![]() ![]() So in 1752 the English rejiggered their calendar by switching New Year’s Day back to January 1st, and then dropping 11 days to line up with the Gregorian calendar. The first page of Inter Gravissimas, the papal bull in which Pope Gregory XIII announced his new calendar in the year MDLXXXII (1582). The English, cranky and anti-Catholic, held out until their Julian calendar was so screwed up that they had to switch. Catholic countries had already fixed this by adopting the new Gregorian calendar in the 1500s. ![]() That may not sound like much, but it was enough to throw things off by 11 days between 45 BC and 1752. Another few millennia and it would have been Christmas in July, as it were. Today we adjust for that with a leap year every four years, but under the Julian calendar the dates and seasons were drifting out of synch by one day every 128 years. England had a problem to solve: the Julian calendar had been doing a good job generally, but it overcorrected just slightly for the extra fraction of a day that exists in every solar year. England switched in that year from the old Julian calendar (ordered by Julius Caesar in 45 BC) to the newer Gregorian calendar (ordered by Pope Gregory XIII in 1582). The key event in the Isaac Newton birthday conundrum is the Great Calendar Adjustment of 1752. And he both was and *wasn’t* born in 1642. Sir Isaac Newton is the Schrodinger’s Cat of birthday boys: He both was and *wasn’t* born on Christmas Day. Sir Isaac Newton in a rather handsome portrait from 1689 by Godfrey Kneller. ![]()
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