Periodic Time is the time taken by any planet to travel once round its orbit. The periodic time of an inferior planet is found by noting the time which elapses between two successive conjunctions, and that of a superior planet by observing two successive oppositions. It was discovered by Kepler that the periodic times of the different planets are connected with their respective distances from the sun, and this relationship he expressed in his third law. [Kepler's Laws.] The fact that the planets move in ellipses under the attraction of the sun makes such a relationship a mathematical necessity, but Kepler found it out by actual comparison of numbers, the mathematics of the subject remaining undiscovered till the time of Newton.