Leap years: Why we have them, why they don’t come every four years

What exactly is a leap year? Why was the need felt for it? When was it first introduced? What adjustments have been made to how it's calculated? We explain

The extra day of the leap year is added to February, the shortest month of the year, as February 29. (Photo: Wikimedia Commons)

February has one extra day this time, as 2024 is a leap year. While it is generally known that a leap year occurs every four years, what exactly is a leap year? Why was the need felt for it? Is every fourth year a leap year? We explain.

What is a leap year?

A leap year has 366 days in a year as opposed to the regular 365 days. The extra day is added to February, the shortest month of the year, as February 29.

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Why was a leap year introduced?

One year in a solar calendar reflects one round the Earth makes around the Sun. The Earth takes 365 days, 5 hours, 48 minutes and 46 seconds to revolve around the Sun. The length of a normal year is thus rounded off to 365 days.

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To account for the extra time, rounded up to six hours, 24 hours — one full day — are added to every fourth year.

If this had not been done, crop cycles and seasons would start occurring at different times of the year gradually, leading to confusion.

When was the leap year introduced?

As The Indian Express has explained earlier, the leap year was introduced by scholars engaged by Julius Caesar in 46 BC, and made more precise from 12 AD.

The Julian calendar had a year that was usually 365 days long, with a 366th day added once every four years. According to the website of the Royal Museums Greenwich, the Islamic calendar Al-Hijra also has an extra day added to the 12th month Zul Hijja on leap years.

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However, even this method was not error-free, because the six hours used for calculation are still different from the actual 5 hours, 48 minutes and 46 seconds, making the calendar year slightly longer than the solar year.

Thus, in the 16th century, it was calculated that the calendar years until then had accumulated 10 extra days. In 1582, Pope Gregory XIII ordered a drastic compensation by dropping 10 days from the calendar, and October 4 that year was followed by October 15 the very next day.

Why does a leap year not come every four years?

Pope Gregory XIII’s one-time action was obviously not enough to solve the problem. Thus, it was decided that some leap years — about one leap year every century — would be dropped to manage the extra day. The years chosen for this were those ending with 00.

However, dropping the leap year from all years ending with 00 would again throw the calculation out of whack. Finally, in the Gregorian calendar, the 00 years that were divisible by 400 ended up as leap years. Thus, 1900 was not a leap year, but 2000 was.