It's leap day. Now why do we do this?

Leap year is an attempt to help people cope with the problem of too much time on their hands. Here is how it works:

Start with what is called the tropical year - the time between vernal equinoxes which is, when all is said and done, just an ordinary year. According to www.infoplease.com, it is 365 days, 5 hours, 48 minutes and 46 seconds long.

You can see that there is too much time there to fit into just one year, but not enough to add a whole extra day. Just ignore the extra bits for a while. If you use the Gregorian calendar, which most of the world does, four years should do it.

OK, after four years have passed, there's a surplus of 23 hours, 15 minutes and four seconds - almost one whole day. Technically, it is called an intercalary day, but regular folks call it a leap day and add it to the end of February. That would be today.

But adding a whole leap day every four years means borrowing time from the years ahead. Eventually, say every 128 years, the Gregorian calendar would be one whole day ahead of itself.

So every century year - 1700, 1800, 1900 - leap day is omitted, unless the century year is divisible by 400 (like the year 2000), in which case the leap day is added.

Got that?

At this point, the calendar year and the solar year are only about half a minute off. But - uh oh - that creates another problem. Every few thousand years (different sources cite different amounts, anywhere from 3,300 to 5,025 years) this builds up to a discrepancy of one day between the calendar year and the tropical year. Again, just ignore it. It will be entirely someone else's problem by the time it rolls around.

If the imprecision of the Gregorian calendar bothers you, consider that Iran has a calendar that will remain nearly in sync with the solar cycle for much longer.

That extra-day problem, according to "Eric Weisstein's World of Astronomy," crops up only once every 141,000 years.

So we're good, right?

Not quite.

Atomic clocks kept by the Navy tell us that the Earth actually loses 2 milliseconds a day because it rotates slower than Coordinated Universal Time would have us think. Because of this, a leap second has to be added every 400 to 500 days. No one ever notices. Perhaps that is why your watch is always wrong.

Diane Tennant, (757) 446-2478, diane.tennant@pilotonline.com