The Laws of Planetary Motion

By the early 1500s, the Ptolemaic and Copernican systems were now both on the table. But the two systems were largely equivalent in their ability to predict the locations of the planets. Galileo’s observations of the phases of venus provided evidence that the sun may indeed be at the center of our solar system, but a complete scientific description of planetary orbits had not yet been developed. This breakthrough came with new observations (as is so often the case in science). Watch the excellent video Planetary Motion, which tells the story of Tycho Brahe and Johannes Kepler - the unlikely pair that is responsible for our modern understanding of planetary motion.
Tycho built the best naked-eye observatory in history to precisely measure the positions of the planets. But he was an arrogant man who tried to use this data to legitimize his own (incorrect) model of the solar system. Ultimately, Tycho’s model couldn’t predict the positions of the planets significantly better than the Ptolemaic or Copernican Systems - they all still used circular orbits. Kepler, however, was able to take Tycho’s incredible data and develop an innovative new model that used elliptical orbits instead of circular ones.
Read Section 3.1: The Laws of Planetary Motion from our textbook.
Watch the video Kepler’s Laws, which demonstrates all three of Kepler’s Laws using a really nice simulation. If you like, you can experiment with this Planetary Orbit Simulator for yourself.
You can see Kepler’s Law in action by creating your own solar system using this simple Gravity Simulator. (Here’s a quick instructor video showing how to use it.)
According to Kepler’s Laws, there is a simple mathematical relationship between the size of a planet’s orbit and the average speed at which it travels around the sun. This confirmed his long-held belief that there must be a force in the sun that drives the planets. A force that is stronger for the inner fast-moving planets and weaker for the outer slow-moving planets. Isaac Newton later identified that force as gravity, answering the fundamental question, “What makes the planets go?”