Starting with Copernicus, this video explains the heliocentric model – especially Kepler’s contribution to it – in some detail. Kepler’s third law of planetary motion can be verified with an accuracy of about 2 percent by timing the period of 5 planets with a stop watch and reading their major axes on a “cosmic ruler” provided in the movie. This table can be helpful in writing the data collected from the movie. Once the (average) constant of the third law is calculated, it is used to find the major axis of a comet whose period is measured using the stop watch. A sketch of comet’s orbit and that of the gray planet are shown here for comparison.