Pre-Kepler: Epicycles And Geocentricism
All those odd years ago, a theory called geocentrism existed, that the Earth is the center of the universe and that everything, including the sun, orbits it. This stemmed from the general human feeling that, of course, our species must be the center of it all; this was later expressed in statements in the Bible, which for centuries was taken as scientific truth.
Of course, the scientists had difficult reconciling canon with what they were observing. Over-elaborate models for planetary motion included some movements that today are most likely to be found on a roller coaster.
Eventually, with much resistance by the religious institutions of the day, geocentricism was overtaken by heliocentricism through the work of astronomers like Copernicus and Galileo. Though this theory simplified models of planetary motion considerably, it still didn't explain a critical kink: a short backwards movement in the orbit of every planet - called retrograde motion, the usual forward motion being known as prograde. This was explained through an older Greek theory that called for epicyclic movement of the planets, or a short circle within the greater circle within the orbit like a loop-to-loop.
This was later to be unraveled to really just be another consequence of our perspective from here on Earth: since we are not, actually, at the center of the universe, nor even our solar system, we are observing the movement of the planets from a skewed angle.
However, serious problems still existed with the theories of planetary motion. Everything was assumed to move in circles, a belief that stemmed from the thought that celestial motion must be perfect, and thus in the clean smooth lines of a perfect circle.
It never really occurred to anyone that all those little discrepancies between observation and theory that they were still finding—planets moving too quickly here, or too slowly there—might be due to non-circular motion. Until Kepler, that is.