One curious fact about Isaac Newton is that you can say he had two birthdays, ten days apart.
You may have previously seen Newton’s birthday as December 25, 1642. That reference is beginning to change, and now it’s more common to see Newton’s birthday as January 4, 1643.
The difference is due to the fact that, when Newton was born, England was in the midst of a 150-year period of using a different calendar from the rest of Europe. The rest of the continent had already adopted the Gregorian calendar, which is the same calendar we use today.
However, at the time of Newton’s birth, the English were still using the Julian calendar, which lagged ten days behind because of a faulty method of accounting for leap years. (Coincidentally, 1642 was the year that Galileo died.)
So Newton himself would have said his birthday was December 25. But everywhere outside of England he was born on January 4.
（earthsky.org)
