Does this mean that all of astronomy is relative, that while the numbers must be right, there is no real way to know whether the planets go around the sun, or vice versa? The thought didn’t trouble most thinkers of Kepler’s day: technical astronomy, involving the calculation of observed planetary positions in the sky, was a branch of mathematics, while cosmography, the theorizing of what might physically be taking place out there, was a distinct discipline. The job of astronomers was to calculate where spots in the sky would appear, but a physical theory was simply not part of that endeavor.
Kepler had long been convinced that the way to resolve the conundrum, and to show the truth of Copernicus’s outlook, was to determine the physical causes of the motion of the planets, and he had thought since his school days that the sun itself caused the motions, instead of just sitting near the center watching them like a couch potato, as Copernicus had it do.
To show the importance of the sun, Kepler proved that there was a difference that was not equivalent, a difference that was not just a matter of opinion. He calculated the difference in the orbit of Mars if the actual sun were used to determine its characteristics, rather than the center of the earth’s orbit. The difference was over a degree, which was definitely observable. Having proved that using the real sun makes an observationally different model, Kepler concludes Part I of his Astronomia Nova, ready to create the first truly sun-centered astronomical model, intent on proving the true role of the sun in the planetary system.