Why our calendar is not perfect despite leap years
Published: Wednesday, Feb 28th 2024, 05:30
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This year is another leap year. But what exactly does that mean? And when do leap year babies celebrate their birthdays? Some fascinating facts.
WHY DO WE NEED LEAP YEARS?
One year corresponds to the time it takes the earth to orbit the sun once. The problem is that the Earth does not need exactly 365 days to do this, but around 365.25 days. Each non-leap year is therefore a quarter of a day too short. This adds up: after four years, the Earth has rotated around its own axis one too many times. To compensate for this, one day is inserted in between.
WHEN DO PEOPLE IN NON-LEAP YEARS CELEBRATE THEIR BIRTHDAYS IF THEY ARE BORN ON FEBRUARY 29?
On March 1. And yes, this is regulated by law. This is because some birthdays play a major role in legal terms, for example in determining when you are allowed to drive a car. The Swiss Civil Code states: "Anyone who has reached the age of 18 is of legal age". People who were born on February 29 have not yet reached the age of 18 on February 28 - so they only count as adults from March 1.
Incidentally, no one can celebrate their 18th birthday on February 29. 18 years after a leap year is not another leap year.
However, this does not affect many people in Switzerland. Just 197 babies were born on the last February 29, in 2020, according to figures from the Federal Statistical Office. In the last 50 years, 2335 children were born in Switzerland on a February 29.
WHAT WOULD HAPPEN IF THERE WERE NO LEAP YEARS?
The calendar days would gradually shift in relation to the actual time of year. After four years, the calendar would lag behind the actual time of year by around one day, and after eight years by around two days. If this is not compensated for, it simply continues as before. After a few hundred years, it would suddenly be summer at Christmas and winter in July.
IS ADDING 24 HOURS EVERY FOUR YEARS ENOUGH TO ALIGN THE SOLAR AND CALENDAR YEARS?
No. Because although the Earth takes a rounded 365.25 days to orbit the sun, it is exactly 365.242190 days, or 365 days, 5 hours, 48 minutes and 56 seconds. So even with the leap years, you are still eleven minutes and four seconds off every year. Over time, even this small difference would accumulate to a shift of the seasons in the calendar.
For this reason, the whole thing becomes much more complicated: some leap years have to be omitted - so not quite every fourth year is a leap year. The rule says that a leap year is skipped if the year is divisible by 100 but not by 400. For example, the year 2000 was a leap year, but the years 1700, 1800 and 1900 were not. The next time a leap year is skipped is the year 2100.
BUT IT STILL DOESN'T WORK OUT EXACTLY, DOES IT?
No, even with all these exceptions, the calendar does not exactly match the Earth's orbit around the sun. But the error is small: it takes over 3000 years to be one day off.
IS IT NOT A GOOD IDEA TO ALLOW 365,242190 WEEKS TO PASS IN ONE YEAR?
Unfortunately, it is not possible to simply add 5 hours, 48 minutes and 56 seconds at the end of each year. Then day and night would shift.
This is the big problem: the day and night cycle has nothing to do with the annual cycle. Day and night are caused by the rotation of the earth on its own axis. If you are on the part of the earth that is currently facing the sun, it is daytime. This rotation of the earth on its own axis works independently of the rotation of the earth around the sun.
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