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  • An object shaped like half a circle is called a semicircle.
  • An object shaped like half a sphere is called a hemisphere.

In my native language we do not distinguish. We are in both cases dealing with half of the object (in Danish they are called "half-circle" and "half-sphere", directly translated.)

Is there a reason for this odd difference between the words in English? Is it correctly understand that I cannot say hemicircle and semisphere? The semi- seems to be a prefix used predominantly, such as in semi-lines, semi-axes in geometry etc.

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    This kind of question is unanswerable. That is just the way it is. hemi is Greek and semi is Latin.
    – Lambie
    Commented Mar 17, 2022 at 14:03
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    Let's not forget the French 'demi'. In music, hemidemisemiquaver (a 64th note) manages to combine Greek, Latin and French
    – Michael Harvey
    Commented Mar 17, 2022 at 14:41
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    @Lambie, and based on the existing answers, it's likely because "circle" is Latin and "sphere" is Greek. This is the best answer to the question. (note the word "likely". It's not a rule that Latin-root word will have Latin prefix, but it's a reasonable explanation why it's more prevalent)
    – justhalf
    Commented Mar 18, 2022 at 12:50
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    @MichaelHarvey And when you need even smaller notes, they start repeating - Hemidemisemihemidemisemihemidemisemiquaver is a thing. And this word is used in English combining 3 other languages. (There was a recent discussion of this on Music.SE about why this naming system is not popular outside of the UK.)
    – Darrel Hoffman
    Commented Mar 18, 2022 at 18:39
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    @Lambie - You have both called the question unanswerable, and answered it.
    – Dawood ibn Kareem
    Commented Mar 19, 2022 at 0:59

2 Answers 2

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The basic answer is because a language is what it is, and not what somebody thinks it ought to be.

But historically, the Oxford English Dictionary has

  • semicircle - cited from 1526
  • hemicircle obsolete or archaic - cited from 1603
  • semisphere now rare - cited from 1659
  • hemisphere - cited from 1585.

So the standard forms are older, and the OED derives them respectively from Latin sēmicirculus and from "late Latin hēmisphærium , < Greek ἡμισϕαίριον", whereas the less common alternatives were coined within English, possibly by people who didn't know or didn't care that "circle" is of Latin origin and "sphere" of Greek origin.

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    "circle" is of Latin origin and "sphere" of Greek origin <-- I guess this is the crux? If a word is of Latin origin, it's more likely to be given Latin prefix, and similarly for Greek. I wonder why answerers don't put this information first. Like mcalex said on another answer, the word origin is something that we can say objectively. The one we can't say is why we use Latin for circle and Greek for sphere.
    – justhalf
    Commented Mar 18, 2022 at 12:48
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    Because it's only marginally relevant, @justhalf. For words of this age it was more relevant - but notice that the rare alternatives I listed above both date from the 1600's, so they're not much more recent. But the coiners of the more recent words television, unprincipled, and automobile didn't know or didn't care whether the roots were Greek, Latin, or something else. Amoral is an interesting case: the word immoral (formed from Latin sources) already existed; but in the 19th century writers wanted to express a different but related concept, and chose to use a Greek-derived prefix.
    – Colin Fine
    Commented Mar 18, 2022 at 13:57
  • Fair point. My point is something like Nick's answer. This likeliness based on the origin is not a rule, but its something reasonable that warrants mentioning as a reason. Of course any explanation on "why" words are like words today all influenced by how people use it, as all aspects of language, as you said. And for me this is exactly why it's not something that I would mention as the main reason. Only if there are no other reasonable reasons, then I'll answer with the disappointing "it's what it is, since people use it that way"
    – justhalf
    Commented Mar 19, 2022 at 7:35
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    Both "circle" and "sphere" were imported into Latin ("circulus", "sphaera") from the Greek ("κίρκος", "σφαῖρα"). So it is misleading to say that one is of Latin origin and the other of Greek origin.
    – TonyK
    Commented Mar 19, 2022 at 11:31
  • @TonyK: you're right: I didn't look further back. That makes that side of the argument (which I was already minimising) rather empy. Having said that, I suspect that circus was naturalised in Latin in a way that sphaera was not, but I have no hard evidence for this.
    – Colin Fine
    Commented Mar 19, 2022 at 13:58
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As is often the case with odd quirks like this, it has to do with where the words came from. Both semicircle and hemisphere entered English from Latin via Old French. That probably traces back to the fact that circle has a Latin root and sphere a Greek one. But as always, there's no rule to that sort of etymology and you can't use it to predict what words will be adopted in a language.

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    +1 Why do we use the different prefixes? Because the source words come from different places, possibly at different times. Asked and answered. Why did they come from different places? That is the 'can't be answered' question.
    – mcalex
    Commented Mar 18, 2022 at 7:32

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