How the leap year keeps our calendars in sync


Tomorrow is February 29th, 2020. A day like any other, you might think. Not quite. In our calendar system, February 29 is only every four years – in a leap year. We explain what this is all about and why there are leap years at all.

The importance of the leap year


Almost five million people worldwide and 55,000 people in Germany should be particularly happy about the leap year 2020: Their birthday is on February 29 – and they do not have to decide whether they would rather celebrate on February 28 or March 1. In contrast to “normal” calendar years, a leap year has one more day; instead of 365 there are 366 days in the calendar. The additional day is “switched on” every four years in February – the shortest month of the year. In technical jargon, switching on an additional day or month is also referred to as intercalation. But why does a leap year have one more day? The Gregorian calendar, which our calendar system is based on, is based on the orbit of the sun. A year has passed when the earth has completely orbited the sun. For this, the Earth needs exactly 365 days, five hours, 48 ​​minutes and 46 seconds – almost six hours longer than a “normal” calendar year.

The Gregorian calendar, which our calendar system is based on, is based on the orbit of the sun. A year has passed when the earth has completely orbited the sun. 

Briefly explained: The history of the leap year


More than 2000 years ago, Julius Caesar ordered a reform of the Roman calendar, according to which every fourth February should have an additional day. To date, the 12 months of a year had only resulted in 355 days, but a full year, as we already know, takes 365 1/4 days. Until Caesar’s calendar reform, the turn of the year did not take place in January, but in March. As the last month on the calendar, only 28 days remained for February – which is why it is still the shortest month of the year to this day. After the introduction of the Julian calendar, the calendar year now included a full 365 days. The remaining quarter of a day should be offset with an additional day in February every four years. So far so good. Over the centuries, however, the calendar got out of step again: in 1582 there was a difference of ten days between the calendar and the solar year. The problem: If every fourth year is a leap year, the average length of a year is 365.25 days. This comes very close to the sunny year with 365.24 days, but is still not exact. Pope Gregory XIII recognized the mistake and, with the introduction of the Gregorian calendar, set up a new concept that would eliminate three leap days in 400 years. The calendar year was adjusted to the solar year up to about half a minute.

And when will the next leap year follow?


Leap year or no leap year? With a simple formula we can also determine without a calendar whether it is a leap year or an ordinary “common year”:


      1. A leap year must be divisible by 4.
      2. A leap year is not divisible by 100, except …
      3. … the year can be divided by 400. Then it is still a leap year.

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