Ptolemeic and Copernican Models of the Solar System -- A Brief History

So, in my previous blog post I touched very briefly on what the ecliptic is and mentioned that the Sun and the planets move along the ecliptic.  Which led me to start rambling on about the evolution of Solar System Models, from Ptolemy to Brahe and Copernicus.  Which I realized was irrelevant to the post, so snipped all that discussion out and put it here instead!

So, the Sun moves along the ecliptic.  As do the planets... And the stars move around a fixed point over the course of a night, and rise and set.  You know nothing of gravity, but notice everything appears to revolve around us... And you therefore assume that we're the center of the solar system, because everything moves around us.  Rather like Ptolemy did.  This is the Ptolemeic or geo-centric model of the Solar System, where everything revolves around the Earth.

But there's a problem with it... Most of the time all of the planets chase each other around the sky moving in the same direction, but sometimes they appear to double back on themselves.  So either the planets are doing their own thing, or as was suggested, and incorporated into the geo-centric model, you have the planets themselves to move on smaller ellipses (around an invisible point I guess), as they travel around the Earth -- a bit like this:

And hey, what do you know, the planets chase each other around the sky and sometimes look like they're going backwards.  Woo!  But that seems needlessly complicated, and besides, what are the planets going to revolve around? (That black point at the center of the epicycle is just there to show you the center!)

So then there was an idea espoused by Tycho Brahe, that the Sun orbited the Earth and all the other planets orbited the Sun.  Brahe wasn't big on the Earth not being the center of the Solar System!  This also worked, but well, why's the Earth so special?  Just because we live here?  Its all rather convenient isn't it?  Looked a bit like this (from Wikipedia):

Well, I guess, if the Earth was about the same mass as the Sun then you could make that argument, but um, yeah, it doesn't really work, not if you want something consistent.

Then of course, Johannes Kepler came along, and building on what others had done (including "borrowing" some of Brahe's observations), popularized Copernicus' idea that hey, if you put the Sun at the center of the Solar System, and have all the other planets, including ourselves, revolving around the Sun, we don't need anything really complicated!  And you get a Heliocentric model  Of course, the Moon still orbits us, but we're no longer the center of the Solar System... What a step down. ;)  And its what we now know.  (Well, technically, we orbit around the center of mass, or the barycenter, or the Solar System, which is dominated by, um, the Sun, and the Sun always stays pretty close to the barycenter, but wobbles around quite a bit because of the planets).  Kepler also had the pretty fantastic idea of letting the planets move around on elliptical orbits rather than circular ones (Copernicus prefered circular orbits, so the planets still needed epicycles!), and did away with the idea of epicycles.  Basically getting the picture of the Solar System as we know it now. :)