though the wisdom of nature can reason it thus and thus . . . : though science can explain it [an eclipse] thus and thus . . . —The phrase, "the wisdom of nature" refers to what we now call "science," and what was called "natural philosophy" in Shakespeare's time. Gloucester's point is that despite the ability of natural philosophers to explain just how an eclipse happens, eclipses are still portents, and are always followed by disasters of one kind or another.