Aristarchus & His Crazy Idea

So here's an easy question. Who first theorized the heliocentric model, the theory that the Earth revolves around the Sun instead of the other way around? Now I think most of us would say Copernicus, or maybe Galileo, who first came up with some concrete evidence. But before Galileo observed the phases of Venus, before Copernicus published On the Revolutions of the Celestial Spheres in 1543, even before Ptolemy suggested that the planets moved around the Earth in tiny epicycles, there was a man called Aristarchus of Samos who asked a very basic question. What if we're not the center of the universe? We don't know that much about Aristarchus. We do know that he lived at the Library of Alexandria a couple of generations after Aristotle, but none of his original writings on his heliocentric model survive today. Instead, we hear about his theories through another source, specifically Archimedes' book, The Sand Reckoner, where he writes, His hypotheses are that the fixed stars and that the Sun remain unmoved, that the Earth revolves above the Sun on the circumference of a circle. That's about all we know. We don't know why he thought that the Earth moves around the Sun, and we don't know what evidence he might have had, because none of his original texts survive to today. We do have one writing by Aristarchus, though, in which he tries to determine the relative distances between the Sun and the Moon. Aristarchus is a really smart guy, and he guessed that by observing the half-moon, when the Moon is on the opposite side of the Earth and the Sun, and the angles between them, he would be able to guess the relative distance between the Sun and the Moon. So he gave it a try, and he concluded that the Sun was probably 20 times further away from the Earth than the Moon. Was he right? Well, no. The Sun is actually about 400 times further away from the Earth than the Moon. Aristarchus was measuring with the naked eye, so he thought that there was about an 87-degree difference between the two of them, when actually, it's 89 degrees and 50 minutes. Even though his numbers were wrong, Aristarchus' work was incredibly important. People didn't realize back then just how big the universe was. You see, one of the reasons the geocentric model lasted for so long was that people figured that if the Earth was moving around the Sun, the stars should appear to move. So if it was summer, and you were looking at the stars from here, they would look different from when you looked at them from here, in winter. This phenomenon is called parallax, and it does happen, but we can only see it with our advanced telescopes, because they are too far away to appear to move. So, how do scientists measure distance from stars today? Well, Aristarchus' methods still apply. By measuring parallax, that phenomenon where the stars look different from one angle than when you're viewing them from another, scientists can measure distance in parsecs, which stands for the parallax of one arcsecond. We can do this today because we're using telescopes instead of the naked eye. And that's what's so important about what Aristarchus did. He started to realize how huge the universe really is. Of course, no one believed what he was saying, because we didn't have telescopes and couldn't see parallax. It wasn't until the 1500s that Copernicus came up with his theories, and Galileo provided some evidence to confirm them, that we finally gave Aristarchus the credit he deserved as a visionary thinker who was right all along.