Why a leap day...

So, today is leap day! Why? Well, it sounds simple- adding an extra day every four years compensates for the fact that a solar year is almost 6 hours longer than 365 days. But not quite, when you look at the details it gets a little more complicated. Some exceptions to the basic rule of adding a day every for years are required. Because a solar year is not exactly 365.25 days. It is slight less. So, how do we account for this????

Years 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. 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. Got it?