Gnomon

Part of speech: noun

Pronunciation: /ˈnəʊˌmɒn/

Definitions

1. The part of a sundial that casts a shadow to show the time ; a device or feature that serves as an indicator or pointer to something else
2. A structure on a sundial designed to project a shadow indicating the hour ; any object or element used to point out or mark a position or direction
3. An upright piece on a sundial that produces a shadow used to measure time ; a tool or symbol employed to signify or highlight a particular spot or concept

Etymology: The most surprising thing about this word is that its core meaning is "discerner" or "one who knows." In ancient Greek, "gnomon" (gnṓmōn) came from "gignṓskō," meaning "to know" or "to perceive," so the same root that gives English "know" sits behind it. The Greek word was used for a carpenter's square, an instrument for judging whether an angle was true, and for the pointer of a sundial, which "discerns" the time from the position of its shadow. The sundial sense came from that carpenter's-square idea. The shadow-casting pin or plate was named for its resemblance to the square's shape, so an instrument for judging angles became an instrument for judging the hour. The mathematical sense developed the same way. A gnomon in geometry is the L-shaped figure left when a smaller square is cut from the corner of a larger one, and it looks like a carpenter's square when drawn. English took the word through Latin, and it has kept both the sundial and geometric uses.