The period of 223 → synodic month, equaling 6585.32 days or 18 years, 11.33 days, after which the Sun, Earth, and Moon return to approximately the same relative geometry. When two eclipses are separated by a period of one Saros, they occur at the same node with the Moon at nearly the same distance from Earth and at the same time of year. Thus, the Saros is a useful tool for organizing eclipses into families or series. Each series typically lasts 12 or 13 centuries and contains 70 or more eclipses (F. Espenak, NASA).
Gk. saros, from Akkadian shār, Sumerian shar "multitude, large number." The ancient astronomers knew the Saros cycle, but they did not use the term Saros. In the Almagest, Ptolemy refers to the Saros as the "periodic time" (periodikos chronos) and gives it the following properties: 223 → synodic months = 239 → anomalistic months = 242 → draconistic months = 6,585 1/3 days = 241 revolutions in longitude plus 10 2/3 degrees. Edmund Halley seems to have been the first to apply this term to an eclipse cycle, in 1691.