How a 2,000-year-old bronze machine predicted eclipses with the number 223

In 1901, divers pulled a corroded bronze machine from a shipwreck off the Greek island of Antikythera. One dial ended in the number 223, written in Greek letters.

That number is the Saros: after 223 new Moons, the Moon is back at the same node of its orbit and the same eclipse returns. Slide the gap yourself and see why 223 months, and almost nothing else, resets the sky.