Probably the most impressive pre-telescope discovery to me is axial precession. It is a 26,000 year long cycle. There are very few "events" to notice that would indicate something is even happening, and those things that you could notice are so gradual that even in an entire human lifetime, the change would be minuscule. With things like predicting eclipses, it's pretty obvious that you're going to notice an eclipse when it happens, and very quickly start to want to predict how they work.

WIth axial precession, there's the two possible states of "having a pole star" and "not having a pole star". People also noticed that stars that never dipped below the horizon in the past now would, and vice versa, or that stars that had never been visible at certain latitudes now were, and vice versa. But it's only a change of about 1 degree every 75 years. Truly impressive.