September 2, 1752, was a great day in the history of sleep. That Wednesday evening, millions of British subjects in England and the colonies went peacefully to sleep and did not wake up until twelve days later. Behind this feat of narcoleptic prowess was not same revolutionary hypnotic technique or miraculous pharmaceutical discovered in the West Indies. It was, rather, the British Calendar Act of 1751, which declared the day after Wednesday 2nd to be Thursday 14th.
Prior to that cataleptic September evening, the official British calendar differed from that of continental Europe by eleven days—that is, September 2 in London was September 13 in Paris, Lisbon, and Berlin. The discrepancy had sprung from Britain's continued use of the Julian calendar, which had also been the official calendar of Europe from its invention by Julius Caesar (after whom it was named) in 45 B.C. until the decree of Pope Gregory XIII in 1582.
Caesar's calendar, which consisted of eleven months of 30 or 31 days and a 28-day February (extended to 29 days every fourth year), was actually quite accurate: it erred from the real solar calendar by only 11.5 minutes a year. After centuries, though, even a small inaccuracy like this adds up. By the sixteenth century, it had put the Julian calendar behind the solar one by 10 days.
In Europe, in 1582, Pope Gregory XIII ordered the advancement of the Julian calendar by 10 days and introduced a new corrective device to curb further error: century years such as 1700 or 1800 would no longer be counted as leap years, unless they were (like 1600 or 2000) divisible by 400.
What factors were involved in the disparity between the calendars of Britain and Europe in the 17th century?