some years can jump

(ler em português: anos que pulam)

why do we call leap years, leap years? + names for them in other languages — by mati ★

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leap years are a really interesting phenomenon. an actual year, which is how long it takes for the earth to circle a full lap around the sun, doesn't fit into a whole number of days (it's more than 365, but less than 366), so we kind of solved this adding extra days to some years in the calendar. every year that's divisible by 4 is a leap year, except the ones that are divisible by 100, except except the ones that are divisible by 400. phew! but i'm not here to talk about the math this involves, i'm here to talk about the words that we use to describe years with extra days like that.


the romans had some really weird calendars

let's start with the words for 'leap year' in romance languages, which i think are more interesting and have a fuller "backstory" behind them. in spanish, you say año bisiesto, in portuguese ano bissexto, in french année bissextile and in italian anno bisestile. in english too, you can say bissextile, although literally no one uses that word and everyone just says leap year instead.

anyway, it really looks like these bissextile words come from the fact that the number of days in a leap year, 366, ends in two sixes — after all, bis is latin for 'twice' and sextus is 'sixth', so it kind of sounds like a bissextile year would be a "bi-sixth" or "two-sixed" year. sadly though, that's not where that word comes from, but the real origin story is much more interesting, i promise.

warning: math.

there's a lot of math here. like, insane amounts. i think you'll probably be okay with understanding everything, but don't get sad if you get lost.

so, in order to understand the word bissextile, we first have to understand how the roman calendar worked. i'll simplify a lot here, but if you want to understand this in depth, you can look for a video by latintutorial explaining it, which i think is really good. but basically, the roman calendar didn't count how many days have passed since the beginning of the month (like our 1st, 2nd, 3rd... 31st days of the month), but how many days were left until the next month. yes, that's like super bizarre, i know. but that's how they counted it. so, in a regular month of february, the 28th day, that is the last day of the month, would be "the eve of the beginning of march", and the 27th would be "the second day before the beginning of march". but they didn't use to say second, they said third. yeah, another really weird thing about the roman calendar and roman counting is that it was inclusive, so they considered that there was three days, not two, between feb 27 and march 1 (the 27th, 28th and the 1st; that is, they counted both ends).

at this point you might be asking yourself where leap years fit in with all of this. so, you see — leap days used to be after the 23rd. i wish i was joking with you right now, but they literally added a leap day in the middle of the month. now, doing all the math in the roman way:

"regular" day roman day
feb 28 eve of march
feb 27 3rd day before march
feb 26 4th day before march
feb 25 5th day before march
feb 24 6th day before march
feb 23 7th day before march

look at the 24th there. when there was a leap year, they considered that the 24th, or the "sixth day before march", lasted for two days instead of one. it's as if there was the 23rd, then the 24th, and after that a 24th-and-a-half day before they got to the 25th. that second 24th day was the "twice sixth day before march". 'twice' in latin is bis, and 'sixth' is sextum, so they called that day bis sextum, 'twice sixth', which became the word bissextile, which exists in english but is much more common in romance.

besides the romance languages, one language that has a word for 'leap (year)' derived from bis sextum is russian, where the word for 'leap' is високосный visokosnyj (see wiktionary for more) — this is the bis sextum derivation that i find the most interesting!

time flies, years leap

okay, now we can finally talk about english leap year. the leap in leap year sounds like it'd be the same word as the verb to leap — and it indeed is the same word! the leap in leap year really is from the idea of a "leap", or a jump. but mati — you might be thinking right now — a leap, or jump, of what? calendars can't jump.

so, i don't know if you've noticed, but 365 days don't really fit that well inside weeks. the first 364 days fit perfectly into 52 weeks, but there's still one day left at the end. this gives us a really interesting property for dates in regular years: if in a given year, your birthday is on saturday, next year it's going to be on sunday, next next year it's going to be on monday, then 3 years later on tuesday and so on. and this would be all fine and dandy, if we didn't have leap years! leap years have 366 days, which adds up to 52 weeks and two days, so those fixed dates end up "skipping" or "leaping" a day instead of sliding from one weekday to the next.

still don't get what i mean? i'll use my birthday as an example — i was born on march 26th. on this year, 2023, march 26 fell on a sunday. if the next year, 2024, was a regular 365-day year, my next birthday would be monday, the day right after sunday. but 2024 is a leap year (and my birthday is after feb 29), so it turns out that my next birthday is going to be tuesday, not monday. and that's what the "leap" in "leap years" is.

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Pub: 19 Sep 2023 21:10 UTC
Edit: 30 Jan 2024 15:07 UTC
Views: 567