What distinguished the work of Johannes Kepler from his predecessors, both ancient and contemporary, was his method. Unlike Ptolemy, Copernicus or Brahe, he employed a very different method in his study of the heavens; one that allowed him to develop a more truthful understanding of the motions of the planetary bodies.
Whereas others simply focused upon the bodies themselves as they moved through space, Kepler stood back, and sought instead to account for their apparent or sense perceived motion set against the Celestial Sphere, through discovery of the underlying causal principles responsible for truly determining their physical action. For this reason his work differed in a true qualitative sense from that of any other.
Indeed, what he understood, that his colleagues throughout the ages did not, was that developing a series of mathematical functions and ‘hanging them’ upon a changing event in the physical world does not lead to knowledge, for it is blind to causes. Knowledge itself is attained only when one discovers the underlying causal principle behind what is observed by the senses. Moreover, the high predictive accuracy that one undoubtedly strives for through study of such bodies as the planets is attained almost naturally as a secondary outcome following on from the initial discovery of the lawful principles found to underlie their activity.
With this firmly understood, Kepler was able to far surpass the work of all those who had gone before him by discovering a set of truly universal physical principles responsible for generating the non-uniform motion of the planets. Indeed, as a direct result of this he was able to falsify the very existence of epicyclical orbits; discarding them altogether.
In addition to even this though, he went on yet further to discover a unique set of harmonic intervals physically operative within the whole solar system, that unified in a most exacting manner all of the known planets about the sun in a way that had never been done before. Today, the primary discoveries of Johannes Kepler are by convention grouped together into 3 Laws of Planetary Motion: