The science behind leap year and why it only happens every four years

We all know that every year consists of 365 days, and that every four years, the calendar gets an additional day added to February. That day is February 29 and it is marked as the Leap Day, falling in the month which also happens to be the shortest of the year. But if you have also wondered why the calendar slightly changes a bit every four years then it's time to take a deep dive into the science and history behind a leap year.

The reason behind the inclusion of an additional day is because of the Earth's orbit. A report by National Geographic states that the number of days it takes for Earth to complete a full revolution around the Sun is not 365, but in reality, it is 365.2422 days in numerical terms. To round up the 0.24222 days, our calendar gets an additional day after every four years. 

Now, a lot of us might wonder who was the first fellow to decode this idea of adding an extra day to the calendar and the real story lies way back in our history. According to the History Channel, some calendars – such as the Hebrew, Chinese and Buddhist calendars – contained leap months, which are also called "intercalary or interstitial months."

As per National Geographic, the famous Roman general and dictator, Julius Caesar, is often credited for inserting the leap days into his Julian calendar after he got the idea from the Egyptians, who used to follow a solar calendar which had 365 days and a leap year every four years. Taking a look back at ancient Roman calendars, we can find that they had a different dating system that included a 23-day intercalary month called "Mercedonius." Mercedonius was added to February to account for the difference between the Roman year and the Egyptian solar year. Under Caesar's rule, the Julian calendar officially began on January 1, 45 BCE. 

Caesar, however, "overestimated the solar year by 11 minutes" and therefore, his calculation was not precise. This meant the Julian calendar would be short a day every 128 years. By the 16th century, the standard calendar underwent a lot of changes. Pope Gregory XIII introduced the Gregorian calendar system that kept a leap day every four years but eliminated it during centurial years not divisible by 400. Hence, years like 1700, 1800 and 1900 were not leap years, but 2000 was. But the Gregorian calendar is not flawless and it falls short once every 3,030 years.

Now, the question is, how do people born on February 29 celebrate their ultra rare birthdays? History Channel reports that about 5 million people have their birthdays on the leap day. The odds of being born on February 29 is 1 in 1,461 people. Most people choose to celebrate it on February 28 or March 1 during a typical leap year. We will get to see the upcoming leap years in 2028, 2032 and 2036. Meanwhile, you can check out Neil deGrasse Tyson's simple explanation about leap year on the YouTube channel of National Geographic.