Why We Have Leap Year

why we have leap year

It’s 2016.  This is the year that Juno will arrive at Jupiter to study the gas giant’s composition, gravity field, magnetic field, and polar magnetosphere.  This year also brings us February 29–the Leap Day.  It happens every four years.  So why is it that we have Leap Year?

Just about every four years we add an extra day to the end of February.  In short, this extra day is put there to ensure that we stay in line with the Earth’s movement around the Sun.  The calendar we know contains 365 days; however, the time it takes for the Earth to revolve around the sun is a tad longer–roughly 365.2421 days.  This might not seem like a big deal, but it adds up over time.  To ensure that we stay in line with the true astronomical year, it’s necessary to add this additional day to make up for the lost time.

The Egyptians were the first to realize that we needed to have a Leap Year, but it wasn’t until the reign of the Roman dictator Julius Caesar that the practice took hold.  Before this, the Roman calendar operated on a messy lunar model that had the addition of an extra month to keep things on track.  Then, in 46 B.C., Caesar and the astronomer reworked the Roman calendar to include 12 months and 365 days.  This became known as the “Julian Calendar” and accounted for the slightly longer solar year by adding a leap day every four years.

So why did you say “just about every four years”?

Well, the Julian calendar didn’t solve all the problems.  Caesar’s model helped to realign the calendar, but the solar year is only .242 days longer than the calendar year–not a nice .25.  Adding a leap day every four years actually leaves a surplus of roughly 11 minutes.  This difference means the Julian calendar drifts off course by one day every 128 years.

To fix this problem, Pope Gregory XIII instituted the revised “Gregorian Calendar” in 1582.  This change has leaps years occuring every four years except for years evenly divisible by 100 and not by 400.  For example, the year 1900 was not a leap y ear because it was divisible by 100 but not by 400.

This new calendar remains in use to this day; however, it’s still not perfect.

Experts agree that the remaining discrepancies will need to be readdressed in about 10,000 years.

What are your chances of being born on leap day?
About 1 in 1,500.

When is the birthday party?
This is an ambiguous question that is decided from state to state. Most states, however, consider March 1st the official day. For instance, the Michigan Vehicle Code states that people born on February 29th “are deemed to have been born on March 1st.”

How many people were born on leap day?
There are about 187,000 people in the US and 4 million people in the world who were born on Leap Day.