I've searched a little bit and found that I can derive kepler's third law from newton's law of gravitation. That's okay. But I want to deduce kepler's second law too: "An imaginary line joining a planet and the sun sweeps out an equal area of space in equal amounts of time". I know it's possible to do that using angular momentum, and from there proving that angular momentum in this case is constant. But how would I do that?