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).

Plato's Timaeus, the philosophical basis of the medieval cosmological model

Plato, according to Raphael Sanzio

Among Plato’s dialogues, Timaeus has always captured the attention of scholars, for it represents the first description in Greek philosophy of a coherent cosmological model, which reached great resonance by becoming part of the medieval model through the partial translation into Latin of this dialogue by the mysterious Roman philosopher Calcidius.

Very little is known about Calcidius. Although he lived in the fourth century, we do not know his date of birth or death, nor the place where he lived. It is not even known whether he was a Christian or a pagan (a Neo-Platonist). His book is dedicated to a certain Hosius, who may or may not be the bishop of Cordoba who participated in the Council of Nicaea.

It is often said that medieval philosophy in Western Europe was based initially on Plato, and from the twelfth century on Aristotle. This happened because, in the realm of the Western Roman Empire, the knowledge of the Greek language had been lost, therefore the Greek classics could no longer be read in their original language. There were no Latin translations, for the illustrated Romans of the imperial period could read the Greek language perfectly, so did not need them. What is not usually mentioned is that Plato’s works had also become inaccessible, with the sole exception of the Timaeus, which in the partial translation by Calcidius knew an unexpected boom during the Middle Ages, even stronger than Calcidius’s work during his life.