What is a leap year? Breaking down the science, and history, behind the ancient phenomenon

Each year has 365 days. Well, almost. Every four years (give or take), our calendars receive an additional day.

Known as "leap day," Feb. 29 is tacked on and becomes the last day of the shortest month of the year. And beyond giving people an extra day to reach their goals, there is some science as to why we have leap days.

So, as you're making leap day plans for this year, here's a quick primer on how it came to be.

What is leap day?

A leap day takes place during a leap year, which is a year containing "an inserted period of time," according to Britannica.

Leap day falls on Feb. 29. Typically, February is the shortest month of the year, spanning 28 days. However, about once every four years, the month will get another date added to its calendar.

Why is leap day every four years?

The reason there are leap days, and years, is because of the Earth's orbit.

The amount of days it takes for the Earth to complete a full revolution around the Sun is not a whole number. The 365 days we experience is actually 365.2422 days, National Geographic reports.

Getting rid of 0.2422 days adds up.

The fraction allows the seasons to correctly line up each year. If it was forgotten, the months during which we normally experience each seasons would shift, according to National Geographic. This would impact other aspects of life, such as the growing and harvesting of crops.

This is why leap years exist. When added, four 0.2422 days roughly equal one full day. Feb. 29 is added to the calendar of most years that are divisible by four, including 2024.

Who made leap day?

The concept of adding leap days is not new and has been around for millennia, Britannica reports. Some calendars – such as the Hebrew, Chinese and Buddhist calendars – contained leap months, also known as "intercalary or interstitial months," according to the History Channel.

While Julius Caesar is often credited for originating leap days, he got the idea from the Egyptians. By the third-century BCE, Egyptians followed a solar calendar that spanned 365 days with a leap year every four years, National Geographic reports.

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 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.

What happens if you are born on leap day?

Feb. 29 is the rarest birthday someone could have. Still, at least 5 million people celebrate their birthday on leap day, according to the History Channel. Your odds of being born on Feb. 29 are one-in-1,461.

Many "Leaplings" (or those born on leap day) will celebrate their birthdays on Feb. 28 or March 1 during a typical 365-day year, even though documents will reflect it is on Feb. 29.

How common is your birthday? Check out the full list, plus uncommon celebrity birthdays.

Upcoming leap days

This year, 2024, is a leap year. Leap day will fall on Thursday, Feb. 29.

The next leap years will be in 2028, 2032 and 2036.

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This article originally appeared on USA TODAY: Leap Day 2024: The science, and history, behind the phenomenon