Are you ready for leap day? Here's where the concept came from

How many days are there in a year? The answer is 365, right? That number is actually rounded down slightly from 365.24. That quarter adds up to a full day every four years, which we observe as a leap year.

This year is a leap year, so February will be an extra day longer than expected.

That's all well and fine, but it doesn't quite get into the reason why our calendar uses leap years. Here's what to know.

What is a leap year and a leap day?

As described above, a leap year is when Feb. 29 is added to the calendar. Feb. 29 is a leap day, and it's the sum of those quarter days we ordinarily miss because our calendar rounds down to 365 days per year instead of 365.24222.

Why is February the month an extra day is added during a leap year?

Choosing February for the leap year and adding an extra day dates back to the reforms made to the Roman calendar by Julius Caesar, who was inspired by the Egyptian solar calendar, according to History.com. The Roman calendar, at that time, was based on a lunar system and had a year of 355 days, which was shorter than the solar year. This discrepancy caused the calendar to drift out of sync with the seasons over time.

To address this issue, Julius Caesar introduced the Julian calendar, a solar calendar, which included a leap year system. The tradition of adding a leap day to February persisted when the Julian calendar was later refined into the Gregorian calendar in 1582.

Was 2020 a leap year?

It may be hard to remember, but the last leap year occurred in 2020. If you forgot, don't worry. The excitement was quickly overshadowed by the pandemic caused by the novel coronavirus just weeks after the leap day was celebrated.

What years are leap years?

Leap years occur every four years due to the sum of the banked quarter days. With a leap year falling in 2024, the next leap years will occur in 2028, 2032, 2036, 2040 and so on.

There is a catch, however. Our calendar isn't 100% accurate, according to the National Air and Space Museum. Adding a leap day every four years causes our calendar to be longer by 44 minutes, which adds up over time and causes our seasons to drift.

A general rule of thumb is employed to help offset this — if a year is divisible by 100 but not 400, we skip the leap year. We skipped leap years in 1700, 1800 and 1900, but didn't in 2000.

According to How Stuff Works, the Y2K, or Year 2000 problem, was the reason we skipped the leap year. The Y2K problem, which subsequently spawned world-ending conspiracy theories, stemmed from how computer software at the time interpreted dates.

Back then, software typically used a two-digit date format, like MM/DD/YY. At the turn of the century, software wouldn't be able to interpret the difference between the year 1900 and the year 2000, so developers had to rewrite software to use 4-digit placeholders for years instead.

Who came up with the leap year concept?

There doesn't appear to be a consensus on who originally came up with the idea for leap years, but history does point to a few potential candidates.

Hebrew, Chinese and Buddhist calendars have all contained leap months, which are also referred to as "intercalary or interstitial months," according to the History Channel.

Julius Caesar is often thought to be the originator of the idea, but even he got the idea from the Egyptians, which followed a solar calendar that spanned 365 days, with a leap year happening every four years, just like ours.

In ancient Rome, their calendar varied and included a 23-day intercalary month called "Mercedonius." But it was not a standalone month. Mercedonius was added to February to account for the difference between the Roman year and the solar year, according to the History Channel.

When making the Julian calendar, Caesar took inspiration from the Egyptians and decided to add an extra day to the month of February every four years. The Julian calendar officially began on Jan. 1 in 45 BCE.

This method would continue over several centuries, but not without issue. Caesar's math of 365.25 days was close, but it wasn't the exact 365.2422 days the solar year contains. To be precise, Caesar "overestimated the solar year by 11 minutes," the History Channel reports. This meant the Julian calendar would be short a day every 128 years, according to National Geographic.

By the 16th century, time had shifted again and not in a good way. Major dates had changed, including Easter. The holiday is supposed to occur on the first Sunday following the first full moon on or after the spring equinox. At the time, Easter's date had moved by about 10 days.

To fix this, Pope Gregory XIII introduced the Gregorian calendar, which kept a leap day every four years but eliminated it during centurial years, not divisible by 400, according to the History Channel. This is why 1700, 1800 and 1900 were not leap years, but 2000 was.

Despite its accuracy, the Gregorian calendar is not flawless. Instead of being off by one day every 128 years like the Julian calendar, the Gregorian calendar falls short once every 3,030 years, the History Channel reports.

Contributors: Olivia Munson and Saman Shafiqu, USA Today

This article originally appeared on Pensacola News Journal: 2024 is a leap year. Ever wondered why we leap every 4 years?