Leap Year: A Deeper Look into the Science Behind the Extra Day

What is leap day?

Leap day is an extra day that gets added to the calendar. During a leap year, which occurs every four years, leap day falls on Feb. 29, giving the shortest month of the year one added day.

Why is leap day every four years?

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

The 365 days we experience is actually 365.242190 days, according to the National Air and Space Museum.

Getting rid of those 0.242190 days adds up.

That fraction allows seasons to correctly line up each year. If leap day was left off the calendar, the months during which we normally experience each season would eventually shift. This would impact other aspects of life, such as the growing and harvesting of crops.

When do we skip leap day?

To make up for decimals of time, we’ll sometimes skip leap years, but it’s rare. Years divisible by 100 but not 400 are skipped, meaning we skipped leap years in 1700, 1800, and 1900 but not 2000. The next leap year we’ll skip is quite a ways away, in 2100.

Who created leap day?

The concept of adding leap days is not new and has been around for millennia. 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 overestimated the solar year by 11 minutes. 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. 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.

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.

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.

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.

25 never looked so good: Oklahoma woman born on leap day celebrates 100th birthday

Meet Mary Lea Forsythe. She turns 100 on Leap Day, but it’s only her 25th birthday.

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