Leap years fascinate people. Maybe that’s because they add a day. February 29 is not an extra day at all, just a mathematical trick to make up for lost time.

It is said in my family that my father was born on February 29, 1936 – on a leap day in a leap year long ago. Maybe it suits a man who likes to say that he is not interested in birthdays: So he has a good excuse to skip three out of almost four celebrations.

But you should know that my father was a passionate storyteller – he started his professional life at weekly markets. In this respect, it could also be that he was not born in 1936, but maybe already in 1935 – in a “normal” year.

**What is the difference between a normal and a leap year?**

According to the Gregorian calendar – which has prevailed worldwide – one year has 365 days. A leap year, on the other hand, has 366 days. This is an attempt to reconcile the accuracy of the Western Christian calendar with the earth’s rotation around the sun – and with other astronomical events: the summer and winter solstices marking the beginning of the calendar summer and winter, and the day and -Night-like, the equinox. These are the two dates on which day and night are of equal length. They mark the calendar start of spring and autumn.

**Where is the problem?**

Quite simply: there is no perfect calendar. Every calendar describes a year, but it is never a mathematically perfect year.

Actually, a year is the time it takes the earth to circle the sun. This period, which astronomers call the tropical year, does not last exactly 365 days. The exact period between two spring day-and-night matches is actually 365 days, 5 hours, 48 minutes and 46 seconds. Precisely expressed as a decimal number, that’s 365.2422 days.

Roughly speaking, each year is almost six hours longer than the calendar year. And the leap years compensate for these 0.2422 days. If we had no leap years, our seasons would get out of sync by 24 days after 100 years.

**How often do we need leap years?**

There is a leap year every four years, with a few exceptions. And this is how it happened: Leap years were originally introduced by the Roman emperor Julius Caesar and his Julian calendar. Back then, the rule was strict: there is a leap year every four years. But that led to overcompensation.

The Gregorian calendar, developed by the astronomer Aloisius Lilius in the 16th century and named after Pope Gregory XIII, had stricter rules. Accordingly, leap years are years in which the year can be divided by four. There are also century years that end in “00”. These are not leaped years – unless they can be divided by 400: Then there are.

You have to skip a few leap years to compensate for the fact that the 5 hours, 48 minutes and 46 seconds are still 11 minutes and 14 seconds shorter than a quarter day. So you correct the correction. But at the end of the day, a less than perfect sum remains.

**Competition of calendars**

The Gregorian Calendar was first introduced in Italy, Poland, Portugal, and Spain in 1582. It is considered one of the most accurate calendars today, but still has a deviation of about 27 seconds a year from the tropical calendar – about one day every 3236 years. It only ranks fourth among all historical calendars when it comes to accuracy.

The Mayan solar calendar from around 2000 BC came to a deviation of one day in 6500 years. The revised Julian calendar from 1923 had a deviation of only one day in 31,250 years and the Iranian solar calendar Hijri, which is about as old as the Mayan calendar, had an error rate of one day in 110,000 years. This calendar achieved its precision by orienting itself on astronomical observations and not on mathematical logic.

**Do other calendars also have leap years?**

The Chinese calendar has leap years with leap months, but no leap days like the Gregorian calendar. The Hindu calendar also inserts a leap month. An Ethiopian calendar has 13 months, with the 13th month having five days in normal years and six in leap years. In Islam, there are 11 leap years within a 30-year cycle. The Jewish leap year has between 383 and 385 days and occurs seven times in a 19-year cycle.

**What is the use of switching?**

We also switch a lot back and forth: World time is adjusted regularly to compensate for inaccuracies in the earth’s rotation. But it is very important for people to feel that they live in harmony with nature. Certain public holidays are certainly not based on astronomical events by chance: Easter, for example, at the beginning of spring.

But if we didn’t have such reasons – would it really make a difference for us if the seasons shifted from month to month or if we lost a few hours or days over thousands of years? Would we even notice?

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