the.com/gnomon

means The upright part of a sundial — or more generally any pointer whose shadow is used to mark position or measurement — that casts the shadow telling the time.

from From Greek gnōmōn, 'one who knows' or 'indicator,' rooted in gignōskein, 'to know' — the same root that gives us 'gnosis' and 'know.' The Greeks used the word for the sundial's shadow-caster, but also for a carpenter's square and for a type of knowledge-marker in geometry, since all three 'know' or reveal something — an angle, a shape, a time of day — by their fixed position.