24. A Saros Mystery
A Saros period is 18 years plus about 10 days. Hence the year of an eclipse in a Saros cycle can generally be obtained by adding 18 to the year of the preceding eclipse. On the rare occasion that an eclipse occurs within 10 days of the end of a year, 19 must be added to arrive at the year of the next eclipse.
Now to the denouement: The last entry of the Saros Tablet is a year that draws to a close with an eclipse. Historians propose that this eclipse marks the finale of an 18-year sequence, and the next entry would require a 19-year jump. In effect, after mulling over the problem for a century, historians settled on a particular Saros cycle and declaredThe Mystery of the Saros Tablet solved.
25. Saros Innards
In addition to tracking a Saros cycle from one eclipse to the next, the Babylonians worked out the internal structure of the period. The 18 years interval, or more precisely 223 months, allows for exactly 38 eclipses possibilities, 33 spaced 6 months apart and 5 separated by 5 months.
The Babylonians organized this scheme in tables of Saros cycles that begin as early as 747 BC. They continued mapping the Saros scheme until the 1st century AD.
27. Grand Saros box score
The primary texts buttressing the Grand Saros (No. 2, 3 & 4 in Lunar and Planetary Texts) list 52 entries that match modern computations and 4 that may be wrongly filed.
Checking all the entries in the eclipse texts, no further mismatches crop up in the other 93 records from the period of the Grand Saros (-746 to -314).
28. Many eclipses, many Saros
A Saros cycle generally peters out after a few centuries and new cycles come into being. Moreover, not all eclipses of a Saros cycle are visible. The changing pattern of eclipses led Babylonian astronomers to organize their observation in a multiplicity of Saros cycle configurations.
Still, the Grand Saros Canon is noteworthy. Except for two low magnitude lunar eclipses that Babylonian astronomers may have missed, the Grand Canon correctly predicts every eclipse possibility over 400 years, from -746 to -314.
A document akin to the Grand Saros may have been source of Ptolemy's eclipse data from Babylon.