Recess4GrownUps

Let your brain out to play.

Three Hundred Sixty Five and a Quarter

Popes, Orbits, Rotations and the Origins of the Modern Calendar

December 31st marks a day most modern people are well acquainted with; the end of the old year, and the beginning of the new. In fact, we’re quite used to the year proceeding as expected; spring, summer, fall and winter all happen more or less on a predictable schedule.

There are 365 days in a year…with the one small (and frequently under-appreciated) exception that occurs every fourth year at the end of February. In its simplest description, a calendar is a system of time-tracking based on astronomy. Why do I say that? Well, because despite the numerous different calendars that have been invented and used throughout the ages, all of them have something in common; a single unit of time, based on the earth’s rotation on its axis.

That unit consists of a period of light and a period of darkness. We call it a “day”. Calendars group these individual units into larger units (weeks, months, years). These larger units are where there is a lot of variability among different calendar systems. The Jewish calendar, for example (which is the modern official calendar of Israel), bases its months and years purely on lunar cycles.

On the other hand, the “Christian” (also called the “Julian” after Julius Caesar, or “Gregorian” after Pope Gregory XII, who declared it the official calendar of Christendom in 1582) calendar, which is the most common calendar in use around the world today and is the standard for business and international affairs, bases its year on the Earth’s orbit around the sun.

Picture: Creative Commons | Earth

Although the Gregorian month is approximately as long as a typical lunar cycle, its months have no connection to the actual cycles of the moon. This difference is why holidays such as Chinese New Year or Hanukkah fall on different Gregorian dates from year to year; the Chinese and the Jewish calendar don’t line up perfectly with the Gregorian calendar, so a Chinese holiday may happen on the same day of the Chinese calendar every year, and yet appear to “migrate” on the Gregorian calendar.

The science of determining the precise length of a year is a tricky one, because the rotation of Earth on its axis doesn’t necessarily correlate perfectly with the Earth’s completing one full orbit around the sun - that is to say, the true length of time it takes the Earth to go around the sun isn’t perfectly 365 24-hour cycles.

So a 365 day year is actually slightly inaccurate - as is every other calendar ever used, in fact. This is why we have “leap years” (or for some calendars, a “leap month”) - because the true time it takes the Earth to complete one full orbit is three hundred and sixty five days…and one quarter day. To prevent that quarter day from accumulating over time and causing months and holidays to round-robin such that December is 120 degrees in Minnesota, we add in an extra day every four years to compensate. The year with the extra day is called a leap year, and the extra day is the cumulative “spare” quarter days that went unaccounted for in non-leap-years.

In what we now call 46 BC, Julius Caesar introduced a calendar reform in an attempt to better measure the year, and avoid the irregular years that came from the inaccuracies of the common-use calendar at the time. In order to put all the seasons back into their appropriate places so that summer was hot, winter was cold, and the summer solstice happened in June, Caesar was compelled to “make” a year last 445 days! This is known as the “Year of Confusion”, or annus confusionis in Latin, and it served the purpose of normalizing the calendar and syncing it with the realities of the Earth’s solar orbit.

The exact length of a complete orbit changes over time, as the sun and our planet both age (on average, the Earth’s rotation slows by about half a second per century, meaning it’s slowed a total of 10 seconds from 1 AD until today). Many different systems of measuring months and years have emerged over the centuries. However, the calendar we use today is not the first calendar to measure a year as 365.25 days.The first was the calendar of ancient Egypt - first recorded, with a 365.25 day-long year, in 4326 BC. So much for the advances of modern technology, I guess.