Why Is There a Leap Day Every Four Years? Here’s the Science Behind Feb. 29th

The last leap year was Feb. 29, 2016, which means we won’t have another one until 2020. However, most people don’t know why we periodically add the 29th day of February.

The easy answer? Leap year is a corrective measure for our calendar system. However, the concept is so much more detailed than that.

ENTITY wants to make it easy for you.

First things first, why is there a leap day?

When it comes to answering this question, you should know that leap year is much more than a fun day – February 29th – that gets added every four years. Leap year actually has a scientific purpose to keep our calendars aligned with the solar year.

The concept arose in Rome while Julius Caesar was in power. While he toyed with the idea of adding a leap month to compensate for calendar shifts, top astronomers at the time advised him otherwise.

They explained that his calculation was based on a year having 364 days, but in reality it has 365.25 days. With this new information, the leap year concept transformed from Caesar’s 15-month calendar to adding a single leap day on Feb. 24 in 45 B.C.

However, their estimates were also slightly off because a year is actually 11 minutes short of 365.25 days. To be exact, a single year is 365 days, five hours, 48 minutes and 46 seconds.

Interestingly enough, this distinction led to some pretty weird leap year rules when it comes to the turn of the century. By 1582, the previous astronomer method caused the calendar to be 10 days off, notable by Easter occurring too late in Spring. Pope Gregory XIII then amended Caesar’s calendar by creating a system that subtracts three leap years every 400 years.

This compensated for history’s earlier mistake.

In the new system, if the turn of the century year is divisible by 400, a leap year occurs. Likewise, if a century year is not divisible by 400, there is not a leap year for another four years. Simply said, the year 2000 is divisible by 400, so a leap year occurred. The year 1700 is not, so the leap year was skipped for an additional four years.

Here’s the science behind leap year.

Let’s start with what we know already. A single year is 365 days, five hours, 48 minutes and 46 seconds. That almost quarter of a day tacked on to the 365 days is the important concept when it comes to answering, “What is leap year?”

We still use the Gregorian calendar today, so we discount the extra five hours, 48 minutes and 46 seconds. Easier said, we have 365 days in a standard calendar year. But, that doesn’t mean the quarter of a day we don’t factor in just magically disappears. Every four years, those five hours, 48 minutes and 46 seconds add up to a single day.

That’s where the concept of the leap year occurring every four years comes into play. We are essentially giving the fourth year 366 days to account for the time we take out in a standard year.

Why does this matter?

So you may be asking, “Why does it matter if we count the extra time every year or every four years?” Aside from avoiding an awkward New Year’s celebration a quarter into the last day of the year, there are real issues relating to early sustainability and time that a leap year reconciles.

This goes back to the concept of our calendars being aligned with the solar year. Our system relies on the sun to dictate how many days make up a year and when the seasons begin. When it comes to determining seasons, scientists observe the sun’s position on the horizon. It moves further south in the winter season and north in the summer.

In the early days, the leap year helped farmers account for seasonal switches that would affect crop growth. Today, it sets our concept of time that helps us accurately and efficiently predict future events.

For example, a 365 day calendar that discounts those five hours, 48 minutes and 46 seconds without adding a leap year, would provide a whole new concept of the seasons as time passes. After 750 years, June would be observed in winter instead of summer.

This means there would be no more “white Christmas” or springtime Easter celebrations. In addition to the predictability of future planning increasing with a leap year, socio-cultural reasons like holidays have pushed the system to continue.

But, why do we always add a day to February?

We’re heading back to ancient Rome when it comes to this portion of the what is leap year question.

February is shorter than the other months. While a popular circulating myth attributes the shortened month to Augustus Caesar stealing a day from February to give his month, August, a 31st day, this explanation is just another tall tale of the Internet.

In reality, February is shorter because it was an afterthought in the traditional Roman calendar.

In 8th century B.C., the Roman calendar had 10 months, starting in March. This meant January and February didn’t exist. They also did not account for the 31st day in their month Sextilis, equivalent to August, and December.

Why did the Romans ignore 61 days of the year? The cold days January and February are known for, aren’t productive to crop growth. At this time, crop growth and seasonal habits were the whole point of having a calendar and they didn’t see the need to consider the first two months.

However, through time and superstition, the cold months were added to the calendar.

Picking a leap day depends on culture.

Calendar days were changed to alternating odd numbers – 29 days or 31 days in 713 B.C. – and were based on a lunar rather than a solar system. February was left the oddball out with 28 days because back then it was considered the last month, not the second.

By the time Caesar stepped in with the solar year system, February was now considered at the beginning of the calendar. Withstanding the changes, it kept its 28 days. Then, February finally stopped getting the short end of the stick, which brings us back to the Romans creating the leap day on Feb. 24.

In the modern world, switching the day to the end of the month was just more practical and convenient. Although, some countries still have not adopted the Gregorian calendar. China, for example, includes a leap month. Meanwhile, Iran does not recognize Feb. 29 as a leap day and instead calculates leap years by seasonal equinoxes instead of mathematics.

So now you know everything you need to know about the leap year. It’s time to spread your newfound knowledge and share this article with your friends!