Lavanya Meesa
We can talk right from the earliest cavemen, who looked up to the sky and wondered what was up there.
But for the sake of brevity, Aristotle was a major proponent of the idea of a so-called “ Geo-centric universe”.
That is, the Moon, Mercury, Sun, and all the planets all were revolving around the Earth, with the Earth at the center of the Universe.
Greeks thought that Earth was round by observing lunar eclipses.
Anyway, it turns out that the idea that the Earth was at the center of the Universe was not 100% shared among all Greek thinkers.
Another Observer Aristarchus of Samos proposed that the sun was the center of the cosmos and everything revolved.
He published his ideas in something called on the sizes and distances and he used lunar eclipses to work out the relative distances and sizes of the Earth, Sun, and Moon.
He also proposed that the stars were distant suns and that the universe was vast. However, his idea that the sun might be at the center of the universe was largely rejected.
The reasoning went something like this: “ If the stars were at varying distances from the sun, and if the sun were at the center, and earth around in an orbit, from one location we should be able to look at a relatively nearby star and see its apparent image projected onto the background stars, and that means 6 months later, we should be able to make a similar observation and see that same star, this time with its position shifted with respect to the background stars”.
So this phenomenon is called parallax, and it’s a phenomenon that everyone is familiar with at all times.
Greeks decided to test this idea at the time by enlisting their best visually acute individuals.
That even pulled soldiers from their ranks who specialized in scouting. And they looked at the brightest stars hoping to detect parallax.
Unfortunately, No parallax could be detected and therefore the Greeks just largely gave up on the idea of heliocentricism in favor of Geocentricism.
Eratosthenes of Cyrene
He measured Earth’s circumference.
He actually made the first calculation of Earth’s circumference, and after a lot of effort, he remarkably got accurate results using just some simple geometry and reasoning.
Hipparchus of Nicaea
He was involved in making a detailed catalog of stars and also measuring the first brightness system, something we call the apparent magnitude.
He cataloged the stars by their brightness. He also proposed the idea of Earth’s precession cycle.
Claudius Ptolemy: In the Next millennium, Ptolemy introduced a new concept called Epicycle which revolved around something called a deferent.
And it would be this epicycle moving about the deferent that would create the apparent retrograde motion.
So you can imagine yourself looking at mars and it appears to be going in prograde motion before executing a slight zig-zag back and forth in the sky, giving us retrograde motion.
It was a good first-order approximation, but the problem though is that the Epicycle that we see here just would not be accurate enough to predict the next retrograde motions would be.
Solution by Ptolemy
To solve this problem, Ptolemy introduced a modification to his ideas.
He introduced an Equant, that is, the Earth is still very much at the center of the cosmos, but the Epicycle, the deferent, and so forth now were centered on an offset point called an equant.
And this is what helped to make the retrograde motion of mars and all the planets a little bit more on time, and do a slightly better job of predicting exactly when these retrogrades would occur.
Even then, sometimes additional modifications could be required, not the least of which was adding an additional Epicycle.
Things got a little bit complex over time and this was a major problem because while the epicycle model did an extremely good of predicting retrograde motions for about 1500 years it was at the same time a little bit messy and it allowed some people to begin to think maybe there were alternatives maybe some of these ideas of the geocentric model should
