Semicircle

A semicircle is half of a circle. It is made by cutting a circle in half, after which there are two semicircles and no circle, making the semicircle, numerically, the more successful shape.^[1]

A semicircle knows which half it is by checking where the other half is not. If the other half is not on the right, it is the right half. If the other half is nowhere at all, it has been a semicircle all along.

Properties

A semicircle has one curved edge and one straight edge, the diameter. When a circle is cut, both halves claim the diameter. A tribunal in 1931 awarded it to both, on the grounds that it had no width and could not be split any further.^[2]

Everyday semicircles

• The Sun at sunset, for about a minute, at the halfway point.
• A protractor, which is a semicircle that has learned to count to 180.
• A rainbow seen from the ground. Seen from an aircraft, it can be a full circle, which rainbows only do when they know they are being watched from above.
• Half a pizza that someone else has been looking at.

Naming

Mathematicians considered calling it a “hemicircle” or a “demicircle”. After a vote, they chose “semicircle”, with exactly half the committee in favour and the other half in favour of lunch.^[3]

See also

• Circle – the whole story
• Sun – a semicircle at sunset
• Left side – where one of the halves may be

References

1. Halfway, H. (2010). Half a Circle. Arc Books. p. 90 (of 180).
2. Tribunal on Diameters (1931). Ruling No. 1. Split decision.
3. Demi, D. (1977). Hemi, Semi, Demi. Prefix Press.