Everything you need to know about Leap Day

Like the Summer Olympics and presidential elections, Feb. 29 happens just once every four years. Here’s everything you need to know about the science and history of Leap Day.

Why is it necessary?

As you might remember from elementary school, a day is the time it takes for Earth to complete a rotation on its axis, while a year is the time it takes the Earth to complete an orbit around the sun.

“None of these cycles divide evenly with one another, or are multiples of one another, so that presents a challenge,” said Kenneth Coles, a professor of geoscience at Indiana University of Pennsylvania. “How do you develop and present your calendar?”

The time it takes for Earth to orbit the sun, called a solar year, actually is 365 days, 5 hours, 48 minutes and 45.41 seconds long. Without some form of Leap Day, the calendar would fall one day behind every four years. In just a few hundred years, Americans would be watching the leaves fall while celebrating the Fourth of July.

A first attempt

The calendar in the Ancient Roman Republic was based on cycles of the moon, which don’t align with days or years. As a result, the Romans would add an entire leap month every few years to keep their calendar in line with the seasons.

Julius Caesar, working with astronomers and mathematicians, proposed a new calendar to fix these problems. It looks a lot like the one we use now, with 12 months, 365 days and a leap day every four years at the end of Februarius (February).

The Julian calendar would go on to be used by most of Europe.

Take two

The Julian calendar was good, but not perfect. It created slightly too many leap days, enough that the date drifted one day ahead every 128 years.

By 1582, the calendar was running 10 days fast. That’s when Pope Gregory III proposed some tweaks to Caesar’s system for more accurate timekeeping.

It took centuries, but the Gregorian calendar was eventually adopted worldwide, and is the one we still use today.

Weird exceptions

There’s a leap year every four years, unless the year is a multiple of 100, in which case there’s not.

However, if the year is also a multiple of 400, the 100-year rule is disregarded and a leap day is observed. That means 1900 was not a leap year, 2000 was, and 2100 will not be.

These were the tweaks necessary to correct the problems with the Julian calendar.

Not perfect … but pretty close

Thanks to the Gregorian calendar, we won’t have to worry about inaccurate dates for a long time. It takes 3,030 years for the calendar to drift by a single day.

That hasn’t stopped people from thinking about how to make it better.

In 1840, British arithmetician John Herschel proposed removing the leap day scheduled for the year 4000, and the ones every 4,000 years thereafter, which would almost completely eliminate calendar drift.

His proposal has yet to be adopted. We’ve still got 1,980 years to think about it.