Feynman's lectures on physics
three volume book, 1963
www.feynmanlectures.caltech.edu
Numerical approach to the proof of Kepler's laws
Chapter 9: Newton's Laws of Dynamics, Section 9-7: Planetary motions
Feynman, R., Leighton, R. and Sands, M. The Feynman Lectures on Physics, 2nd ed., Volume 1, Addison Wesley, USA, 2005. (pages from 9-6 to 9-9)