If you are not aware already, the year 2016 is a leap year! This means that instead of 28 days in February, we would have 29 days. That said, what is the origin of the leap year? What are the criteria for a leap day, and how is it calculated?

Origin of the Leap Year

The entire earth for a while modelled the calendar after lunar cycles, but this required adding an intercalary month every now and then, to maintain solar consistency. But in 46 B.C, Roman Emperor, Julius Caesar, together with Sosigenes, the astronomer changed the Roman calendar to include 12 months and 365 days. Why?

The answer lies in the fact that the Earth takes approximately 365.25 days to orbit around the sun every year. During non-leap years aka common years – like 2013, 2014, 2015 – when the calendar year is 365 days, the calendar doesn’t take into account that extra quarter of a day actually required by Earth to complete a single orbit around the sun.

In essence, the calendar year, which is a human creation, is faster than the actual solar year as defined by our planet’s actual motion through space. It is that .25 that creates the need for a leap year every four years, because adding up that extra 6 hours over the span of 4 years gives a full day. If we didn’t add a day on February 29 nearly every 4 years, we would lose almost six hours off our calendar every year. After only 100 years, our calendar would be off by approximately 24 days!

But this is not the whole story, there is more.

Criteria for Naming a Leap Year

When Julius Caesar introduced leap years, he introduced the rule that any year evenly divisible by 4 would be a leap year. This led to way too many leap years, which was corrected in the Gregorian calendar introduced more than 1500 years later.

According to this calendar, which we use to this day, three criteria must be taken into account to identify leap years. These are:

  • The year can be evenly divided by 4;
  • If the year can be evenly divided by 100, it is NOT a leap year, unless;
  • The year is also evenly divisible by 400. Then it is a leap year.

This means that in the Gregorian calendar, the years 2000 and 2400 are leap years, while 1800, 1900, 2100, 2200, 2300 and 2500 are NOT leap years. 

How is it Calculated?

In real fact, a solar year occurs over a period of 365.2422 days or more simply – 365 days, 5 hours, 48 minutes, and 46 seconds. So, adding a leap year every four years leaves a surplus of roughly 11 minutes.

And because this fact was not taken into consideration by Julius Caesar, this meant that the Julian calendar drifted off course by one day every 128 years, and by 1582, it had strayed 10 days off the solar year. 

Hence, in 1582, Pope Gregory XIII revised the Julian calendar by creating the Gregorian calendar with the assistance of Christopher Clavius, a German mathematician and astronomer. The Gregorian calendar further stated that leap days should not be added in years ending in “00” unless that year is also divisible by 400.

The Pope’s updated calendar remains in use to this day, but it is still not perfect (because it leads the solar calendar by 26 seconds). Experts note that the remaining discrepancies will need to be addressed in around 10,000 years, and by the year 4909, the Gregorian calendar will be a full day ahead of the solar year.

See Also: What Happened in the Year Our Calendar Lost 11 Days?