There are 6,585 days between one solar eclipse and another, which is 18 years, 11 days, and 8 hours. This has been known, observed, and calculated for many thousands of years, but was probably first chronicled in ancient Babylon (in Mesopotamia, in modern-day Iraq). It would later be disseminated by the Greeks as the Saros Cycle.
Excerpted from: Rogerson, Barnaby. Rogerson’s Book of Numbers: The Culture of Numbers–from 1,001 Nights to the Seven Wonders of the World. New York: Picador, 2013.