What is a leap year? Why the concept of leap year is necessary; All you need to Know

Every four years, our annual 365-day journey around the sun expands to 366 days.
The year 2024 is a leap year, entailing the addition of an extra day to the end of February, thereby extending the year by one day. Leap years occur every four years, with the most recent instances in 2020 and 2016, and the upcoming leap year scheduled for 2028.

Understanding Leap Year: What is Leap Day?

Leap day, occurring once every four years, serves to make February one day longer, contributing to the scientific precision of our calendars. The rationale behind leap years is rooted in science.

Typically, our calendar years consist of 365 days, representing the time Earth takes to complete its orbit around the sun. However, according to the National Air and Space Museum, 365 days is a rounded figure. The Earth actually requires 365.242190 days to complete a full orbit, equivalent to 365 days, 5 hours, 48 minutes, and 56 seconds.

What defines a leap year?

A leap year signifies the inclusion of an additional day in the calendar. According to NASA, "It takes approximately 365.25 days for Earth to orbit the Sun - a solar year. We usually round the days in a calendar year to 365. To make up for the missing partial day, we add one day to our calendar approximately every four years. That is a leap year."

Why is the concept of leap years necessary?

The concise explanation for the necessity of leap years is that our calendar must stay synchronized with astronomical seasons. Earth's orbit around the Sun spans about 365.25 days, slightly more than the neat, rounded number of 365 days in our Gregorian calendar. Since our calendar doesn't precisely account for the extra quarter of a day Earth requires for its solar orbit, it gradually falls out of sync with the solar year.
Due to this 0.25-day difference, our calendar starts to deviate from the seasons. The addition of an extra day, commonly known as a "leap day," to the calendar every four years serves to realign it with the seasons.

Without leap days, the calendar would accumulate an error of 5 hours, 48 minutes, and 45 seconds each year. Over a century, this would result in a misalignment of the seasons by 25 days. Eventually, what we currently identify as February and March would feel like summer in the Northern Hemisphere.

How do you determine a leap year?

There are specific rules for identifying whether a year qualifies as a leap year. As per the Farmer's Almanac, these rules are as follows:
A year may be a leap year if it is evenly divisible by 4.
Years divisible by 100 (century years like 1900 or 2000) cannot be leap years unless they are also divisible by 400. (This is why 1700, 1800, and 1900 were not leap years, while 1600 and 2000 were.)

Is a leap year truly every 4 years?

Not precisely. The formula accommodates a six-hour annual calendar discrepancy, exceeding by 11 minutes and 14 seconds. Therefore, the rule is as follows: Leap year occurs every four years, except if the year is divisible by 100, unless it's also divisible by 400. This implies that we typically omit a leap year once every century, but every fourth century, we include it.

In which years does February have 29 days?

The years 2024, 2028, and 2032 are leap years, resulting in February having 29 days. Extending through the end of the 21st century, the list of leap years also encompasses 2036, 2040, 2044, 2048, 2052, 2056, 2060, 2064, 2068, 2072, 2076, 2080, 2084, 2088, 2092, and 2096.