What is a leap year?

You may be used to hearing that our planet takes 365 days to complete a complete revolution around its star, but in reality this journey takes 365 and a half days. After four years, these extra hours add up to one full day. In a leap year, we add this extra day to February, making it 29 days instead of the normal 28 days.

The idea of ​​the annual catch-up dates back to ancient Rome, where people had a calendar of 355 days instead of 365 because it was based on the cycles and phases of the moon. They realized that their calendars changed with the seasons, so began adding an extra month called Mercedonia every two years to make up for missed days.

In 45 BC, Roman emperor Julius Caesar introduced a solar calendar based on a calendar developed in Egypt. In this situation February has an extra day every four years. In honor of Caesar, this system is still known as the Julian calendar.

But this was not the last change. Over time, people realized that the year is not actually 365.25 days, but 365.24219 days, or about 11 minutes less. So adding a full day every four years was actually a bit more of an adjustment than necessary.

In 1582 Pope Gregory XIII signed an order making a minor change. A leap year occurs every four years, except for “centennial” years (years divisible by 100, such as 1700 or 2100), unless the year is divisible by 400.

It may sound like a bit of a puzzle, but this adjustment made the calendar even more accurate. From then on it became known as the Gregorian calendar and, despite some objections, it spread throughout Christendom.

What if we didn’t have leap years?

• If the calendar did not make this small adjustment every four years, it would gradually lose harmony with the seasons.
• Over centuries, this can cause solstices and equinoxes to occur at different times than expected.
• Winter weather can occur when the calendar indicates summer, leaving farmers confused about when to plant seeds.

Other calendars around the world have their own ways of telling time. The Jewish calendar, ruled by the Moon and the Sun, is like a big puzzle with a 19-year cycle. A leap month is occasionally added to ensure that special holidays occur at the right time.

The Islamic calendar is even more unique. It follows the phases of the Moon and does not add extra days. Since the lunar year has only 355 days, important dates in the Islamic calendar move 10-11 days earlier than in the solar calendar each year.

For example, Ramadan, the Islamic month of fasting, falls in the ninth month of the Islamic calendar. In 2024, it will last from March 11 to April 9, in 2025 – from March 1 to 29, and in 2026 – from February 18 to March 19.