Aristotle and Ptolemy

Aristotle, a Greek philospher, was the founder of a famous school in Athens, known as the "Lyceum" or "Peripatetic School" in around 335 B.C.. Amongst his works the following are remembered especially: the Organon, which contains his writings on Logic, the Metaphysics, Physics, On the Soul, Nicomachean Ethics, Economics, Politics, Poetics, Rhetoric, For Aristotle "physics" signified the qualitative study of all natural phenomena, carried out without the aid of a store of mathematical knowledge. In Aristotelian cosmology the Earth was imperfect and situated at the centre of the Universe. It was composed of the four elements: earth, water, air and fire, which were characterised in terms of a rectilinear and sporadic motion. Conversely, the movement of the celestial bodies (the Sun, planets and stars, composed of ether or quintessence) was continuous and circular. In order to explain the independent motion of the planets, Aristotle proposed that they rotated on concentric spheres (see Ptolemaic System). After the physics of Aristotle had been "Christianised" in the thirteenth century, it became the foundation of university instruction in this subject. This system was put into serious doubt with the advent of the heliocentric theory developed by Copernicus.

Ptolemy (87-150) was an Alexandrian astronomer, mathematician, and geographer who based his astronomy on the belief that all heavenly bodies revolve around the Earth.  In his world system the Earth was immobile at the centre of the Universe, as the centre for the planetary motions. This system actually dates from antiquity and was earlier proposed by Aristotle.  Ptolemy's main contribution is a mathematical treatment of plantary motions, as well as a large collection of information about astronomical phenomena known at the time.  The Ptolemaic System can be schematically described as follows: Earth (said to be immobile at the centre of the universe), the Moon, Mercury, Venus, Sol (the Sun), Mars, Jupiter, Saturn. The planetary spheres were enclosed by the heaven of the fixed stars which rotated due to the impulse which it received from the primum mobile (the ninth heaven, which moved extremely quickly and was devoid of stars), with the aid of God.