Aristarchus of Samos (c. 310 - c. 230 BCE) was an ancient Greek mathematician and astronomer from Ionia who came up with a revolutionary astronomical hypothesis. He claimed the Sun, not the Earth, was the fixed centre of the universe, and that the Earth, along with the rest of the planets, revolved around the Sun. He also said that the stars were distant suns that remained unmoved and that the size of the universe was much larger than his contemporaries believed.
This Sun-centred view of the universe is often referred to as “heliocentric”. It is fair to say that while the term heliocentric seems to be relatively adequate to classify Aristarchus view, it nevertheless fails to address the fact that Aristarchus’ hypothesis had a number of additional interesting implications far beyond just a Sun-centred planetary system and a rotating Earth. In fact, a rotating Earth was already held by Heraclides Ponticus (390 BCE - 310 BCE) some time before Aristarchus and the Pythagorean tradition believed that the Earth was not the centre of the universe, but that it revolved around the “Central fire”, an imaginary body believed by them to be the actual source of the light of the universe.
Just like Egyptian influence caused Greek mathematics to flourish a number of centuries before, the influence of Babylon had an equivalent stimulus over Greek astronomy. Alexander the Great's opening of the East led to an exchange of ideas that had a critical impact on Greek’s astronomical notions. This is the context in which Aristarchus developed his theory.
Except for a few rare exceptions, the general consensus among Greek astronomers during the time Aristarchus lived was that the universe was Earth-centred. During the 4th century BCE, Plato and Aristotle defended the geocentric model but both philosophers did so using mostly mystical and mythical arguments. The stars and planets were carried around the Earth on spheres, arranged in a concentric fashion. Plato even described the universe as the Spindle of Necessity, attended by the Sirens and turned by the three Fates. There was little room in Plato’s views for the idea of a universe governed by natural laws since he rejected any form of determinism. In fact, the unpredictable motions of some planets (especially Mars), were seen by Plato as proof that natural laws could not account for all the changes in the universe. Eudoxus, a student of Plato, challenged the views of his teacher by working on a more myth-free mathematical model, but the idea of concentric spheres and circular planetary motion still persisted.
Despite the general consensus on the Earth-centred model, there were a number of reasons that led some to believe that the model was not fully accurate and needed some corrections. For example, it was not possible for the geocentric model to explain either the changes in the brightness of the planets or their retrograde motions. Aristarchus' hypothesis was developed as an alternative explanation for the Earth-centred models’ flaws.
Development of Aristarchus' Hypothesis
The only extant work of Aristarchus is called On the Sizes and Distances of the Sun and the Moon, and it contains no hint of the heliocentric model. In fact, it adheres to the geocentric view. There are several possible explanations for this. It could well be that, for the purpose of the works, it makes no difference which theory is adopted and, therefore, Aristarchus decided that presenting a view in contradiction to the general consensus would have been unwise. Another option would be that he may have arrived at the heliocentric view after writing this work. Some historians who have studied this matter in detail, such as Sir Thomas Heat, believe the latter. In this work, by means of careful geometrical analysis based on the size of the Earth’s shadow on the moon during a lunar eclipse, Aristarchus concluded that the Sun must be much larger than the Earth. It is possible that the idea that tiny objects ought to orbit large ones and not the other way around, motivated his revolutionary ideas. Aristarchus also suspected that the stars we see in the night sky are actually nothing more than distant suns.
Unfortunately, Aristarchus works where the heliocentric model is presented are lost. His theories on the universe have been pieced together from later works and references. One of the most important and clear is the one mentioned by Archimedes in his book The Sand Reckoner:
[...] ‘universe’ is the name given by most astronomers to the sphere whose centre is the centre of the Earth and whose radius is equal to the straight line between the centre of the Sun and the centre of the Earth. [...] But Aristarchus of Samos brought out a book consisting of certain hypotheses, in which the premises lead to the result that the universe is many times greater than that now so called. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun in the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same centre as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the centre of the sphere bears to its surface.
Rejection of Aristarchus’ View
Some historians have suggested that Aristarchus himself may have abandoned his theory as a result of failing to reconcile it with the supposedly circular movements of the heavenly bodies all Greek astronomers took for granted. Whether Aristarchus himself ended up rejecting his own hypothesis is not totally clear. What seems to be clear is that Hipparchus of Nicea, in all probability the greatest astronomical genius of antiquity among Greek astronomers, concluded that the geocentric model better explained the observations than did the model of Aristarchus. The only way that Aristarchus' view could stand mathematical analysis was by supposing an elliptical orbit of the Earth, and this supposition was almost a blasphemy to Greek science. On top of this, this new model expanded the size of the universe far beyond the accepted size, which was also difficult to accept.
Aristarchus planetary model was discarded only to be rediscovered almost two millennia later during the years prior to the rise of modern science that took place during the Renaissance.