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Jun 9, 2015 Tags: programming
This post is at least a year old.
In 1582, under the inter gravissimas papal bull, the Catholic world transitioned from the Julian Calendar to the Gregorian Calendar. This new calendar shortened the length of the year from 365.25 days to 365.2425 , a reduction of just 0.002% .
One side effect of this is the modern leap year system, in which every fourth and 400th year is a leap year, but no other year divisible by 100 is. For example, this means that 1900 was a normal year, 1904 was a leap year, and 2000 was also a leap year.
Another side effect (and the topic of today’s post) was that the Julian and Gregorian calendars no longer agreed on the date. Calculating each from 1 to to 1582 AD, the Julian calendar had slowly accumulated “drift” relative to the Gregorian calendar, lagging eleven days behind it.
To correct for this drift, ten days had to be removed from the 1582 year, converting the entire system from Julian to Gregorian. Selecting an appropriate 10-day span took nearly 20 years (owing in no small part to the Catholic church’s reluctance to skip any holidays and desire to correct the Easter drift), but eventually the span of October 5 to October 14 was chosen.
The end result? On the fourth of October, 1582, citizens of the Catholic world* went to sleep and woke up ten days later, on the fifteenth of October, with a new calendar.
In essence, then, those ten days in 1582 never happened and simply do not exist within the Gregorian calendar system used almost universally in the West today.
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