Plural of gnomon: the part of a sundial (usually a rod or triangle) that casts a shadow to show the time of day.
From Greek gnomon meaning 'interpreter' or 'one who knows,' from gignoskein 'to know.' In geometry, it also refers to a portion of a parallelogram.
The word 'gnomon' is related to 'gnostic' and 'know'—all from the same Greek root—because ancient people saw the shadow cast by a gnomon as something that 'tells' or 'knows' the time.
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