The Sun’s Place before Copernicus
Alessia Tami (University of Zurich)
The Aristotelian Model
Aristotle’s geocentric model, which placed the Earth at the center of the cosmos, was expounded by the Christian Church and popularized in influential treatises such as, Tractatus de sphaera (Treatise on the Sphere) by the English astronomer, Johannes de Sacrobosco.
Click here to learn more about Johannes de Sacrobosco.
Geocentrism was based on the notion of the perfect heavens, a series of concentric spheres rotating around the unmoving Earth. The heavens moved according to a principle of eternal, uniform circular motion that stemmed from the first mover, the sphere that was thought to generate the motion of the universe. The stars were fixed on the outer sphere of the firmament, beyond the seven spheres that contained the five known planets, along with the Sun and Moon, which were also considered planets.
From Aristotle to Copernicus
Copernicus left the Aristotelian principle of perfectly circular heavenly spheres intact. Instead, he rearranged the spheres of the Earth and the five planets visible with the naked eye, which had been known since antiquity, so that they orbited the Sun. Copernicus’s order was convenient because it aligned the planets from fastest orbit (Mercury) to slowest orbit (Saturn).
The Sublunary Spheres
In the Aristotelian model, the Moon marked the boundary between the corruptible Earth, made by the four classical elements (earth, water, air, and fire), and the unchanging heavens. The heavens were thought to be made of a fifth everlasting element called celestial aether. The composition of this fifth element is unclear, but there is one additional characteristic particular to celestial aether: This element alone was thought to have a natural tendency towards movement and was therefore responsible for generating the motion of the heavenly spheres.
According to the Aristotelian model, the sublunary spheres made by water, air, and fire situated between the Earth and the Moon were the only possible site of changeable phenomena, such as comets. In 1577, Danish astronomer Tycho Brahe observed a comet in the western sky over Zeeland. He calculated that the comet must have been considerably higher up in the heavens than the moon, a fact that hinted at a more changeable universe than Aristotle’s model could account for.
Aristarchus of Samos
Copernicus was not the first to suggest that planets rotate around the Sun. Greek astronomer and Pythagorean, Aristarchus of Samos had argued as much around 330 BC, but he did not provide proof to corroborate his theory.
Although the results of Aristarchus of Samos’s calculations were incorrect, they remain one of the earliest examples of humanity’s attempt to understand its relation to the Sun. In his manuscript, Copernicus referred to Aristarchus as someone who might have believed the Earth to be mobile, but this statement was never printed.