Factismal: A year is 365 days, 5 hours 49 minutes, and 12 seconds long.
Today is Isaac Newton’s 370th birthday. Then again, so was Christmas Day of last year. Old Isaac didn’t get two birthdays because he was all that large a baby or that special a person; it happened because the calendar in use when he was born wasn’t very good.
Don’t get me wrong, it was better than the one that came before it. You see, until the time of Julius Caesar (yes, that Julius Caesar), the Roman calendar was a mess. The original Romulan (no, not those Romulans!) calendar had just ten months and covered just 304 days of the year – the period between December and March were just considered to be one long, cold, winter of despair. Obviously, this wasn’t a very good way of keeping track of time. So Numa, the second king of Rome, changed it.
Numa added two more months between December and March, and brought the length of the year up to 355 days. But, as every schoolkid knows today, the year is actually closer to 365.25 days long. As a result, the calendar slowly slipped ahead of the actual year, with the embarrassing result of the equinox being declared several weeks before it actually happened! In order to fix this, the chief priest (called the Pontifex Maximus or “Chief Bridge Builder”) would slip in after February an intercalendary month known as a Mensis Intercalaris every so often. Because of the extreme difference between the length of the calendar and the length of the actual year, this had to be done roughly every other year.
But then Roman politics came into play. You see, the Pontifex Maximus was usually also the person who was in charge or one of his friends. As a result, the Pontifex Maximus could add in a Mensis Intercalaris or two when his party was in power (thus making the year and their term in power longer) or withhold them when the other guys were (thus making the year shorter). The worst offender for this was, you guessed it, Julius Caesar who made the year of his third consulship 445 days long!
This move offended just about everyone. In order to push it through, Caesar had to promise to reform the calendar so that nobody else could play that sort of trick again. (For a more modern example of this sort of political shenanigans, consider FDR’s four presidential terms.) He did it by making the year 365 days long and adding in an intercalendary day at the end of February every four years, starting with the year 46 BCE.
Now this would have been the end of the story, other than the various maneuverings over the names of months, except for one important fact: the year is not 365.25 days long. Instead, it is 365 days, 5 hours 49 minutes, and 12 seconds long, or a difference of 10 minutes and 48 seconds. Though the difference might not seem like much, it added up over the course of a few centuries. By 1582 CE, the calendar was nearly eleven days behind, throwing everything out of whack.
So it was up to the new Pontifex Maximus, Pope Gregory XIII, to fix the mess. He did it by jumping the calendar forward ten days and by changing the number of leap years. Under Gregory, every fourth year would be a leap year unless it fell on a century (i.e., 1000, 1200); only every fourth century year (i.e., those divisible by 400) would be leap years. That neatly fixed the lagging calendar and patched the problem so that another intercalendary day wouldn’t be needed until the year 10,000.
But Europe in 1582 CE isn’t the same as Rome in 46 BCE, and though the Pope might call himself the Pontifex Maximus, he didn’t have complete control over the world’s calendars. As a result, many countries didn’t adopt the new calendar until much later. Italy, of course, adopted it immediately. France took up the new calendar less than a year later. But it wasn’t until 1752 that England finally adopted the new calendar. And that is why, though Isaac Newton was born on Christmas day in England, he was really born on January 4.