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There are many instances in Google Books of this expression which sounds very unfamiliar to me:

Is a third of an hour a valid current expression? or is it used just within specific contexts, scientific for instance? or is it just an archaic expression substituted by the more common 20 minutes?

Edit:

@close-voter: what is opinion based? Which of my three questions?

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    Your first source seems math-related, and outlining an exercise in fractions, which might explain the one third. The second occurrence is a translation from Italian, and may have been influenced by the original's style and word choice. The third excerpt is from a fantasy novel, a genre often rife with poetic variations on language. I don't think I have ever come across a third of an hour in the wild, but it is of course readily understood to mean the more common twenty minutes. – oerkelens May 5 '15 at 11:09
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    Anecdotally, I’ve definitely never heard anyone say it. Sounds most odd to me. Half an hour, sure. A quarter of an hour, all right. Three quarters of an hour … if you must (but I’d always just say “45 minutes”). But a third of an hour? That’s stretching it. It’s easily understandable, of course, but I’d say its currency is practically nil. – Janus Bahs Jacquet May 5 '15 at 11:09
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    "A third of an hour" is rare, but there is certainly nothing wrong with it. One may even say "three-fifths of an hour" if they wish. Clearly, as the expression gets more complicated comprehension suffers, but that's between the speaker and the listener. (The thing that used to drive me crazy was my grandmother and aunt who used expressions like "ten to", with no "what" after the "to". You don't hear that much anymore.) – Hot Licks May 5 '15 at 11:19
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    @Mari-Lou - Time related questions are unloved (apparently). – user66974 May 5 '15 at 11:19
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    @Area51DetectiveFiction. N-grams are so easy to manipulate. If you change the search phrase to "third of an hour" (without "a") and increase the time-scale you will see that between 1700 and 1750 "third of an hour" is considerably more frequent that "20 minutes". – fdb May 5 '15 at 15:03
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I think that "a third of an hour" may be another relic of the Pre-Digital Clock Era, just as "a quarter of an hour" (notably in the forms "a quarter after" and "a quarter till") and "a half hour" (in the form "half past") are.

Most people who grew up when all clocks had hour and minute hands sweeping through 360 degrees every 12 hours or every 60 minutes, respectively, are familiar with the custom of describing the time by dividing the clock face into 30-minute halves and 15-minute quarters. Dividing the clock face into 20-minute thirds is somewhat less common, I believe, but there is no reason it couldn't be used to convey precise time information.

Come to think of it, you could also refer to "a fifth of an hour" (12 minutes), "a sixth of an hour" (10 minutes), "a tenth of an hour" (6 minutes), "a twelfth of an hour" (5 minutes), "a fifteenth of an hour" (4 minutes), "a twentieth of an hour" (3 minutes), "a thirtieth of an hour" (2 minutes), or a sixtieth of an hour" (1 minute)—although at some point the fraction becomes so small that people cease to look to the corresponding sweep of the minute hand across the clock face as a visually meaningful representation of the fraction involved.

Objectively it shouldn't be harder to imagine a pie divided into three equal pieces than to imagine one divided into four equal pieces; but the reality is that a simple perpendicular division yields four equal pieces while an accurate trisection of a pie takes more thought and judgment (or practice). Perhaps something of the primitive human preference for fourths owing to the simplicity of obtaining them carried over to clock division, even though in that case the dividing points for true thirds were already clearly in place.

  • Thanks, so you think that the expression has actually never been a common one (btw 1/3 referring to an hour is a much more sensible and usable measure than 1/6 or 1/10) and whenever it is used it is just a playful way to express a portion of an hour? – user66974 May 6 '15 at 8:04
  • Yes, I think that people rarely looked at a clock and said "It's a third after noon"; and not coincidentally (I think) the habit of naturally subdividing an hour into thirds was never common. It may have been standard practice for some people to think in terms of one-third hours, but I don't recall ever having met one of them. – Sven Yargs May 6 '15 at 8:10
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"A third of an hour," is not a common expression - probably because it's much easier to say, "twenty minutes." (US)

Also, the average person can divide something in half, (like, an hour) then half it again - into quarters - much more adeptly than messing around with "thirds". (It's just easier on the brain.)

Conceptualizing "a third of an hour" is too taxing, especially when compared with the easy-peasy "twenty minutes".

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    "1200 seconds" is likely to get you funny looks too. – Wayfaring Stranger May 5 '15 at 12:07
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    That's all very clear and who could contradict you? So all the instances of usage that can be easily found in science, literature and other books are just playful exceptions to the more common 20 minutes? – user66974 May 5 '15 at 12:32
  • @Josh61- Uh... yup. – Oldbag May 6 '15 at 0:11
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This is like asking about the choice of "fifty cents" versus "half a dollar". There is nothing wrong with either, both will be understood, and which you pick is determined by where you want to put the emphasis, if any. If I'm discussing billable hours, a third of an hour may be more convenient. If I'm discussing recipes, 20 minutes may be easier to work with. If it doesn't matter, use whichever seems most appropriate at the moment and don't worry about it.

  • If I'm discussing billable hours, a third of an hour may be more convenient. By this you mean that you would use the expression, it just depends on the context and the emphasis you want to give? Have you ever used it? – user66974 May 5 '15 at 15:44
  • Yes. Both are perfectly idiomatic, as far as I know. – keshlam May 5 '15 at 15:49
  • I don't think a third of an hour is to 20 minutes like half a dollar is to 50 cents. – user66974 May 5 '15 at 15:52
  • Can you explain why you don't think so? It's less common because our usual division of hours is into quarters and halves, but that's habit, not linguistics and barely idiom. We wouldn't generally say two dekaminutes mostly because folks aren't used to applying metric prefixes to that unit (though scientists are comfortable with seconds scaled that way)... and business accounting in tenths of an hour confuses some folks until they get used to it... but a third of an hour is a nicely intuitive unit, when it's relevant. – keshlam May 5 '15 at 17:59
  • I agree with you that a third of an hour is a nicely intuitive unit ( being an hour 60 minutes easily divisible by 3) . But that is the point!! The expression is rarely if not used at all ( see comments above) . The evidence I provide shows that there are actually instances where it was used, mainly in the past. So I am asking if it was a more common expression in the past or in specific contexts ( math, science, literature etc.) and why it not used like a quarter or half an hour. – user66974 May 5 '15 at 18:07
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There is absolutely nothing wrong with saying "a third of an hour" rather than "twenty minutes" when describing a time duration. Yes, it is less common, but there is no law that the most common phrase for a given meaning must always be used. And in some cases (the "billable hours" scenario, for example) "a third of an hour" may be more "intuitive".

Unless there is some "style guide" or such directing the author differently, he's completely free to choose either phrase, as it suits him.

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