A long time ago, in a galaxy far far away……too much of Star Wars (inside joke). I bet most of us in the entire world probably heard of Copernicus. However, only little of us knew about Aristarchus of Samos, ( Ἀρίσταρχος ὁ Σάμιος, Aristarkhos ho Samios-his name in Greek) an ancient Greek astronomer and a mathematician I would say who first proposed a ‘Sun-Centered Solar System’ eighteen centuries before Copernicus.WOW! ( I can see the face of the enthusiasm among all of you).
However, the Greeks rejected his astronomical ideas that the Earth goes around the Sun especially from the two famous astronomers: Aristotle and Ptolemy, which are now known to be incorrect. Do you know why did they reject the real explanation for planetary motion?
Although there were many reasons the Greeks were reluctant to abandon their original idea (Earth-centered universe), the most ultimate reason was their inability to detect stellar parallax. Stellar parallax is a parallax on an interstellar scale, means that there is slight apparent shifts in stellar positions over a course of the year. Since stellar parallax is only detectable with telescopes, his accurate astronomical theory was unprovable at the time. Besides, his theory did not gain wide acceptance until almost 2000 years later.To know more about the stellar parallax, there are few links that I shared in this post.
Since there is no observable stellar parallax, he suggested that the Stars were extremely very far away.Why? This is because parallax is a function of distance. In other words, distant objects exhibiting smaller parallax than nearer objects. To further strengthen his argument, he estimated the sizes of the Moon and the Sun. He estimated the Moon’s size by observing the shadow of Earth on the Moon during a lunar eclipse and measured the angle between the Moon and the Sun at first and third quarter phases of the Moon. Based on these experiments, he concluded that the Sun must be larger than the Earth hence he believed that Earth should be the one that is orbiting the Sun.
Today, we can detect stellar parallax with the aid of the telescopes. Thus, Aristarchus of Samos was right and we have a direct proof that the Earth really does orbit the Sun.