Wonder if he also accounted for the effect of the earth being closer/farther to Io having an additional/less pull by R^2
Would likely be miniscule-but the calculations they used to do were amazing considering they had no computers.
They had the ultimate computer, the human mind..................
It was timing, apparently, not computing power that was important here. The pendulum clock was invented in the mid 17th century, and was unrivaled in accuracy until the advent of the atomic clock. In any case, a computer is only as smart as the person who programs it. Mathematically, Isaac Newton could do with quill and parchment what any modern supercomputer can do. It would just take a lot longer.