The next question was what makes them go around? Or how can we understand this in more detail? Or is there anything else to say? In the meantime, Galileo was investigating the laws of motion. Incidentally at the time of Kepler, the problem of what drove the planets around the sun was answered by some people by saying that there were angels behind here, beating their wings and pushing the planet along around the orbit. As we'll see that answer is not very far from the truth the only difference is that the angels sit in a different direction, and their wings go this way.. But the point that the angels sit in a different direction is the one that I must now come to. Galileo in studying the laws of motion and doing a number of experiments such as seeing how balls rolled down incline planets and how pendulums swung. and so on. Discovered an idealization, a great pinciple called the principle of inertia, which is this that if a thing has nothing acting on it, if an o...