Science

# The origin of leap year: Why is there an extra day on February 29th only every four years?

## Julian calendar, Gregorian calendar, leap years, February 29th... why there is an extra day every four years goes back to the time of Julius Caesar.

**February 29th** is a day that **“exists” only once every four years**. This means that every four years there is a year with 366 days instead of the usual 365, which are called “leap years”. Although this is something that practically everyone knows, what is not so well known is the exact **origin **of this anomaly in the calendars.

### February 29: Why do leap years only occur every four years?

The origins of this extra day in the year go back to the **Julian calendar**, which was introduced by **Julius Caesar** in 46 BC. The dictator **reformed the Roman calendar**, which was the first system devised to divide time in ancient Rome, by **adding an extra day to February every four years**. This means that after this reform, there would be **three consecutive years of 365 days and a fourth year of 366 days**.

What was the reasoning behind this decision? Quite simply, **a solar year** - the time it takes the Earth to go around the Sun - is **slightly less than 365.25 days**. In other words, **this phenomenon does not last for a full number of days**,** causing a discrepancy in the calendar** that grows larger as time goes on. And since calendars were designed precisely to keep track of the events of the year, such as the four seasons, **this discrepancy would render them useless**.

### From the Julian Calendar to the Gregorian Calendar

However, the Julian calendar was not perfect, since it **introduced a delay of about one day every 128 years to the solar year**. Therefore, since 1582, we have been governed by the **Gregorian calendar**, introduced by Pope **Gregory XIII**.

To correct the mismatch of the Julian calendar, **the Gregorian calendar introduced a new rule: a year is considered a leap year only if it is divisible by 4, except if it ends in “00″, in which case it must also be divisible by 400**. For example, the year 2000 (divisible by 4, by 100, and by 400) had a February 29, while the year 1900 did not.

### The Gregorian calendar is still inaccurate for the solar year

This stricter rule introduced by the Gregorian calendar meant that the days and the advance of the seasons could be measured more accurately; **the mismatch is only one day every 3324 years**, whereas, with the Julian calendar, **there was a mismatch of one day every 128 years**. In the Julian calendar, the years lasted 365.25 days, while in the Gregorian calendar, they lasted 365.2425 days.

The new Gregorian calendar rule to compensate for the Julian calendar mismatch divided time into **cycles of 400 years, with 97 leap years and 303 common years**. Today, as we continue to use the Gregorian calendar, **there is still an inaccuracy in synchronizing the advance of days and the change of seasons with the solar year.**