George Parker, Earl of Macclesfield, was the astronomer-president of the Royal Society who helped steer Britain’s adoption of the Gregorian calendar. His Remarks upon the Solar and the Lunar Years lays out the underlying arithmetic — the 19-year cycle, the Golden Number, the epact, and the finding of Easter — as presented in the run-up to the reform of 1752.
When Britain finally dropped eleven days from September, this was the mathematics that justified it. Download the free EPUB for the calculation behind one of history’s oddest bureaucratic acts.