Tuesday, February 28, 2012

Leap Years


Why a leap year? 

It takes the earth about 365.25 days to go around the sun.  To be more precise, it takes 365 days, 5 hours, 48 minutes, and 46 seconds. This means that every 4 years the calendar will be one day out.  To stop that happening a year is added each fourth year to counteract the difference between the calendar year and the astronomic year. This method of fixing the problem was first imposed by Julius Caesar in 45 BC.

So a day is added every February 29, easy peasy. 

Well, that’s not all.  The extra day every four years doesn’t exactly equal the 6 hours x 4, there is also the matter of the missing 11 minutes x 4 when rounding up.  After 128 years, that 11 minute difference will equal a full day that the calendar is out, 3 days each 400 years, but this time in the other direction.  The adjustment of one extra day each 4 years counteracts the additional 6 hours but the rounded off 11 minutes needs to come off.  Enter Pope Gregory in 1582 with his Gregorian Calendar. Dispensing with the Julian Calendar, he decided that every 100 years there would not be a leap year.  Therefore 1700, 1800 and 1900 were not leap years, to allow for the missing 11 minutes added by rounding up.

Okay, I can understand that, but then why was 2000 a leap year?

There are also the 14 seconds that are added each year for which allowance must be made.  As noted above, every 100 years there is no leap year.  However, to adjust for the seconds, every 400 years is a leap year.  Therefore 2000 was a leap year, as was 1600.

And so that’s it.  A bit complicated but I think I understand it.  Thanks for explaining it, I’ll be off now.

Err, well, there is one other thing.  To fine tune it even more, every 4000 years the century years will not be leap years.  Therefore 4,000, 8,000 etc will not be leap years.

If all that’s too much, use this guide:

To sum up:
·         Every year divisible by 4 is a leap year
·         EXCEPT the last year of each century, such as 1900, which is NOT a leap year . . .
·         EXCEPT when the number of the century is a multiple of 4, such as 2000, which IS a leap year
·         EXCEPT the year 4000 and its later multiples (8000, 12000, etc) which are NOT leap years.

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