11. How far back?

The Almagest logs nearly one hundred observations to which a precise date can be ascribed. The most ancient observations are from Babylon - three eclipses of the moon in the years 721 BC and 720 BC.

Although Ptolemy possessed Babylonian observations of older eclipses, he notes the triplet of 721/20 was the oldest recorded in an unambiguous way. Moreover, they conform to calculations based on an eclipse triplet that Ptolemy had personally observed.