What really happened in the history of cosmology

To complete last week’s post, I will offer here a summary of the history of Cosmology, from the Greeks to the paradigm shift that took place in the sixteenth and seventeenth centuries.

The basic elements of Ptolemaic astronomy, showing a planet on an epicycle (smaller dashed circle), a deferent (larger dashed circle), the eccentric (×) and the equant (•).

• Greek cosmology (with the exception of Aristarchus of Samos) put the Earth at the center of the universe. Plato and, above all, Aristotle established the idea that, since the sky is perfect, the orbits of the planets must be exactly circular, because, for them, the circumference is the most perfect curve of all.
• The Greek model explained well the movements of the sun and moon, and therefore made it possible to predict eclipses, but had a problem with the retrograde movements of the planets then known (Mercury, Venus, Mars, Jupiter and Saturn). Three centuries before Christ, Apollonius of Pergamum proposed that the orbits of these planets are epicycles, circumferences centered on another circumference (the deferent), which in turn revolves around a point located near the Earth, but apart from its center (the eccentric).

Dark matter or a new theory

Urbain Le Verrier

Science studies facts and tries to explain why they occur. Scientific theories are the more credible, the more facts they explain or predict. A single fact in opposition to a theory, or a single unconfirmed prediction, is enough to make us consider revising the theory. With the scientific method, theories are never final and facts must always take precedence.

We have a classic historical example in the theory of universal gravitation, which allowed Newton to explain events like the fall of bodies and the movement of planets and satellites. Its first achievement, by Newton himself, was the mathematical deduction of Kepler’s three experimental laws, obtained empirically from the observation of the orbits of the planets. But the greatest success of the theory was a correct prediction when discrepancies were detected between the orbit of Uranus deduced from the theory and the observed orbit. When something like this happens, the problem can be solved in two ways: