Leap Day 2012

Ever wonder why we have an extra day every 4 years? In my early school years I learned a song that went something like this:

Thirty days has November, April, June and November. All the rest have thirty-one. Except for quite contrary February, which has only 28 - except in Leap Year, twenty-nine.

The simple explanation is that this extra day is added to the calendar every four hears as a corrective measure, because the earth does not orbit around the sun in precisely 365 days. 

But to learn more, I checked with Wikipedia, and this is what I learned:

In the Gregorian calendar, the current standard calendar in most of the world, most years that are evenly divisible by 4 are leap years. In each leap year, the month of February has 29 days instead of 28. Adding an extra day to the calendar every four years compensates for the fact that a period of 365 days is shorter than a solar year by almost 6 hours.

Some exceptions to this rule are required since the duration of a solar year is slightly less than 365.25 days. Years that are evenly divisible by 100 are not leap years, unless they are also evenly divisible by 400, in which case they are leap years. For example, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. Similarly, 2100, 2200, 2300, 2500, 2600, 2700, 2900 and 3000 will not be leap years, but 2400 and 2800 will be. Therefore, in a duration of two millennia, there will be 485 leap years. By this rule, the average number of days per year will be 365 + 1/4 − 1/100 + 1/400 = 365.2425, which is 365 days, 5 hours, 49 minutes, and 12 seconds. 

The marginal difference of 0.000125 days between the Gregorian calendar average year and the actual year means that, in 8,000 years, the calendar will be about one day behind where it is now. But in 8,000 years, the length of the vernal equinox year will have changed by an amount that cannot be accurately predicted. Therefore, the current Gregorian calendar suffices for practical purposes, and the correction suggested of making 4000 a non-leap year will probably not be necessary.

Happy Leap Year Day! Enjoy it - it only comes around once every 4 years. (Except on years divisible by 100, unless they are also divisible by 400.)

(With thanks to my sister, whose Facebook post made me consider this and look it up!)