So here we are at the dawn of a new year… which is as good a reason as any to take a look at an instrument that we all use and which allows us to divide the year into days, weeks and months: the calendar. On that note, can someone tell me what happened between October 4th and 5th 1582? In actual fact, nothing did, because these ten days never existed. Here’s why …
A complete year lasts exactly 365 days, 5 hours, 48 minutes and 45 seconds, or 365.2422 days if you prefer. Very early on, man decided to divide it into shorter periods, which was exactly what gave rise to the calendar. Depending on the civilisation, it is lunar or solar – the latter being the case with our Gregorian calendar which is the most widely used in the world.
As its name indicates, it was introduced in 1582 by Pope Gregory XIII, assisted by the astronomer Lilio. The aim was to correct the Julian calendar, developed in 46 BC by Julius Caesar and the astronomer Sosigenes. This was based on an inexact calculation of the length of the year, resulting in a loss of 0.078 of a day. This may not seem like much, but as use of the Julian calendar spread from the year 325 of our era, it accumulated a ten-day loss over the 1582 solar year. The Gregorian calendar corrected this error when it was decreed that October 5th 1582 would be followed by the 15th and not the 5th.
Pape-Gregoire-XII

Pope Gregory XII © DR

Just like the Julian calendar, the Gregorian calendar incorporates three ordinary 365-day years followed by a leap year with 366. However, in order to correct the above-mentioned 0.0078-day error, a leap year is deleted every hundred years, except for of which the number is divisible by 400. In this way, 1900 and 2100 are not leap years, whereas 2000 and 2300 are.
While Catholic countries adopted the Gregorian calendar in 1582, it took longer for Protestant countries (1700), Great Britain (1752), Japan (1873) and China (1912).
Amazing isn’t it? In 2014, we use exactly the same system that was developed in the 16th century. What is even more fascinating is the precision with which our ancestors were able very early on to measure the length of a year. Take the example of the Egyptian calendar that is also solar: in use since 4236 BC, its error amounted to 6 hours per annum or just 0.07%!
As one might imagine, horologists attempted from a very early stage to reproduce the calendar, including the 1,461 days of irregular cycles between February 29th from one leap year to the next, four years later. Such a display is known as a perpetual calendar and was to take its place in steeple clocks, followed by smaller clocks, then pocket watches and finally wristwatches. I will come back to this in a future article.


Willy Schweizer is the curator of the Girard-Perregaux Heritage