gnomon - n.
1 the rod or pin etc. on a sundial that shows the time by the position of its shadow.
2 Geom. the part of a parallelogram left when a similar parallelogram has been taken from its corner.
3 Astron. a column etc. used in observing the sun's meridian altitude.

gnomonic adj. [F or L gnomon f. Gk gnomon indicator etc. f. gignosko know]