Overview
The Saros is a period of roughly 18 years, 11 days and 8 hours after which the Sun, Earth and Moon return to very similar relative geometry, producing a repetition of solar or lunar eclipses with comparable characteristics. Because of the extra one-third of a day, each recurrence is visible about 120 degrees west in longitude from the previous one.

How it works

The Saros is based on near-integer relationships between three lunar cycles. One Saros is equal to 223 synodic months (new Moon to new Moon), and closely equals 242 draconic months (node to node) and 239 anomalistic months (perigee to perigee). These coincidences mean that after one Saros the Moon is at a similar phase, at nearly the same node of its orbit, and at a similar distance from Earth — the conditions needed for similar eclipses.

Series and evolution
Each Saros series is a family of eclipses that begins with a sequence of partial events near one of Earth's polar regions, evolves into central (total, annular or hybrid) eclipses as the geometry shifts, and ends centuries later as partial eclipses near the opposite pole. A typical Saros series spans many centuries and includes on the order of 70 or so eclipses.

History and usage

Patterns equivalent to the Saros were recognized by ancient astronomers, notably in Mesopotamia, who used them to predict eclipses. Modern astronomy adopted the concept as a basic organizing principle for eclipse catalogs. Observers and researchers use Saros numbering to group and compare eclipses and to anticipate when a similar eclipse might recur.

Notable facts and distinctions

  • Solar and lunar eclipses each have Saros series; corresponding solar and lunar Saros cycles are separate but related.
  • The extra one-third day shifts each successive eclipse westward by about 120° longitude.
  • A half-Saros (about 9 years and 5.5 days) links certain lunar and solar eclipses separated by opposite node geometry.

Because the Saros is an approximation — not a perfect integer relation — successive eclipses in a series change slowly in latitude, duration and type. Nevertheless the Saros remains one of the most useful long-term patterns for organizing and predicting eclipses.