LEAP YEAR. 2020 has a February 29th! But why is it a leap year? Origin, calendar, definition, algorithm … Linternaute.com takes you behind the scenes of this atypical date!
Julius Caesar, astronomical phenomenon, algorithm, calendar and solar time … All these keywords have one thing in common: they have something to do with the leap year and its February 29! And 2020 is there, since it is a leap year, which comes to take over from 2012 and 2016 over the past decade. February 29, 2020 “falls” on a Saturday, a boon for people born on this day, who will for once be able to celebrate their birthday on the same day, moreover the weekend.
But to understand the leap year phenomenon, it is not enough to know that it returns every four years. Exceptions do indeed exist for the end of the century (see here and here), and the setting up of leap years has a very precise origin: limiting the gap between the solar and calendar rhythms, since the revolution of the Earth around the Sun is done in slightly more than 365 days, in contrast to the duration of the calendar. What if you were told that a historical figure played a key role in this story, in the person of Julius Caesar, helped by a great scientist? Good exploration!
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2020 is a leap year! It is indeed divisible by 4 and not divisible by 100 (see the calculation method, alias leap year algorithm). It therefore has a February 29, last day of the month “added” to the calendar, and which falls on a Saturday this year.
The leap year as such therefore corresponds to a calendar phenomenon which occurs every 4 years. The reason is astronomical, in the strict sense of the term. It is the revolution of our Earth around the Sun. The latter does not actually take place in 365 days, as is often believed and as the Egyptians of Antiquity agreed in their calendar, but in 365.2422 days. By rounding down to the nearest whole number, we therefore create a delay between our human calendar and the Earth’s cycle of revolution around the Sun. The result is a shift in the seasons which must be regularly corrected if one does not want to one day have summer in winter and vice versa (read the details of the explanations for leap years). It is to make up for this delay that every four years, one more day sneaks into the year.
But where does the word “leap” come from? According to the service-public.fr site, it comes from the Latin “bisextus”, literally meaning “twice the sixth”. So that the year does indeed last 365.25 days (as close as possible to the time the Earth takes to circle the Sun), Julius Caesar had already added a 366th day every 4 years in his calendar, on the advice of an astronomer, Sosigenes of Alexandria. This great scientist from ancient Greece is behind the Julian calendar, nothing less. To introduce this additional day without changing the rhythm of Roman festivals, he had suggested to the Roman general to create a “sixth bis day” in March (“bisextus”).
If the rules have changed a lot today (read below), we must therefore give the illustrious Jules his contribution. It is partly to him that we owe our leap year in 2020.
Before the current Gregorian calendar, it is therefore the Julian calendar – introduced by Julius Caesar in 46 BC – that governed the days, the months and the years, with a leap year every 4 years. Now the Julian calendar was slightly biased, even if the nuance will seem imperceptible to you: an average year counted 365.25 days, instead of 365.2422 days that in reality a terrestrial cycle of revolution of the Earth lasts around the Sun (it is even 365.242,199 days, according to the IMCCE, the Institute of Celestial Mechanics and Calculation of Ephemerides). Leap years occurred every 4 years, with no exceptions. The calendar simply did not take into account the ends of centuries (divisible by 100 and sometimes not divisible by 400, according to the calculation of use, they should most of the time not have had to be leap). As a result, ten days of delay have been accumulated in fifteen centuries.
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To restore the fragile balance, the introduction of the Gregorian calendar – named after Pope Gregory XIII, who set it up in 1582 – was accompanied by a deletion of ten days from the calendar (to fill at once the accumulated delay), but also the cancellation of three leap years every 400 years (three end of centuries out of 4, the famous end of century divisible by 100 and not by 400). In 1582, with the Gregorian calendar, we therefore got closer to natural time. For rather pragmatic reasons, like keeping Easter in the spring! In the current Gregorian calendar, the average year therefore now lasts 365.2425 days (when an earth cycle takes 365.2422). A micro-offset therefore persists with a year of hair that is too long. But only translates into an advance of 3 days in 10,000 years for the Earth’s cycle of revolution …
Except in special cases, leap years occur every 4 years, frequency with which a “February 29” is added to the usual 28 days of February in our Gregorian calendar. The only trap concerns the end of the centuries (see how we check if a year-end is a leap year or not).
The next leap year falls in 2020. But what other leap years are we headed for?
