Kepler’s New Astronomy: The ‘Apparent’ Sun

Kepler concludes Part I of his New Astronomy by contrasting the "mean sun" of Copernicus and Tycho Brahe (which is really related to the Earth's orbit) with the "apparent sun" of Kepler (the actual Sun itself). Kepler proves that if the actual Sun is used to make an astronomical model (which Copernicus did *not do), it can be tested to be fundamentally different than those that use the "mean" sun.