The calendar year is based on the time it takes for the Earth to orbit the sun, which is 365.242 days. The leap year system was created to add an extra day every four years to account for the .242 days, but this still caused inaccuracies. In 1582, Pope Gregory XIII removed 10 days from the calendar and established a new system where leap years occur every four years, except for years divisible by 100 unless they are also divisible by 400. The new Gregorian calendar is more accurate but still not perfect.
A year is not an arbitrary period of time, it is an astronomical reality as it represents the time it takes for the earth to orbit the sun. A good calendar, therefore, must correspond to this time period. Earth takes 365,242 days to complete the journey. Instead of adding 242 of a day (5 hours 48 minutes 30 seconds) to a year of 365 days the leap year system was devised. Every four years, one more day was added to the 365 days of the regular calendar, thus creating years of 366 days. This system is relatively accurate, but .242 isn’t exactly the .250 of a day needed for leap day to work perfectly. Over the centuries, the .008s in a day (11 minutes and 30 seconds) began to accumulate.
The continued buildup caused by the inaccuracies of the Julian calendar ended in 1582. Pope Gregory XIII declared that 10 days in October should be removed from the calendar. Specifically, he rang from the 5th to the 14th of the month which made for a very bizarre October 1582. You can imagine what people’s schedule was like during that month:
October 1582
Dom.
Lun.
Mar.
Mer.
Thur
Ven.
Sab.
1
2
3
4
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Pope Gregory XIII, after whom the modern (Gregorian) calendar is named, also established a system to prevent accumulation from developing again. Every four years divisible by four would still be a leap year, but years divisible by 100 would not be, unless they were also divisible by 400. The year 2000 was one of these special years that comes once every four centuries. The year 2000 is divisible by 100 but also divisible by 400 and therefore is designated as a leap year.
The new Gregorian calendar is vastly superior to the imprecise Julian calendar it replaced, but it’s still not perfect. There are still tiny accumulations of time and thousands of years from now another correction may need to be made.
Perhaps the most interesting event of 1582 (aside from the change of calendar), is that William Shakespeare (aged 18) married Anne Hathaway in Stratford-upon-Avon.
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