Johannes —, German scientist born in 1571 regarded as one of the fathers of modern astronomy (6)
I believe the answer is:
kepler
(Other definitions for kepler that I've seen before include "German astronomer who first stated the laws of the movements of planets" , "Johannes ---, found laws of motion" , "Johannes -, sixteenth and seventeenth century German astronomer" , "German astronomer who first stated the laws of planetary motion" .)