When you assay a situation, you look at all the elements that created the problem in order to come up with a solution. Whatever else a philosophical explanation might do, it must account for the way things seem like to us. We should expect science to not just save the phenomena, but also to provide us with the truth of how it came about and explain all the mechanisms involved in the production of the phenomena. It should tell us a true story about any and all unobservable entities that are not detected by the human eye.
Sometime between 1508 and 1514, Nicolaus Copernicus (1473-1543) a Polish mathematician and astronomer ushered in the age of science by developing his own celestial model, a heliocentric or “Sun-centered” planetary system. Copernicus proposed that the Sun was at the center of the universe and he backed up his theory with observations and trigonometric calculations. Copernicus’s theory was the first hint that perhaps the nature of reality depended on the position of the observer. He thought that all the planets’ orbits should be plotted with respect to a fixed point being the Sun, and that the Earth itself is a planet that turns once daily on its own axis, and that when the axis changes directions with respect to the stars this will account for the precession of the equinoxes, thus causing the North Pole star to change over time. Copernicus was not the first to come up with this idea, as Aristarchus of Samos maintained that Earth rotated on its axis and revolved around the Sun, but his idea never caught on, because others in his day judged that it was physically impossible for the Earth to move.
Plato understood that the Earth had to be a sphere since its shadow was always circular and that the planets had to be spheres, because they were in the heavens and that was the most perfect solid. Plato felt that the heavens were perfect, so the planets could only move in continuous uniform circular motion. There were problems with the model of motion of the moon, planets, Sun and stars that Plato conceived, so he gave his students a major problem to work on. Their task was to find a geometric explanation for the apparent motion of the planets, especially the strange retrograde motion. It had been observed that as a planet undergoes retrograde motion (drifts Westward with respect to the stars), it becomes brighter. At periodic times, the planets seemed to reverse their direction of motion around the Earth, which is technically known as retrograde motion. Planets sometimes seemed brighter and sometimes less bright, which was interpreted to mean that they are sometimes closer and sometimes further from the Earth. The solar seasons were not quite equal, as should be expected for perfectly circular motion. It seemed that the Sun was speeding up and slowing down at various points in its orbits. This project became known as “saving the phenomena” and it dominated cosmology until the time of Newton.
Aristotle who studded under Plato created a model of the universe, with planets moving in perfect circles around the Earth, but it did not account for real planetary motions, because he made the wrong assumptions that the Earth was the center of the Universe, and that the heavens exhibited uniform circular motion. Ptolemy and others explained uniform circular motion using a system called epicycles, which had the planets moving in little circles within their greater orbits. Copernicus knew that some things didn’t make sense, like when Mars occasionally appeared to move backward with respect to the stars before moving forward again. He was disturbed by the failure of Ptolemy’s geocentric model of the universe to follow Aristotle’s requirement for the uniform circular motion of all celestial bodies and he became determined to eliminate Ptolemy’s concept, concerning an imaginary point around which the bodies seemed to follow that requirement.
Copernicus decided that he could achieve his goal only through a heliocentric model, which remained simple and was more elegant, but it still required perfect circular motion and it did include the added complexity of epicycles to try and explain retrograde motion. Copernicus presented his sun-centered model of the heavens where Earth is depicted as the third planet orbiting the Sun. Copernicus’ system was able to explain both the varying brightness of the planets and retrograde motion. The brightness of the planets would naturally vary because the planets were not always the same distance from the Earth and the retrograde motion was explained in terms of geometry and a faster motion for planets with smaller orbits. Copernicus anticipated that his ideas would be controversial, so he waited more than 30 years to publish his book and only did this when he was on his deathbed.