Tuesday, October 04, 2005
This Day in Ancient History
On this day, once, there really was no tomorrow, kinda (patience--all will be revealed in time). On October 4, 1582 AD, the old Julian calendar (named after Gaius Julius Caesar who adopted it when he visited Egypt in 48 BC) was abandoned (in much of Europe) and the new Gregorian calendar (named for Pope Gregory XIII who issued the papal bull which declared the reform calendar adopted) took over and immediately omitted 10 days so that the very next day was October 15, 1582.
Here's why they had to do that: As this admirable little website shows, the Julian year was pretty close--365.25 years long, so pretty much just like now, every four years there was an extra day which was in February (go to the link for the little details). The problem was that the year is not on average 365.25 days long. Rather, the real solar year is between 365.2422 and 365.2424 days long (more details at the link--go!). So each Julian year was right around 11 minutes too long. That's almost 2 hours extra each decade and a day too long every 131 years or so. Over centuries this adds up and after roughly 16 centuries, the Julian calendar was approximately 12 days out of sync. Hence the Gregorian reform. Why just 10 days when it's 12 days out of sync? (Link!)
The Gregorian calendar handles the difference between real year and calendar year by foregoing the leap year's extra day every so often. The formula for the Julian calendar was every fourth year was a leap year (as it turned out, every year divisible evenly by four). The Gregorian Calendar kept the first rule, divisible evenly by four, but skipped a leap year evenly divisible by 100 except where the year is also divisible by 400. In other words, a year which is divisible by 4 is a leap year unless it is divisible by 100 but not by 400 (in which case it is not a leap year). Thus the years 1600 and 2000 were leap years, but 1700, 1800, 1900 were not. And 2100 will not be either.
Because the length of the real year is changing as the earth slows in its rotation (largely due to global warming--just kidding--due to tidal forces from the moon's gravity) there will have to be another calendar reform in about 10,000 years. Pope Gregory's calendar's accuracy will last far longer than Caesar's, proving once again the superiority of Christianity over paganism.
Here's why they had to do that: As this admirable little website shows, the Julian year was pretty close--365.25 years long, so pretty much just like now, every four years there was an extra day which was in February (go to the link for the little details). The problem was that the year is not on average 365.25 days long. Rather, the real solar year is between 365.2422 and 365.2424 days long (more details at the link--go!). So each Julian year was right around 11 minutes too long. That's almost 2 hours extra each decade and a day too long every 131 years or so. Over centuries this adds up and after roughly 16 centuries, the Julian calendar was approximately 12 days out of sync. Hence the Gregorian reform. Why just 10 days when it's 12 days out of sync? (Link!)
The Gregorian calendar handles the difference between real year and calendar year by foregoing the leap year's extra day every so often. The formula for the Julian calendar was every fourth year was a leap year (as it turned out, every year divisible evenly by four). The Gregorian Calendar kept the first rule, divisible evenly by four, but skipped a leap year evenly divisible by 100 except where the year is also divisible by 400. In other words, a year which is divisible by 4 is a leap year unless it is divisible by 100 but not by 400 (in which case it is not a leap year). Thus the years 1600 and 2000 were leap years, but 1700, 1800, 1900 were not. And 2100 will not be either.
Because the length of the real year is changing as the earth slows in its rotation (largely due to global warming--just kidding--due to tidal forces from the moon's gravity) there will have to be another calendar reform in about 10,000 years. Pope Gregory's calendar's accuracy will last far longer than Caesar's, proving once again the superiority of Christianity over paganism.
# posted by Roger Fraley @ 12:12 AM 
