What does it mean to say Kepler's laws are empiracle in nature?
We can see them simply by observation, not by mathsy magic.
For something to be empirical means you can just see it happening around you, you don't have to figure out why it's happening or what's going on or deduce it with a load of equations.
Something being rational, on the other hand, means it is something deduced or figured out.
For example, a glass of water being transparent is empirical. The orbit of planets around a star is empirical.