It's been a windy weekend here. David is referring to the Saros 6585 days. [www.skyandtelescope.com]
You know it has always made me wonder just how the ancients could have spent 18 years or so studying this, god they must have been doing it for a very long time to come up with these numbers.
What follows is that they kept records and this is something we don't really have a handle on. You don't study something for 18 years and then out of the blue not keep a record.? But suddenly then come up with a number 6585.???
They kept the records