Kepler’s Laws of Gravitation – Law 2
So now you understand that planetary orbits are ellipses.
Kepler's First Law
So what more can be added to this. We now know the shape in which planets move around stars, but what does their motion look like?
To answer this, we need an understanding of the Conservation of Angular Momentum. This conservation law which is responsible for governing planetary motions is responsible for actions we see on Earth. For example, when an ice skater spins, they tuck their arms in to spin faster. Why is that?
As with before, let’s look at the math
http://csep10.phys.utk.edu/astr161/lect/solarsys/angmom.html
So to have a planet change a parameter such as distance from a star, one of two other parameters, either mass or velocity must change. Since the mass of a planet does not seem like a likely parameter to change constantly, we can hypothesize that the velocity of a planet will change as its distance from a star changes.
What Kepler found out was that a planet sweeps out equal areas in equal times as it orbits around a star. This is explained perfectly by using the Conservation of Angular Momentum.
