Michelle Dickinson: Extra day to make our calendars stay in line

February 29, leap year's extra day, has a rich history. Picture / AP

Opinion

This week is a special week for leaplings - people born on February 29 who can only celebrate their true birthday on a leap year. If you ask anyone how many days there are in a calendar year, they will likely say 365.

This year, however, like the leap years before, will have 366 days. That extra day was created to help solve the complex issues created by the historic events that have shaped the calendar we use today.

For millennia we've relied on the Sun and the solar calendar to tell us how long a year is and when the four seasons begin. This allowed our ancestors to plan their farming and to visually observe the passing of time.

We class a day as the time it takes the Earth to spin around its axis once, and a year as the time it takes to complete an orbit around the Sun.

The number of days it actually takes the Sun to return to the same position as seen from Earth is 365.2421897. That's an extra 0.2421897 days more than the basic annual calendar allows for in a year, and that can add up pretty quickly. The additional quarter of a day is significant enough that, if left unchecked, 750 years from now December would occur in the middle of winter. The leap year's extra day provides the solution.

Leap years don't always happen every four years. If they did, our calendar would shift by an average of 11 minutes a year, leaving it a full day out of alignment with the orbital period within a century.

Instead, leap years occur every year that is divisible by four, except for those years that are both divisible by 100 and not divisible by 400.

If that isn't complicated enough, this rule creates an average calendar year length of 365.2425 days, which doesn't meet the required target of 365.2421897 - we're still off by one whole day every 4000 years. As a final adjustment, irregularly spaced leap seconds are added at the end of either June or December as determined by the International Earth Rotation and Reference Systems Service. These are irregular because, due to the complexity of the solar system bodies acting on the Earth, the Earth's rotation is unpredictable in the long term. As a result, leap seconds are typically only announced six months in advance.

The idea of a leap year was conceived by Sosigenes of Alexandria, the mathematician and astronomer of Julius Caesar, in 46BC. Caesar embraced the concept, and introduced the Julian calendar with a leap year added every four years, replacing the previous Roman calendar: a complex 355 day system with an extra 22 day month added every four years!

In the Julian calendar, February originally had 30 days. July, named for Julius Caesar, had 31 and August, named for Caesar Augustus, had only 29. When Augustus became Emperor in 27BC he took two days from February and added them to August, making his month the same length as Julius'.

In 1582, Pope Gregory XIII and his astronomers corrected for the fact that a year was not exactly 365.25 days long by skipping three leap days every 400 years, resulting in the Gregorian calendar we know and use today.

So leaplings are actually the product of millennia of astronomy and mathematical calculations, sprinkled with the history of a couple of emperors and a Pope.