Talk about space oddities: Tuesday is "leap day," because 2000 is one of those weird, mostly-every-fourth annums we call a leap year.

From varying month lengths to missing seconds, our calendar is full of inconsistencies. And while its quirks have something to do with the twists of human history, the real reason for our strange calendar is simple: Our clocks are based on space rocks.

Since the dawn of civilization, most timekeeping has been based on the motions of the Earth, moon, and sun. The Earth and moon rocks orbiting the sun are like spinning clocks, their motions repeating with great precision. The daily motion of the sun became the clock, the monthly and yearly motions of the moon and sun became the calendar.

But of course, a year is not 354 or 360 days long, but more like 365. For that matter, a lunar period isnt 30 days long either, but about 29 and a half days. So those early calendars would "creep": Because the calendar year was too short, the seasons arrived earlier and earlier each year.

This created havoc with planting, harvesting and other important calendar functions, so some very creative fixes were put in place to keep the calendar in sync with the seasons. The Babylonians threw in an extra month every few years, while the Egyptians held a five-day feast at the end of each year to make up the difference. (Not such a bad idea, a five-day party at the end of the year.) Later civilizations such as the Romans adopted similar schemes.

And so the calendar remained until that great breaker of traditions, Julius Caesar. Caesar thought the Roman calendar was much too complicated and often inaccurate, so he commissioned an Egyptian expert, Sosigines of Alexandria, to come up with a new calendar.

When all was said and done, what Julius Caesar put in place was the first calendar that resembled a modern one: a 12-month year -- some months 30 days long, others 31 days long, with February 29 days in length (30 days on "leap years"). Another innovation: the year started on January 1. (Before Julius Caesars calendar the year started in March, which is why the last four months of the year are named September, October, November, December derived from the Latin for seventh, eighth, ninth and tenth, which they were if one starts counting from March.)

This was known as the Julian calendar, and it became the standard of the world for a time. It was also the last time that February ever had 30 days, and it was to lose the 30^th day very quickly.

The Julian calendar was put in place in 46 BC and, never one to be shy, Julius Caesar even named a month after himself: July. But only two years later Julius was dead at the hand of Brutus, and Augustus Caesar took the throne.

Augustus was not one to be bested by Julius, so he named a month after himself too: August. However, he did not like the fact that July had 31 days while August had only 30, so to make his month as long as July he stole a day from February, reducing it to 28 days, 29 on leap years. (So much for February 30.)

The Julian calendar was the standard of the Western world for 1600 years, but eventually it developed problems because it too had inaccuracies. It assumed the year is 365 and a quarter days long, which it almost is but not quite: The solar year the time it takes the Earth to circle the sun is really 11 minutes shorter than that. Eleven minutes may not seem like much, but over 1600 years it had added up to an error of about two weeks.

For the Catholic Church that presented a real problem, because it played havoc with the timing of Easter, which in turn is tied to the Jewish lunar calendar. In order to re-synchronize the calendar, in 1582 Pope Gregory XIII decreed that 11 days be snipped from the middle of the calendar: In 1582, throughout much of Europe people went to bed on the night of October 4 and woke up the next morning on October 15. The days in between never happened; they just disappeared.

And Pope Gregory had more up his sleeve: He also decreed a change in the calendar, a new system of leap years that would compensate for the 11-minute difference between the solar year and the calendar year.

To do this, the Pope decreed that years divisible by 100 (e.g., 1700, 1800, 1900), though they are divisible by four, would not be leap years. That would have the effect of slightly shortening the length of a century, thereby accounting for each solar year being 11 minutes shorter than a calendar year. But because that adjustment actually over-compensated a bit, every year divisible by 400 (1600, 2000, 2400) would be a leap year.

Thats the Gregorian Calendar, which is what we use today. Though a bit complicated in its rules, it works remarkably well. But over time it too was discovered to have some inaccuracies, and in the last few years it has led to a clever little fix of our own.

The dawn of the atomic age also brought with it a revolution in timekeeping: The atomic clock. For the first time mankind had a timekeeper more accurate than the spinning rocks, and the atomic clock made obvious something very interesting: The spinning rocks the motions of the sun, moon and Earth are not exactly precise either. The Earths rotation has wobbles and lumps, and it is very, very gradually slowing down.

So just as in the time of Julius Caesar and Pope Gregory, our calendar has again been found to be inaccurate. The inaccuracy is very tiny (less than a second per year), but its there and it will grow over time. So is it time for a new calendar?

No, and heres how the problem has been solved:

Using super-accurate time sources such as atomic clocks, scientists at places such as the U.S. Naval Observatory monitor the creeping error in our calendar. When the error approaches one second, an extra second known as a "leap second" is inserted, and the calendar is back in sync. The leap second can be inserted at the end of June or December, and since 1972 there have been 22 leap seconds added, the last one at the end of December 1998.

(Note that this does not mean the Earths rotation has slowed down by that much, only that Pope Gregorys calendar has gone off by that much. The Earths rotation is slowing down, but only by about one second every 70,000 years.)

So perhaps we have reached the ultimate calendar: Its not perfect, but by using leap seconds we can always keep it accurate.

With that in mind, enjoy this rare centenary February 29, and think about how strange it is that we have it, but even stranger that because of a jealous Roman emperor, February 30 is now in August.