What led Aristarchus to come up with a Sun-centered idea more than 2,000 years ago?
That’s a good question! Remember that all books were handwritten in ancient times, so most existed only as single copies (unless someone made another copy by hand). As a result, when a book was lost or destroyed, it was usually lost forever. (Look up the “Library of Alexandria” to learn where most ancient books were stored and how they were lost with the Library’s destruction.) As it turns out, most of Aristarchus’s original writings were lost, so we know of his work primarily through the accounts of other ancient Greek writers whose books survived and who mentioned Aristarchus and his Sun-centered view. These works give some hints of what may have motivated Aristarchus, but we will probably never know the full story.
Still, a few things are clear. First, although Aristarchus may well have been the first person to suggest that Earth orbits the Sun, he was building on suggestions previously made by others. For example, Heracleides (c. 388–315 B.C.) had previously suggested that Earth rotates, which offered Aristarchus a way to explain the daily circling of the sky in a Sun-centered system. Heracleides had also argued that the fact that Mercury and Venus always stay fairly close to the Sun in the sky meant that these two planets must orbit the Sun. So if Aristarchus accepted this idea, then it was only a small step further to suggest that Earth might also orbit the Sun.
As to why he would have taken that further step, Aristarchus was almost certainly motivated by the fact that a Sun-centered system offered a much simpler explanation for the apparent retrograde motion of the planets than was possible with an Earth-centered system. In addition, he apparently came to the conclusion that the Sun was much larger than Earth, so it probably seemed to him more natural for the larger Sun (rather than the smaller Earth) to be at the center of motion.
Aristarchus gained some support among his contemporaries (for example, some evidence indicates that the mathematician Archimedes agreed with him), but most of them rejected his ideas. Again, the full story of why is lost to time, but many probably disagreed with this conclusion that the Sun was larger than Earth, and as discussed in this section, the lack of detectable stellar parallax also played a significant role. (Aristarchus himself was apparently untroubled by the lack of observable parallax, probably because he was comfortable with the idea that the stars might be too far away for it to be detectable.)
Fortunately, Aristarchus’s ideas never died, and Copernicus was aware of them when he proposed his own version of the Sun-centered system. Thus, our modern understanding of the universe owes a debt to the remarkable vision of a man born more than 2,300 years ago.