All calendars have 365 days. This comes from the observations of the star Sirius by ancient Egyptians who noted seasonal repetitions every 365 days. However, if we let this reckoning guide us for too long, spring will not be recurring every March, nor winter every December in the northern hemisphere. The reason for this is that the earth actually takes a little more time to go once round the sun. This was recognized by Egyptians, and later by Romans. Knowing a solar year to be a quarter of a day more than 365, the matter was remedied by adding an extra day every fourth year. Thus began the leap years.
But there is a problem: The length of the year is not precisely 365.25 days, but actually only 365.2424 days. Thus, by rounding it off to 365.25, we are adding about 0.0076 days to every year. This may not be significant for a century, but in a thousand years, it adds up to more than a week. And in 1500 years, to about ten full days. Already in the 13th century, some date-watchers became aware of this problem, and in the next two centuries the matter was becoming more and more serious. Not surprisingly, calendar reform was a topic of discussion at the Council of Trent in 1545.
On 24 February 1582, Pope Gregory XIII issued a bull asking for a reform of the calendar, taking into account the exact duration of the year, as revealed by more precise astronomical observations, and the perceptible changes in the equinoxes. The Jesuit mathematician Christopher Clavius played an important role in the related calculations. He also put out the precise dates for all movable Christian festivals until the year 5000.
The net effect of the calendar reform resulting from Gregory’s bull was the following: In order to compensate for the excess resulting from the fact that a year is a little less than three hundred and sixty five and a quarter days, every hundred years, one will have to skip a leap year. But this would result in a slight loss again in the long run. To compensate for this, every four hundred years one would have a leap year again. Thus, the years 1700, 1800, and 1900 were not leap years, but 2000 was.
All this is based on good astronomy, but religion also played a part in some of the decisions. If the reforms had been formally initiated from, say 1600, everything would have been fine, except that the Pope who had started it all was already 80 in 1582, and he wished to see the new calendar in his terrestrial life-time. Also, the date of the spring equinox would be 11 March or so. From a purely secular perspective, there is really nothing wrong with this. However, this does interfere with the reckoning of Easter, bringing it too early in the season. In order to avoid this, when the Gregorian calendar was initiated in 1582, the date of 4 October was followed by 15 October. In other words, there was no 5 October in the year 1682, nor any 6, 7, … 14. This caused problems with wages for the month of October in some countries .
Today, the Gregorian calendar is universal, except for various religious lunar calendars, as in the Hindu, Jewish, and Islamic traditions. The Eastern Orthodox church did not adopt it, and Protestant countries did so only after more than a century. For a time, they used both systems, as may be seen in many records and letters of the time. History has recorded that Ugo Buoncompagni (as Gregory XIII was called in his pre-pope phase) was somewhat of a bon vivant, susceptible to good food and the female form, and ruthlessly anti-Protestant. He was not much of a success in many of his other undertakings, but by sponsoring the new calendar he has made his name immortal in calendrical references.
October 5, 2012