As I was putting together my 2016 Year Recap, I got to thinking about our concept of a year, specifically a calendar year.  I’ve always been somewhat fascinated and intrigued by celestial and planetary astronomy.  However, like most people my understanding only went about as deep as this: “A year is 365.25 days so we add a day every four years to account for the extra quarter day, and the earth’s axis is tilted which creates the four seasons.”   What else does one need to know?!

A few years ago I read a book (“Fingerprints of the Gods” by Graham Hancock) which opened my eyes to a phenomenon called precession of the equinoxes (or axial precession) – the rotation of the earth’s axial tilt that is linked to the origin of zodiacal signs.   This concept fascinated me, and sparked a deeper curiosity about astronomical processes and their linkage to cultural constructs.

But I knew there was more to it than this, and as I pondered it a few days ago – more and more questions surfaced.  The initial one that sent me down the rabbit hole was “Why does our calendar start 10 days after the winter solstice?”   I did some surface-level internet research (thank you Wikipedia and Timeanddate.com) and I quickly found out that it is an extremely complex topic – both from astronomical and historical points of view.  It almost left me with more questions than before, but if nothing else I’m more aware of the extent of the complexity…. “I do know what I don’t know.”

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Here’s a quick regurgitation of a few of the facts I’ve been able to absorb:

The time it takes the earth to make one full rotation around the sun is known as an “astronomical year”.   The average measurement of this time is 365.2422(…) days.    I say average because there are almost infinite variables and forces at play, and depending on how it’s measured and what it’s measured against, there are micro-variations in the actual value.  Effects of gravity from the moon, other planets, slowing over time of the earth’s daily rotation, rotation of the earths orbital ellipse, axial precession …these all cause minor changes that make nice clean measurements and predictions extremely difficult.   They are minor enough that we don’t notice them in our short lifetimes, but over cycles of thousands or millions of years – they can cause a calendar to be off by days.

Our current civil calendar – our functioning “calendar year” – is an adoption of the Gregorian calendar, instituted in 1582 by Pope Gregory XIII.  The Gregorian calendar is a slight modification of the pre-existing Julian calendar which was developed and instituted by Julius Caesar in 45 BC.    The impetus for Pope Gregory’s change was apparently to reign in variations in the date of Easter, moreso than to create a more astronomically accurate calendar.    Regardless, the Gregorian calendar slightly shortened the average year length from 365.25 days (Julian calendar) to 365.2425 days (a shortening of 10 minutes and 48 seconds per year), which brought it more in line with a true astronomical year.

The Gregorian and Julian calendars, like most astronomically accurate calendars, consist of cycles of “common years” consisting of 365 days, and “leap years” of 366 days.  The variations and accuracy of different calendars is a function of the ratio of leap years to total years over a given time period.

Whereas a Julian calendar uses leap years 1 out of every 4 years (a ratio of 0.25), the Gregorian calendar has 97 out of 400 years as leap years (a ratio of 0.2425).   The next 4th year in the Gregorian calendar that won’t be a leap year will be the year 2100.

Another way to look at is this: over 400 years, the Gregorian and Julian diverge by 3 days.  Right now there is a 13 day discrepancy between the two calendars (01 January 2017 on the Gregorian calendar is 19 December 2016  on the Julian calendar).  You’ll note though, that the 0.2425 Gregorian leap year/total year ratio is still slightly different from the extra 0.2422 days in an astronomical year. This equates to being off by 27 seconds per year.  As a result of this small difference – every 3236 years the Gregorian calendar is off by a whole day compared to the the astronomical year.  I don’t know if the powers that be have a plan to deal with that lost (or extra?) day.

There are astronomically more accurate calendars in existence, and interestingly two of them (the Mayan and the Persian) are much older than either the Julian or Gregorian versions.   The Mayan calendar has an error of approximately 13 sec per year, and the Persian has less than 1 sec per year (the most accurate known calendar, it takes 110,000 years to be off by a day).   In the early 20th century some orthodox churches devised and adopted a “Revised Julian Calendar” which very closely matches the astronomical year – this calendar observes 218 leap years in a 900 year cycle (pull out your calculator and do that one – it matches almost exactly the extra portion of a day in the astronomical year).   Still – it is not perfect, it’s off by an average of 2 sec per year.

So…. if this all seems complicated, don’t worry:  it’s even way way more complicated than it seems.  You could almost devote an entire graduate course to the study of how human cultures have sought to make predictable date systems that are in line with natural cycles.  What I think we see though, is that while we’ve gotten close, we have yet to mirror our civil and calendar structures to natural, astronomical cycles with absolute precision, and on a long enough timescale we’ll always be a little off.   It’s a bit humbling, and maybe a bit metaphorical – mankind’s systems will never be fully in synch with the laws of nature.

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Back to my original question: why does our calendar year start on 01 January?  01 January seems to be, from an astronomical point of view, a somewhat arbitrary point in time, roughly ten days after the 21 December winter solstice.   Wouldn’t it make more sense to start on a fixed, astronomically based point in time (like a solstice or equinox)?   Well, it turns out it used to be.   The beginnings of many ancient calendars aligned with the vernal (spring) equinox on 21 Mar.   With the advent and spread of the Julian calendar, the date of the calendar year shifted left approximately 80 days, to what is now known as 01 January.

Why the shift?  The Julian Calendar was essentially an attempt to combine the pre-existing Roman calendar – which consisted of months based on moon cycles – and the measured astronomical year.  In the Roman Calendar, January was the first month of the year – named after the two faced god Janus, who looked both backward into the past and forward into the future.  Ole Julius Caesar wanted to preserve some aspects of the old Roman calendar system, while instituting a new yearly cycle based on the 365.25 day astronomical cycle of earth’s rotation around the sun.   He adopted the new length, but kept the start date the same (01 Jan), and adjusted the month lengths slightly to make it fit into a 365/366 day cycle.   As the Roman empire spread through Europe and western Asia, so did the Julian calendar.  And later, as Christianity spread around the world, so did the Gregorian calendar.

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In his book “Sapiens,” one of author Yuval Harare’s main themes is that human culture is largely based around cultural and social “myths” – concepts that don’t exist in nature but that we create and believe in in order to give structure and order to society.   So is the year a myth, or an objective natural process of the universe?  Well I think it’s both (cop out answer, I know).

The cycle of the earth around the sun, it’s axial tilt, the resulting seasons, and the effects on the movements of stars in our night sky – these are all objective natural processes that took place millions and billions of years before there were humans to observe them, make sense of them, and build civil systems (like calendars) around them.   And they will continue long after we are gone.

But we take these natural cycles, and shape them into concepts that we can understand and use.  These systems – these “myths”- are completely a construct of mankind.  The civil calendar is man’s adoption of a repeating natural process, tweaked to be predictable, and to provide a globally accepted reference system of measuring time.  The fact that our calendar year looks the way it does is as much a product of mankind’s history as it is astronomical phenomena.  Had things gone differently several thousand years ago, we could be celebrating new year’s eve the night before the vernal equinox, or could even base our long term time system on seasons, vice years (I would be 140 “seasons” old right now).
I’m sure to disappoint with this, there’s no big revelation here.  The takeaway for me is that every day is a new year.  01 January is no more special than any other day in the year, other than the fact that most people in the world think it’s special.  If you want to observe truly relevant and important “dates” from a natural or astronomical point of view – observe and celebrate the equinoxes and solstices!

All of this begs the question: what about our 24 hour based day? or the 60 minute based hour?  I’ll spare myself and all of you another rambling for now, I’ll save that one for another day.