What is a Saros Cycle?

A Saros Cycle is approximately 6585.3211 days, or 18 years, 11 days, 8 hours in length. 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. The Moon will have the same phase and be at the same node and the same distance from the Earth. In addition, because the saros is close to 18 years in length (about 11 days longer), Earth will be nearly the same distance from the sun, and tilted to it in nearly the same orientation (same season). Given the date of an eclipse, one saros later a nearly identical eclipse can be predicted. Each total solar eclipse track looks similar to the previous one, but is shifted by 120 degrees westward.  The August 21, 2017 total solar eclipse is part of the Saros 145 series. The previous total solar eclipse in this series occurred on August 11, 1999. The next one will be on September 2, 2035. The first cycle in this series occurred on January 4, 1639, and the last one will be on April 17, 3009.