Why do leap years occur and how do they occur?

(CNN) — Leap years exist because we need to move forward a bit astronomically.

The time taken by the Earth to revolve around the Sun is not the total number of days. According to NASA’s calculations, a solar year or tropical year is actually 365 days, 5 hours, 48 ​​minutes, and 46 seconds. As a result, each year lags behind the normal 365-day calendar solar year by about a quarter day. Although this may not seem like a big difference, over four years it is equivalent to almost a full day. This is where leap years come in. Without them, the summer weather we typically experience in June would be in December 700 years from now (as just one example from the Northern Hemisphere).

Many ancient populations, including the Sumerians, Chinese, and Romans, created calendars based on the phases of the moon. While lunar calendars do a good job of keeping track of months, they are not good at keeping track of seasons.

This caused some problems in ancient Rome. Around the year 500 B.C. c., the Roman Republican calendar consisted of 12 lunar months and a total of 355 days, about 10 days less than a solar year. To keep the calendar in sync with the seasons, the Romans added an extra month of 27 or 28 days, called the Mercedonian, every few years. However, due to political maneuvering, this was done so inconsistently that the system caused widespread confusion.

Around 46 BC, the Roman emperor Julius Caesar proposed a solution: the Julian calendar. This new 12-month calendar would always have 365 days, except every four years when an extra day was added. This gives the average length of the Julian year as 365.25 days, which is very close to the solar year of 365.2422 days.

Unfortunately, it wasn’t close enough. The average year in the Julian calendar was still 11 minutes and 14 seconds longer than the solar year, so errors began to accumulate in the calendar.

By the end of the 16th century, the calendar was behind by about 10 days and the Catholic Church was having difficulty fixing the date of Easter, which had to fall on the Sunday after the first full moon after the spring equinox.

This is where the modern calendar comes in. Pope Gregory XIII rearranged the seasons in 1582 by eliminating an additional 10 days. That year, Thursday, October 4 was followed by Friday, October 15. To fix the calendar in the future, he introduced what we use now: the Gregorian calendar. It is basically the Julian calendar, but with a new rule: all years divisible by four must be leap years, except centennial years, which must be divisible by 400 to be leap years.

What’s next for leap years?

Therefore, the years 800, 1200 and 2000 were leap years, but 1700, 1900 and 2100 are not leap years, because although they are divisible by four, they are not divisible by 400.

This increased the average length of the Gregorian year to 365.2425 days, only 26 seconds less than a solar year.

Whereas under the Julian calendar, a day shift occurs every 129 years, it occurs every 3,333 years under the modern calendar.

But this is a problem for the second millennium.