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Middle: (Camera follows Earth keeping Sun always behind it.) Demonstrates how the inclination of the moon's orbit precludes eclipses most of the time, leading to distinct eclipse seasons. Bottom left: (Camera follows Earth keeping its initial orientation, “Sun passes Earth“ once per year.) Shows that the periodicity and recurrence of eclipses is governed by the Saros cycle, a period of approximately 6,585.3 days (18 years 11 days 8 hours). It was known to the Chaldeans as a period when lunar eclipses seem to repeat themselves, but the cycle is applicable to solar eclipses as well. Bottom right: View from the top shows how the moon orbital elements (argument of perigee PE and longitude of ascending node AN) change with time, returning to a similar configuration after one saros, like clockwork. One saros period after an eclipse, the Sun, Earth, and Moon return to approximately the same relative geometry, a near straight line, and a nearly identical eclipse will occur, in what is referred to as an eclipse cycle. Blue line: Earths orbit Grey line: Moon orbit Green line: Line of nodes Yellow line: Line to perigee (astronomy)
