Kepler was a German astronomer, most famous for his Laws of Planetary Motion. He added valuable information to Copernicus' heliocentric theory: the sun pulls the planets into non-circular (elliptical) orbits.
Newton was an English mathematician and physicist. He is known for discovering the Three Laws of Motion, as well as for the invention of calculus. He wrote "Philosophiae Naturalis Principia Mathematica" in which he describes universal gravitation and planetary motion.