The cycle of Saros is the most famous known periodicity of the eclipses. It is well known in fact that the solar eclipses are repeated in very similar way every 223 lunations, equal to 6585 days and 8 hours; this shift the region of visibility approximately of 120░ in longitude on the terrestrial globe. Examples 11/07/1991 - 22/07/2009 Saros: a cycle of Saros contains from 69 to 86 eclipses and lasts from 1226 to 1532 years. It has a beginning and an end characterized by partial eclipse and the first ones they are always located near the poles. But the cycle of Saros is not the only one. Said T the duration of a cycle, S the duration of a Saros and I that of an Inex (you see under), exist cycles with superior and inferior duration. Inex: it is equal to 388.5 draconic revolutions, from node to node; because of the 12 hours of discard the eclipses of an Inex alternately happen in the two hemispheres. Semester: lasts exactly 6 lunazionis from which the name. As in the Inex the eclipses are alternate because of the change of node from one to the other. A semester generally contains 7-8 eclipses. Hepton: the interval is of 41 lunations, and also in this case they are alternate. It usually contains 13-14 eclipses and lasts 40-43 years. Tritos: it has a period of 11 year minus one month. It contains about sixty eclipse and it lasts many centuries. The series are still alternated. Metonic cycle: it is well known. It lasts 19 years and the lunar phases almost repeat in the same date of calendar. Octon: it is a fifth exact of the metonic cycle. Tetradia : to see tetrads; it has a period of 586 years.