In 1612 the sun's mean sidereal rotation rate was only 13.3 degrees per day based on a series of 199 unpublished drawings by Harriot. By comparison with the rates in 1625 to 1626 and 1642 to 1644 it appears that the solar rotation was accelerating significantly in the cycles leading up to the Maunder minimum.
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Richard B. Herr (1978) studied this question.
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