Image: Newtons proof of Keplers second law

Description: Isaac Newton's proof of Kepler's second law of planetary motion. At left, a large yellow disk represents a star, and at right, the smaller blue disk represents a planet. The blue arrow represents the planet's instantaneous orbital velocity. Assuming no force acting upon it, it will move in a straight line at a fixed rate. After some fixed amount of time, it will have reached a new position. Then, at this position, an instantaneous centripetal force (red arrow) is acted upon the planet towards the star, altering its path as shown with the addition of the blue and red arrows (violet arrow). The planet is shown traversing this new path, as well as a "shadow" (faint, gray disk) of the planet describing its trajectory during the same amount o