After 223 synodic months the Sun–Earth–Moon geometry almost repeats — a similar eclipse returns.
Symbols
Tsyn
synodic month
[d]
Where it comes from
It is a shared "near-multiple" of the synodic, draconic (nodal) and anomalistic months. Each successive eclipse in the series is shifted by about 1/3 of a day, i.e. about 120° of longitude.
When to use it
For predicting eclipses, grouping them into Saros series, and explaining the rhythm of their recurrence.