The Quanzhou macrocycle, whose nadir was reached ca. 1500......one of the coldest periods in Chinese history occurred during the middle decades of the seventeenth century --near the nadir of the Maunder solar minimum.
GWS is very big on cycles (one of three 'kinds' of Time, which can be said to go (1) back and forth [think tides? commuting?], (2) round and round [annual, dynastic, etc.], and (3) on and on [evolution? one's own life?]), and so needs language to describe points in a cycle.
So how can we investigate this word? The OED is one possibility, and tells us
What other ways to investigate? An AltaVista search gets 62,000+ hits... too many... "nadir NEAR cycle" gets 351, a bunch of them to do with satellites... and "nadir NEAR historical" gets 35 hits, none all that revealing.