Was Isaac Newton born on December 25, 1642 or January 4, 1643?

The short answer is YES.

How can someone have two birthdays, especially two that are so far apart?

From the calendar that we follow today, Isaac Newton was born on January 4, 1643.  That is to say that if we counted backward from today and subtracted the number of days he was alive and the number of days he has been dead, we would come up with January 4th as Newton’s birthday.

But if we rely on the calendar that England was using at the time of Newton’s birth, the day changes to December 25, 1642.  When baby Isaac Newton first popped out into the world the calendar on the wall of every house in England said December 25.  

The reason for this discrepancy is that we don’t use the same calendar today as England used in the 1600s.  The calendars look the same and have the same name for the 12 months and the 7 days of the week but they kept time differently.  The English in the 1600s used the Julian Calendar which was named for Julius Caesar who proposed the calendar in 45 BC.  This calendar had 365 days in each year and there was one leap day every 4 years on February 29th.  The length of the year on the Julian calendar is 365.25 days and this is the main fault with the Julian calendar.  The actual length of a solar year is 365.2425 days.  The Julian calendar was off by 0.002%.

What difference does 0.002% make in the grand scheme of things?  Aren’t there more important things to worry about?  There may be more important things to worry about but that still does not mean that we can ignore this problem because, over time, the calendar starts to drift.  When I say drift, I mean that the Spring and Fall equinoxes start to change days the longer the calendar is in effect.  Since the Julian calendar was enacted in 45BC, the calendar has ‘drifted’ 13 days.  That means that the Spring Equinox on the Julian calendar would occur on March 7th/8th instead of March 20th/21st when it is supposed to happen if an accurate calendar is followed.

Newton was born when England and her colonies were using the Julian calendar to measure time and the day that he was born was December 25, 1642 according to the calendar his culture was using at the time.  

The Gregorian reform of 1582 corrected the mistakes of the Julian calendar by doing two things.  First, days were skipped to make up for the days that were never counted on the Julian calendar.  For example, people went to bed on January 1st and woke up the next morning on January 14th.  The second reform was to change the occurrence of leap days.  The new rule is best summarized as every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.  There are still leap days on February 29th every four years except for those exceptions.  This brings the calendar in line with the actual solar year which is 356.2425 days long.  The calendar that we observe now is called the Gregorian calendar which was named after Pope Gregory XIII who initiated the reform in 1582.

Countries with Catholic governments adopted the calendar sooner than their Protestant counterparts which is why it took England 170 years to abandon the Julian system for the more accurate Gregorian calendar.

It is our view here at Merry Newtonmas that Sir Isaac Newton was born on December 25, 1642 because the English did not recognize and use the Gregorian calendar until 1752, well after his death in 1727.

Fun facts

England and her colonies in the Americas switched to the Gregorian calendar in 1752 so the American Revolution occurred while the Gregorian calendar was in use.  So breathe easy, the American Independence Day will always be on the Fourth of July.

Greece was the last of the European countries to use the Gregorian calendar. They switched in 1923.  One reason they waited so long is that the Greeks depended on the Orthodox Church’s calendar which was the Julian model.

Gregorian Calendar 1 solar year = 365 days 5 hours 49 minutes 12 seconds

Julian Calendar 1 solar year = 365 days 6 hours

Many countries who are outside of the European Christian cultural tradition eventually adopted the Gregorian calendar for civil purposes and international trade.