The orbit of Iapetus, the outer satellite of Saturn

This satellite of Saturn was discovered by Jean Dominique Cassini, at Paris, October 25, 1671. The discovery was made with a small glass of 17 feet focal length. Cassini soon discovered the variation in the brightness of this satellite when it is near its conjunctions, and his explanation of this variation has been generally adopted. It is assumed that opposite sides of the satellite have different reflecting powers, and that Iapetus, as is the case with our moon, turns once on its axis during a revolution round its primary. The satellite disappeared entirely in Cassini's telescopes when it was on the following side of the planet.