Leap Year February 29, 2012

I always wondered why we have leap year and I finally decided to try and find some answers. I thought this was pretty good.

The leap year is a contrivance so that the calendar year (usually 365 days) doesn't get too far away from the solar (astronomical) year. You say: huh? Well, the astronomical year - the time it takes the earth to go exactly once around the sun - is not precisely 365 days. The ancients estimated it as 365.25 days. That wasn't bad as calculations go; it's actually 365 days, 5 hours, 48 minutes, and 46 seconds.

Now, you may think that crummy little fraction (almost 6 hours or 1/4 of a day) doesn't matter much. But every four years, the calendar would lose a full day against the seasonal year. Christmas (Dec. 25) would start to come a little earlier each year. After about 20 years it would come before the winter solstice; after 200 years or so, Christmas would come in the autumn (since the seasons are tied to the astronomical year, because they depend on the earth's slant relative to the sun) . . . and then in summer . . . and . . .

To prevent this drift between the calendar year and the astronomical (seasonal) year, we add one extra day every four years. Thus, over the four year period, we have 1461 days, not 1460, for an average of 365.25 days per year. That pretty much makes it come out right.