Today – January 13, 2019 – marks the last day of the year by the Julian calendar (December 31, 2018), which was first introduced to the world by Julius Caesar in 46 BC. Tomorrow – January 14, 2019 – marks the first day of the Julian New Year (January 1, 2019). Historians and other chronologists care about the Julian calendar because it was used worldwide for over 16 centuries. Some – for example, the Christian Eastern Orthodox Church – still use the Julian calendar to this day.
But most of us don’t use it. Since 1582, nearly everyone around the world has converted to the Gregorian calendar. However, chronologists usually give the dates of astronomical events that occur before the introduction of the Gregorian calendar on October 15, 1582, by the Julian calendar date. For instance, equinoxes and solstices and any lunar and solar eclipses happening before October 15, 1582, are dated by the Julian calendar.
The transition from Julian calendar to Gregorian calendar wasn’t quick or easy, but it was needed. In the old calendar system, there was an accumulated discrepancy between the calendar dates and the actual time of the Northern Hemisphere’s spring equinox. Pope Gregory decreed that October 4, 1582, on the Julian calendar was to be followed by October 15, 1582, in the newly established Gregorian calendar.
That transition is what’s illustrated by the calendar at the top of this post.
But not everyone converted all at once. England, for example, with its large empire and separate church kept its separate calendar, too – the Julian calendar – for another two centuries. But, as explained on mentalfloss.com:
… it all got rather confusing: People often headed up letters they wrote with two dates—one using the new Gregorian calendar in fashion in mainland Europe, and the other using the old-fashioned Julian calendar.
Eventually, Britain adopted the new Gregorian calendar – the calendar we use today – with its Calendar (New Style) Act of 1750. That adoption – promoted the English calendar riots of 1752!
Here’s a bit more about why the new Gregorian calendar was so sorely needed. The Northern Hemisphere’s spring equinox is supposed to fall on or near March 21 every year. However, the discrepancy between the Julian calendar and the spring equinox put the calendar out of step with the seasons. In the year 1582, for instance, the spring equinox came on March 10 by the Julian calendar.
The old style Julian calendar declared every fourth year a leap year of 366 days. This made the average length of the Julian year at 365.25 days, which was about 11 minutes too long relative to the year as measured by the spring equinox.
Eleven minutes doesn’t sound like much. But over the long period of time – over centuries and millennia – the accumulative error of one day in about 128 years is difficult to ignore.
The Gregorian calendar tweaked the rules a bit, to bring the calendar in closer agreement with the seasonal year. Century years which are not equally divisible by 400 are not considered leap years, in the Gregorian calendar. The year 2000 was a leap year, and the year 2400 will be a leap year, but the years 2100, 2200 and 2300 will be common years of 365 days.
Bottom line: Today – January 13, 2019 – marks the last day of a year in the Julian calendar. This post explains why we switched from this calendar. Happy Julian New Year!