In our day-to-day life we tend to use the concept of “year” as our time gauge. It is used to measure the length of events and sort important ones in orderly fashion. Although it seems a straightforward concept to understand and use, it is nonetheless worthwhile to make a brief reflection on exactly what we mean by it and how it has evolved over time.
What is a year?
Our first idea of what a “year” means may seem simple but it might in fact be misleadingly so. According to the model of the Polish scientist Nicolaus Copernicus we all understand that the earth goes round the sun and takes one year to make each orbit. On this basis a year would be the time running from the earth’s return to exactly the same starting point in relation to the sun. This is known as the sidereal year; measured by state-of-the-art technology, it comes out as 365 days, 6 hours, 9 minutes, 9.7632 seconds (365.25636 days). This yardstick comes in handy for astronomers and astrologers who wish to find out and forecast the sun’s position against the firmament as seen from earth.
Nonetheless, the real interest in this concept of the year is to find out how the seasons occur and forecast associated climate changes. It turns out that this bears no relationship to the earth’s position in space but the sun’s position in relation to the inclination of the earth’s axis. The crux of the matter, in other words, is to find out exactly the interval of time running between two identical line-ups of the earth’s axis and the sun. This interval is measured between two vernal equinoxes and it comes out as 365 days, 5 hours, 48 minutes, 45.98 seconds (365.2422 days). This is known as the tropical year.
We can now begin sorting out these initial ideas. The small difference between these two concepts of a year is due to a phenomenon called axial precession. Precession is a gradual shift in the orientation of the earth’s axis due to a spin effect like a wobbling top. This spin is very slow, the complete cycle taking about 25,776 years. The upshot is that the sun lies in a slightly different position at each vernal equinox. A curious side effect of this phenomenon is that the sun’s position no longer corresponds exactly to the zodiac charts drawn up several thousand years ago. On 21 March, for example, the sun should rightfully be entering the constellation of Aries whereas it in fact lags behind in Pisces.
Julian and Gregorian Calendars
Sticking for now with the tropical year, we find that the goal throughout history has been to get the equinoxes to occur always on the same date. In about 46 BCE, on the basis of the experience built up by the Egyptians, Julius Caesar thought a reasonable year length would be 365.25 days. It was therefore decreed that three out of every four years would have 365 days and the fourth would have 366 (the leap year). This model is known as the Julian calendar, in reference to the Roman emperor rather than any more modern Julian like Lloyd Weber or Assange.
As the centuries slipped by the system was seen to move slightly out of sync. The vernal equinox now stole a considerable march on March 21. At the end of the sixteenth century, therefore, Pope Gregory XIII decided to solve this discrepancy by establishing a brand new year model in 1582, called Gregorian ever since. This year would last for 365 days but with three caveats:
1- One leap day was to be added every four years (corresponding to the Julian year of 365.25 days)
2- Every one hundred years the leap day would not count (now the year would be 365.24 days)
3- Every 400 years the centennially subtracted leap day would be added back in (whereby the year would finally consist of 365.2425 days)
Thus, according to the second caveat, the years 1700, 1800 and 1900 lasted only 365 days. According to the third caveat, however, the years 1600 and 2000 remained leap years to keep things on track.
The cumulative error of this model, still in place today, comes out at about 3 days every 10,000 years. Obviously an error so miniscule will not make its presence felt in this era. One quite astonishing fact is that the Mayan civilization had apparently managed to organize a year of 365.242 days, more exact than the one we are working with in the twenty-first century.
Let’s go back to Pope Gregory XIII. After establishing the new leap-year criterion, he still had to deal with the accumulated time lag. It was therefore decided in 1582 that the year would pass straight from October 4 to October 15, scrapping the intervening days. The mind boggles to imagine the chaos if this were done today. Even back then there was some fallout. Saint Teresa of Ávila apparently died on 4 October and was buried the very next day on October 15.
Viewed from today’s mindset, it might seem shocking that the calendar was changed at the behest of a pope. Firstly, we have to bear in mind that at that time there was no international organization capable of orchestrating such a far-reaching change. Secondly, the calendar was vitally important to the church because it ruled so many religious festivals and events. Christianity’s benchmark date for establishing all mobile feasts is Easter Sunday. The Council of Nicaea established that this day would fall on the first Sunday after the first full moon occurring on or after the vernal equinox. This is why we need to work out the moon’s phases to find out when Easter weekend is to be held each year.
Another curiosity bound up with this story is that the model, brought in by a pope, was therefore initially accepted only by Catholic countries. Anglicans did not opt in until 1752. Up to that date, therefore, events occurring on the same day are dated differently according to the particular country’s standpoint. Even worse was the case of some Orthodox countries that did not adopt the new calendar until well into the twentieth century. Russia, in particular, did not make the changeover until 31 January 1918. This is why the so-called October Revolution of 1917 in fact occurred on 6 and 7 November.
Some links of interest
- Sidereal and tropical year (Spanish) I
- Sidereal and tropical year (Spanish) II
- Equinoctial precession (Spanish)
- Easter Sunday (Spanish)
- Julian and Gregorian Calendars (Spanish) I
- Julian and Gregorian Calendars (Spanish) II
Author: Crescencio Lucas Herrera
Las opiniones vertidas por el autor son enteramente suyas y no siempre representan la opinión de GMV
The author’s views are entirely his own and may not reflect the views of GMV