What did thales use to predict a solar eclipse?
Thales lived around 600 BC, so the historical details are hazy; not only is it not known how he predicted the eclipse, it is not even known with certainty whether he made this prediction! Therefore, we can only speculate. There are various cycles of eclipses, and some of these were known to the ancient Babylonians. The most famous eclipse cycle is the “Saros”, which has a period of 223 lunations (i.e., counting 223 new moons following one solar eclipse). This is about 18 years and 11 days. If you see a solar eclipse, there will be very similar solar eclipse 223 moons later, but the next eclipse will be shifted about a third of the way around the world to the west (so you may or may not see it). This pattern isn’t obvious at first, but a society that keeps careful records of eclipses for many decades will probably eventually discover the Saros cycle. (This isn’t quite as hard as it sounds. If you watch the sky carefully, you’ll see that solar eclipses happen only at the new moon, and ec
