In the interest of public welfare, we feel compelled to warn readers that next month will be abnormally long. The reason is that, in addition to its other celebrated oddities, Y2K is a leap year.

That happens only every 400 years. As you may recall, leap years -- in which poor February picks up a 29th day -- occur every four years except when the year is divisible by 100. But there's one other exception buried back in the fine print: Years divisible by 400 are leap years. So 2000 is one, but 1800 and 1900 weren't.

Complaints should be forwarded to the Vatican; the 400-year business was instituted as part of the nifty new calendar introduced by Pope Gregory XIII in 1582. And in truth, it was a major upgrade.

The old Julian calendar had been close enough for Roman goverment work, providing an average of 365.25 days a year -- that is, 365 days in each normal year and 366 once every four years during leap year.

The problem is that Earth completes a cycle of the seasons once every 365.2422 days. So the Julian calendar made the year too long by a shocking .0078 days, or 11.23 minutes. Sure, that may not seem like much. But this stuff adds up.

By the mid-16th century, the first day of spring was way too early -- about March 11 -- and the date of Easter was about 10 days out of whack.

So the Gregorian reform commission threw in the 400-year exclusionary rule. Instead of 99 leap years in every 400, there were only 97. That brought the length of the average year down to 365.2425 days. That's pretty good but not perfect. We'll be having problems again before you know it -- in 3,000 or 4,000 years at the latest. See David L. Book's warning in Letters at left.

In the meantime, for further details, consult the U.S. Naval Observatory web site at www/usno.navy.mil.