gnomon (n.)

"vertical shaft that tells time by the shadow it casts" (especially the triangular plate on a sundial), 1540s, from Latin gnomon, from Greek gnōmōn "indicator (of a sundial), carpenter's rule," also, in plural, "the teeth that mark the age of a horse or mule," literally "one that discerns or examines, interpreter, expert," from gignōskein "to come to know," from PIE root *gno- "to know." In geometry from 1560s, from a use in Greek. In early use in English sometimes folk-etymologized as knowman. Related: Gnomonic.