The leap years to come:
And so on every four years, until 2400 (2400 will be a leap, like 2000 and unlike 2100, 2200 and 2300) because 2400 is divisible by 400, like 2000 was a leap. On the other hand, 2100, 2200 and 2300 are all divisible by 4 AND by 100 (which invalidates the first condition to be fulfilled). And are also not divisible by 400 (which corresponds to the second condition). As a reminder, a year is a leap year if it is divisible by 4 but not by 100 and / or if it is divisible by 400 (read the details of the explanations for leap years).
Finally, the additional day of leap years could be added to different months of the year on our calendar, but it is not! Even at the time of Julius Caesar, this extra day was placed in February, although it was added just before February 24 and not all at the end of the month (it was a kind of “February 23 bis”) . With the change of calendar and the switch to the Gregorian calendar day counting system, the leap day was set to February 29, and did not move any more.
What is the common point between the actor Gérard Darmon, the novelist Géraldine Maillet, the top model Lena Gercke, the artist Saul Williams, the singer Khaled, but also before them Michèle Morgan or the Italian composer Gioacchino Rossini? All were born on February 29. They can therefore only celebrate their birthday every 4 years. Unfortunately for them, they age however as quickly as the others. In general, people born 29 February celebrate their birth on February 28 in non-leap years. During the real D-Day, a very special gift is reserved for them: the satirical newspaper La Bougie du Sapeur does not appear until February 29, therefore every four years. In 2016, a very special issue was offered on newsstands since it was the 10th. The team of the newspaper, originally a group of friends, had promised nice surprises for this anniversary, including its supplement on the history of the Candle of the Sapper. “Who are we kidding? Of us, of course!” Could be read on the front page.
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In certain latitudes like in Taiwan, a person born on February 29 is considered as legally born on February 28! It will therefore become major on February 28 of the year of its 20 years and not on 29, even if that year were to include a 29. Same thing on Facebook, where subscribers born on February 29 receive their notification of birthday on the 28th.
Others also take the thing with humor, like James, this Belgian father born in 1968 who chose to celebrate his eleven years in 2012 (the age of his son at that time), as the site tells 7sur7.be.
However, being born on February 29 is not trivial. As Slate tells us in a long investigation into the issue, it happens in maternity wards that patients whose delivery is scheduled for February 29 want to postpone their cesarean or their induction. Nothing massive, however: when the baby arrives spontaneously, that is to say in the majority of cases, expectant mothers generally accept the situation willingly. Which makes Slate say that there is no drastic drop in births on February 29. These are all the same “slightly lower than the monthly average, or the number of births recorded on the 28th”.
To find out if a year is a leap year without a calendar in front of you, there is a simple calculation. The year must meet one of the following two criteria:
1. First option: the year in question is divisible by 4 and not divisible by 100
2. Second option: the year you are thinking of is divisible by 400
2020 fulfills for example the first of these two conditions necessary to be a leap (the year does not need to fulfill both): 2020 divided by 4 = 505 and 2020 divided by 100 = 20.2
If we take the opposite example of a non-leap year: 1900, on the other hand, does not meet any of these conditions. It is divisible by 4 but also by 100, which invalidates the first option. The second condition does not work either, because 1900 divided by 400 = 4.75. On the other hand, 2008 fulfills the first rule (divisible by 4 and not 100) so it is a leap year (it has 366 days instead of 365).
Last example: the year 2000, leap year or not? Yes because we can divide it by 400. So it meets one of the possible conditions necessary. Are you still following?
The “python” is a computer language, and more specifically, as JDN explains, the open source programming language that is most used by computer scientists. It is possible to check whether a leap year or not by writing a python program. Several websites give the procedure for coding this verification: Waytolearnx is one of them, as are Wikibooks or even python.developpez.
Leap year counts 366 days instead of the usual 365 days. The average year of the Gregorian calendar (leap and non-leap years included) also offers 365.2425 days, versus 365.2422 days for a cycle of rotation of the Earth around the Sun. Enough to cause only three days of lag (more precisely in advance) in 10,000 years.
“Bank holiday”? Missed ! This English term means “holiday”, not a leap year. In English, “leap year” is said “leap year”. As for the leap day of February 29, present every four years in our calendar, it is said, in the same way, “leap day” in the language of Shakespeare.
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