Leap year: why we have a need for the occasional Feb. 29

The leap year is a testament to the tough time that humans have trying to organize 365 days, 5 hours, 49 minutes, and 16 seconds into a year.

Extend a welcome to Feb. 29, the official leap day in the Year of Our Leap 2012.

If you were born on Feb. 29, 1988, you can claim with a wink that you're only 6 years old. And if you're a single woman with your eye on a special guy, this is the day to pop the question. If he refuses, by tradition he owes you.

The leap year is a testament to the tough time that humans have trying to squeeze the 365 days, 5 hours, 49 minutes, and 16 seconds of a year – as measured by one trip around the sun – into a yearly calendar containing 365 days.

Knowing when to tack an extra day onto a year isn't as simple as “add a day to February every four years.” That would be too easy.

For those who have trouble remembering the spelling rule, "I before E, except after C," and its caveats, try this: If the year is evenly divisible by four, add a day to February – unless you can divide the year evenly by 100, then no leap day, unless the year is evenly divisible by 400, then you add a leap day.

It could be worse. If Julius Caesar hadn't reformed the old Roman calendar (and no one else had in the meantime), we might still be adding a month to the calendar every two or three years. The Roman calendar was based on a lunar month, which averages 29.5 days. If you were big on festivals and a slave to your calendar, your spring planting festival could end up in the dog days of summer unless you brought the calendar back in sync with solar time.

Around 46 BC, Caesar stepped in and, one can imagine, said, “Ist es ridiculum!” He moved the calendar onto a solar year.

But even Julius didn't get it quite right.

The Julian calendar is based on a year that is 365 days, 6 hours long. So you could add one day to February every four years from now until Brutus stopped by and be happy. However, if you did that, the equinoxes, as marked on the calendar, arrived earlier every year. The spring equinox, always a favorite among festival planners, arrived on March 25, when Caesar had completed his conquest of the calendar. By the 1500s, the spring equinox was cropping up on March 10 or 11.

Keep this up long enough, and Easter Sunday as marked on a calendar eventually would take place in the dead of winter. Easter Sunday is the first Sunday after the first full moon following the vernal equinox, keeping it close to the Jewish observance of Passover. In the late 1500s, a continued shift of Easter and other Christian holy days did not sit well with the Vatican.

The result: The Gregorian calendar, which has since become the international standard as a general-purpose calendar for scheduling business meetings, mortgage payments, and Little League games.

Still, variations on the lunar calendar – and the periodic additions of leap months – remain in place for religious and cultural purposes, largely within cultures that trace their roots to Asia and the Middle East.

But back to marriage. Remember we mentioned marriage? According to an Irish tradition, back in the day, St. Bridget cut a deal with St. Patrick allowing women to propose marriage to men on a leap day. In some areas that adopted the tradition, men were penalized if they refused the proposal. Small consolation, perhaps, but a “no” would leave the forlorn miss in line for a new gown or several pairs of new gloves.

These days, international time is determined by the vibrations of atoms in incredibly accurate atomic clocks, rather than by Earth's rotation. This has added another kind of leap to the time-keeping lexicon: the leap second.

Keepers of atomic clocks periodically add or subtract one or two seconds a year to keep the clocks synced with a 24-hour day as measured by Earth's rotation – which on average is gradually slowing. The first leap seconds were added in June and December 1972. The next leap second – the 25th since 1972 – is slated for this coming June.

Earlier this year, international time gurus meeting in Geneva considered a proposal to abolish the leap second. A final decision is expected in 2015.