Saros ساروس Sâros (#)
*Fr.: saros*
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 month*s = 239
→ *anomalistic month*s = 242
→ *draconistic month*s =
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. |