Eddy mentions many of the noted astronomers of the day. Reports of sunspots seen without visual aid can be traced back to 28 B.C. and before. These sightings were made when the sun was obscured by heavy haze or smoke, or was low in the sky during early evening or morning hours. As naked-eye sightings are relatively rare, the likelihood of their discovery is statistically greatest during times of high sunspot activity. Although a number were observed prior to, and later than the Maunder Minimum, apparently none were recorded during the period from 1639 to 1720.
Additional evidence comes from a generally unrelated field.
It concerns the abundance of carbon 14 in tree rings. For technical reasons, we would expect to find a relative abundance of the element when solar activity has been at low levels. During the period of Maunder's minimum this is exactly the case. In fact, in his extensive analysis, Eddy found virtually no evidence to dispute the minimum. Thus it appears that the sun did undergo major changes, with possible terrestrial effects, during the 17th century.