Like many other fields, science has its own style of communication full of specific jargon and guided by unwritten rules. Most of the posts on this blog focus on breaching this barrier to the public’s understanding and appreciation of science. In this series, I’d like to take another approach by highlighting scientific words which have escaped the confines of jargon to reach a broader appeal because of their sound or their evocative power as metaphors. Today’s uncommon but useful word is nychthemeron. [Previous words: petrichor, alluvium]
Nychthemeron (pronounced nik-themaron and sometimes spelled nycthemeron) simply means a 24 hour period — that is, a day and a night. In fact, it literally translates as night-day, being an amalgam of the Greek words nyct (night) and hemera (day). Nychthemeron has primarily been used in various kinds of technical writing to overcome the ambiguity inherent in the word “day”. Interestingly, it seems that “day” doesn’t come from the Latin dies, but rather from the Sanskrit verb dah, meaning “to burn”. Although the Online Etymology Dictionary claims that day originally meant “daylight hours”, one of the earliest quotation in the Oxford English Dictionary is from Ælfric’s Anglo-Saxon Bible, where it’s used to mean a nychthemeron. The earliest use of nychthemeron cited in the OED is from 1682 in Henry More’s annotations of Lux Orientalis: “Onely the shadowy Vale of the Night will be cast over them once in a Nycthemeron.” Five years earlier, Robert Cary used an alternative spelling in his Palæologica Chronica: “They came to hit upon the number of 360 Days in the first place, concluding..that in so many Nuchthemerinal Revolutions, the Sun came to the same τροπή,..where he was the Year before.” It’s continued to be used in technical contexts since then; just last year, Clément Bougard and his colleagues wrote: “Although several studies have reported that postural control is influenced by the time-of-day, only a few were conducted across an entire nychthemeron.”
I think it’s a shame that nychthemeron is mainly confined to technical treatises. It really is a wonderful word, perfectly filling a gap that I never quite realized existed. Discovering it was like eating an exquisite hors d’oeuvre: it seems insignificant at first, but turns out to be full of splendour and richness. Now that I know it exists, I’m eager to use it in a story or a poem!
“The dawn and gloaming most invite one to Musement; but I have found no watch of the nychthemeron that has not its own advantages for the pursuit.”
Charles Pierce
The Hibbert Journal: a quarterly review of religion, theology, and philosophy; vol 7 93, 1908
(via Oxford English Dictionary)
Finnish has an equivalent word, vuorokausi (vuoro=”turn”; kausi=”period”), though unlike nychthemeron, it isn’t considered a technical term and is very commonly used. Do you know any other languages with a word for a nychthemeron?
Inspiring Science 
It is interesting to note that in nature circadian rhythms are physical, mental, and behavioral changes driven by an internal body clock over a roughly 24-hour cycle; while nychthemeral rhythms are the result of the external environment over a 24-hour period..
I hadn’t really thought of that distinction, but you’re right. Another use for this great word. 🙂 Thanks, Jo Ann!
Actually, yes, Dutch uses the word “etmaal”, which Wiktionary links to “nychthemeron” (http://en.wiktionary.org/wiki/etmaal). Nowhere near as evocative though…
And from that I stumbled upon http://en.wikipedia.org/wiki/Nychthemeron
Quite a few languages seem to have words for this it seems : )
Thanks! The Dutch word sounds to me like it might actually be derived from something like “one time”. Is there any chance of that?
The list on Wikipedia is interesting since the Scandinavian languages and Dutch constitute much of the list. I wonder if this is some kind of historical accident of linguistics or simply sampling bias on Wikipedia…
Actually, now that I think about it, I realize that Arabic also makes this distinction. يوم (yawm) is a nychthemeron while نهار (nahaar) is the sunlit period. It never really struck me before, though, since the words are so prosaic. I use يوم in Arabic all the time! I guess it’s the foreignness of nychthemeron that gives it such a rich, evocative feeling.
Interesting you would ask. I had a look at http://www.etymologiebank.nl/trefwoord/etmaal which is obviously all in Dutch. The entries in different etymological dictionaries all point in the same direction.
Etmaal consists of “et” and “maal”, with “et” coming from old-Germanic and meaning something along the lines of “returning” or “recurring”. Apparently it is also used in the old Dutch word “etgras”, meaning grass that has grown for the second time. The second half, “maal” refers to “time” or “period of time”. Originally “etmaal” meant something along the lines of “recurring times”, referring to recurring religious feasts. This usage was supplanted by the Latin loanwords “feria” (to celebrate) and “festus” (feast), leaving only the narrower meaning of “recurring period of 24 hours”.