| Criterion | Sun-centred model | Success |
|---|---|---|
| 1. Common sense | It requires a leap of imagination and logic to see that the Earth might circle the Sun |
|
| 2. Awareness of motion | We do not detect any motion, which is not easy to explain if the Earth is moving |
|
| 3. Falling to the ground | There is no obvious explanation for why objects fall to the ground in a model where the Earth is not centrally located |
|
| 4. Stellar parallax | The Earth moves, so the apparent lack of stellar parallax must be due to huge stellar distances; hopefully parallax would be detected with better equipment |
 |
| 5. Predicting planetary orbits | Good agreement, but not as good as in the Earth-centred model |
|
| 6. Retrograde paths of planets | A natural consequence of the motion of the Earth and our changing vantage point |
|
| 7. Simplicity | Very simple – everything follows circles |
|
Then, in the fourteenth century, Nicole d’Oresme, chaplain to Charles V of France, openly stated that the case for an Earth-centred universe had not been fully proved, although he did not go as far as saying that he believed it to be wrong. And in fifteenth-century Germany, Cardinal Nicholas of Cusa suggested that the Earth is not the hub of the universe, but he stopped short of suggesting that the Sun should occupy the vacated throne.
The world would have to wait until the sixteenth century before an astronomer would have the courage to rearrange the universe and seriously challenge the cosmology of the Greeks. The man who would eventually reinvent Aristarchus’ Sun-centred universe was christened Mikolaj Kopernik, but he is better known by his Latinised name of Nicholas Copernicus.
The Revolution
Born in 1473 into a prosperous family in Torun, on the banks of the Vistula in modern-day Poland, Copernicus was elected a canon at the cathedral chapter of Frauenburg, largely thanks to the influence of his uncle Lucas, who was Bishop of Ermland. Having studied law and medicine in Italy, his main duty as canon was to act as physician and secretary to Lucas. These were not onerous responsibilities, and Copernicus was free to dabble in various activities in his spare time. He became an expert economist and advisor on currency reform, and even published his own Latin translations of the obscure Greek poet Theophylactus Simocattes.
However, Copernicus’s greatest passion was astronomy, which had interested him ever since he had bought a copy of the Alphonsine Tables as a student. This amateur astronomer would grow increasingly obsessed with studying the motion of the planets, and his ideas would eventually make him one of the most important figures in the history of science.
Surprisingly, all Copernicus’s astronomical research was contained in just 11/2 publications. Even more surprising, these 11/2 publications were hardly read during his lifetime. The 1/2 refers to his first work, the Commentariolus (‘Little Commentary’), which was handwritten, never formally published and circulated only among a few people in roughly 1514. Nevertheless, in just twenty pages Copernicus shook the cosmos with the most radical idea in astronomy for over one thousand years. At the heart of his pamphlet were the seven axioms upon which he based his view of the universe:
1. The heavenly bodies do not share a common centre.
2. The centre of the Earth is not the centre of the universe.
3. The centre of the universe is near the Sun.
4. The distance from the Earth to the Sun is insignificant compared with the distance to the stars.
5. The apparent daily motion of the stars is a result of the Earth’s rotation on its own axis.
6. The apparent annual sequence of movements of the Sun is a result of the Earth’s revolution around it. All the planets revolve around the Sun.
7. The apparent retrograde motion of some of the planets is merely the result of our position as observer on a moving Earth.
Copernicus’s axioms were spot on in every respect. The Earth does spin, the Earth and the other planets do go around the Sun, this does explain the retrograde planetary orbits, and failure to detect any stellar parallax was due to the remoteness of the stars. It is not clear what motivated Copernicus to formulate these axioms and break with the traditional world-view, but perhaps he was influenced by Domenico Maria de Novara, one of his professors in Italy. Novara was sympathetic to the Pythagorean tradition, which was at the root of Aristarchus’ philosophy, and it was Aristarchus who had first posited the Sun-centred model 1,700 years earlier.
The Commentariolus was a manifesto for an astronomical mutiny, an expression of Copernicus’s frustration and disillusionment with the ugly complexity of the ancient Ptolemaic model. Later he would condemn the makeshift nature of the Earth-centred model: ‘It is as though an artist were to gather the hands, feet, head and other members for his images from diverse models, each part excellently drawn, but not related