the time of Aristotle in the 5th century BCE, western learning advocated that heavenly bodies revolved around the earth at the centre of a stable universe.

This theory was developed through observations of the stars and the calculation of orbits by the Roman astronomer Ptolemy (second century CE) whose Almagest (The Great Compilation) was one of the most important astronomical works of the Middle Ages. To reconcile his observations with an earth-centred view of the universe, Ptolemy described the heavenly bodies as moving in complex orbits.

Nicolaus Copernicus (1473–1543) studied astronomy at the University of Cracow before being elected a canon of the Cathedral Chapter of Frombork (Frauenberg) through his uncle, the Bishop of Warmia (Ermland). He then studied law and medicine in Italy before returning to Warmia. The Lateran Council sought Copernicus’s opinion on calendar reform in 1514 and around this time he circulated a manuscript criticising the Ptolemaic system for not adhering to the principle of uniform circular motions. Copernicus argued that the positions of the stars and planetary orbits could be better explained by the sun being at the centre of the universe with the planets rotating around it in a circular motion, as shown in his iconic diagram.

Copernicus was writing his main work On the Revolutions of the Heavenly Spheres from about 1515, and although by 1532 it was completed, he continued to revise it. With the help of Georg Rheticus, an astronomer and professor of mathematics at the University of Wittenberg, Copernicus made the final revision of his manuscript, which Rheticus had copied and subsequently arranged for it to be published in Nuremberg in 1543.

Copernicus’s ideas were not widely accepted until the 17th century. Some argued against them on religious grounds. However, reception of the heliocentric theory was not a simple controversy with religion on one side and science on the other. The observational astronomer Tycho Brahe (1546–1601) held the Copernican system to be mathematically superior to earlier theories, but did not believe in its physical reality. Instead he argued that the earth remained fixed at the centre of the universe while the other planets orbited the sun.

Brahe’s distinction between mathematical and physical explanations drew on contemporary views about the roles of different kinds of experts. It was the role of astronomers to develop mathematical models that did not necessarily reflect physical reality, whereas natural philosophers explained physical causes. Only with the work of Brahe’s assistant and successor Johannes Kepler would these two roles be brought closer together.

Full title:
De revolutionibus orbium cœlestium (On the revolutions of the heavenly spheres)
1543, Nuremberg
Nicolaus Copernicus
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Public Domain
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British Library

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