David K. Israel explained the leap second the other day, so in the last few hours of this leap year, let’s also look at that extra day we had way back in February.

Our average calendar year (the usual 365 days) is a little out of sync with the astronomical year – the 365 days, 5 hours, 48 minutes and 46 seconds it takes the earth to go once around the sun. That little chunk of extra time doesn’t seem like much (you could probably let 5 hours, 48 minutes and 46 seconds slip by while reading this blog), but over time it adds up. Every four common calendar years, the calendar would be about a full day behind of the astronomical year. As time went on (we’re talking a few hundred years), our calendar months would start to fall earlier in the year. We’d have Christmas in the summer and Fourth of July barbecues in the dead of winter. It would be chaos.

To prevent that drift and keep the calendar and astronomical years in sync, we created the leap year and add one extra day to the calendar every four years (most of the time, we’ll get to that in a minute). Over a four year period, then, we average 365.25 days per year and just about keep pace with the astronomical year. But a solar year is just shy of 365.25 days – 365.2422 days, actually – so if we added a leap day strictly every four years, we’d eventually get ahead of the astronomical calendar and months would fall later in the year.

To prevent that other drift, we space our leap years out by the following rules:
1. Years divisible by 4 are leap years (e.g. 2008).

2. Years divisible by 4 and 100 are not leap years (e.g. 1900), unless…

3. Those years are also divisible by 400 (e.g. 2000), in which case, they are leap years.

In the long run, then, we average 365.2425 days in a year, which is close enough to the astronomical year that it will take over 3,000 years for the calendar and astronomical years to be off by a day.

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