The origin of leap years: to whom the idea belongs

Yulia PoteriankoLife
We explain what the leap year idea and the legendary Julius Caesar have in common

The year 2024, which has just begun, will be somewhat unusual - it has 1 more day in the February calendar, i.e., it is a leap year. But do you know the history of this phenomenon?

The Live Science publication told us more about it. The Roman Emperor Julius Caesar and Pope Gregory XIII took part in this story. And, of course, astronomy was involved.

What is a leap year?

In a leap year, there are 366 days rather than 365. According to the modern Gregorian calendar, this happens once every four years. Then an extra day appears in February - the 29th. This happens in years whose serial number is divisible by 4 without a remainder, like 2024. Almost all years that are divisible by 100 are exceptions.

Leap years exist not only in the generally accepted Gregorian calendar but also in local chronological systems, such as the Jewish, Islamic, Chinese, or Ethiopian calendars. However, they are calculated differently and do not occur every four years and may have several extra days and even entire short insertion months.

In addition to leap years and leap days, the Gregorian calendar also has several leap seconds that are added to certain years from time to time. The last time this happened was in 2012, 2015, and 2016. However, the International Bureau of Weights and Measures (IBWM), the organization responsible for global timekeeping, will abolish the extra seconds starting in 2035.

Why do we need leap years?

If you don't delve into the astronomical details, leap years may seem like a meaningless idea. But, in fact, they are very important for balancing time.

That's because, in terms of Earth days, a Gregorian calendar year is slightly shorter than a solar or tropical year, which is the amount of time it takes for the Earth to make one complete revolution around the Sun. A calendar year lasts exactly 365 days, while a solar year has 365.24 days, or 365 days, 5 hours, 48 minutes, and 56 seconds.

If this difference is not taken into account, then for each year, the gap between the beginning of the calendar year and the solar year would increase by the same 5 hours, 48 minutes, and 56 seconds. This would gradually shift the seasons. Thus, if we abandon the practice of leap years from today, in 7 centuries, summer in the Northern Hemisphere will begin in December instead of June.

Adding a leap day every four years largely eliminates this problem, as the length of the extra day is roughly equal to the length of the difference accumulated over this time. However, even this system is not perfect. In one year, about 11 additional minutes are still accumulated, and in 129 years, an additional day is thus gained. To solve this problem, we skip leap years in every centennial year, except for those that are divisible by 400, such as 1600 and 2000. But even in this case, there is still a small difference between calendar years and solar years, so IBWM experimented with the extra seconds.

The history of leap years

But when did leap years appear? Actually, a long time ago - even before B.C. They were officially introduced in 45 B.C. by the Roman Emperor Julius Caesar. The calendar he introduced was called the Julian calendar. It consisted of 365 days in a year, divided into 12 months. We still use this system today.

According to the University of Houston, the Julian calendar included leap years every four years. They were appointed without exception and synchronized with the Earth's seasons due to the "last year of confusion." It was 46 BC. It consisted of 15 months and lasted 445 days.

For centuries, the Julian calendar seemed to work perfectly. But by the middle of the 16th century, astronomers noticed that the seasons had shifted by about 10 days and began to start earlier than expected. This affected the calendar of important holidays such as Easter. It no longer fell on the vernal equinox.

To fix this, Pope Gregory XIII introduced the Gregorian calendar in 1582. In addition to the Julius Caesar's calculation of time, he introduced certain exceptions for years whose number is divisible by 100 (except for years divisible by 400).

For centuries, the Gregorian calendar was used only by Catholic countries such as Italy and Spain. Eventually, however, it was adopted by Catholic states for its convenience. For example, Great Britain adopted it in 1752 when it became clear that its years were deviating greatly from those of Catholic countries. To do this, it had to skip 13 days. So that year, September 2 was immediately followed by 14.

At some point in the distant future, the Gregorian calendar may have to be revised, as it is not perfectly synchronized with the solar years either. But it will take thousands of years for this to happen. For now, this system works quite well.

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