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Going through a specification sheet for an engineering device, I glanced upon this phrase:

...the angular scanning range of the device is a semicircle of 300 degrees...

A semicircle is usually defined as a proper half of a circle - in mathematical terms a circle of angle 180 degrees. Clearly 300 degrees is more than a half, it is closer to being 5/6th of a circle. Is it grammatically correct to refer to non-full circles as a "semicircle", even though they are not exactly a half-circle.

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    I would say the word arc can be used: "an arc of 300 degrees," or perhaps: "the scanning range of the device is an angle of 300 degrees". – Weather Vane Mar 13 at 17:24
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    No, you can't use it. A semi-circle means half of a circle. – Ubi hatt Mar 13 at 17:31
  • I presume it is clear from context that you are talking about geometric angle measure, not temperature. Thus, I would get rid of the "angular scanning range" as completely superfluous non-information. Instead, you may want to add actual information, like that it is scanning range in horizontal (or vertical) plane. Or is a sphere? Or half-sphere? Look at it from the point of view of a user of the device. – Rusty Core Mar 13 at 21:11
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    It is 5/6 of a circle. – user207421 Mar 13 at 21:23
  • @WeatherVane The word should needs to be used in "... the word arc can be used," not can. Arc it is, after all. – Kris Mar 14 at 8:24
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The answer by Steven is correct, you cannot use semicircle in this case. Here are some alternatives I would use:

the angular scanning range of the device is an arc of 300 degrees...

Or if we want to give a sense of an area being scanned:

the angular scanning range of the device is a circular sector of 300 degrees...

Here's the definition of circular sector

Or simply:

the angular scanning range of the device is 300 degrees...

I think this is the most technically accurate one. The angular range should be described as an angle, hence measured in degrees (or rads).

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In English, the prefix semi- usually means partly, as in semiconscious or semiautomatic.

But it can also mean half, as in semiannually, which always refers to something that happens every half-year (twice yearly).

The word semicircle has a specific defined meaning: a half of a circle or of its circumference. (Source: New Oxford American Dictionary)

So no, semicircle cannot be used to refer to any part of a circle that isn't 180 degrees.

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  • This might be an unusual opinion, but when I hear "semiannually" I think "probably once every 2 years, but inconsistently so". I'm Australian, in case that's relevant. – Clonkex Mar 14 at 3:05
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    Biannual is the term for something that happens every six months. Not to be confused with biennial which is something that happens every two years. – mcalex Mar 14 at 5:53
  • @Clonkex "semi-" means half, and "bi-" (as in bicycle, which is how I remember the difference) means two. – RonJohn Mar 14 at 8:10
  • Please read the FAQ. Good Luck. – Kris Mar 14 at 8:26
  • @RonJohn I understand that, but that's not how I naturally interpret "semiannually". I only point this out because I may not be alone. – Clonkex Mar 15 at 3:55

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