Changes In Solar Rotation

John A. Eddy has continued his historical studies of sunspots from the earliest records to date. Analysis of sunspot drawings suggest that, between 1625 and 1645, the equatorial velocity of the sun was significantly higher than it was earlier and is now. Eddy believes that this acceleration presaged the onset of the peculiar Maunder Minimum, 1645-1715, when the sun was virtually clear of spots.

(Eddy, John, et al; "Anomalous Solar Rotation in the Early 17th Century," 198:824, 1977.)