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.

  • 9
    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 '19 at 17:24
  • 4
    No, you can't use it. A semi-circle means half of a circle. – Ubi hatt Mar 13 '19 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 '19 at 21:11
  • 3
    It is 5/6 of a circle. – user207421 Mar 13 '19 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 '19 at 8:24

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).


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.

  • 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 '19 at 3:05
  • 1
    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 '19 at 5:53
  • @Clonkex "semi-" means half, and "bi-" (as in bicycle, which is how I remember the difference) means two. – RonJohn Mar 14 '19 at 8:10
  • Please read the FAQ. Good Luck. – Kris Mar 14 '19 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 '19 at 3:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.