This animation is of the Copernican system. In this system the planets orbit the sun in perfect circles. An important ingredient of the success of the Ptolemaic system was that it accounted for apparent phenomena such as retrograde motion. The Copernican system accounted for observational data in a much more elegant manner. The second half of this animation shows the apparent backward motion of the outer planets.
Galileo’s experiment showing uniform acceleration was an important step toward understanding physical laws. It showed that the weight of an object did not alter the rate of gravitational acceleration.
Johannes Kepler was a German mathematician and astronomer. His major contribution to the feild of astronomy came when he discovered the elliptical nature of celestial orbits. Kepler’s three eponymous laws provided mathematical evidence for profound scientific claims about planetary motion.
Kepler’s Second Law states that planets have elliptical orbits.
Kepler’s Third Law establishes a relationship between distance and orbital period.