Gno"mon (?), n. [L. gnomon, Gr.
? one that knows, the index of a sundial. See Gnome.]
1. (Dialing) The style or pin, which
by its shadow, shows the hour of the day. It is usually set parallel
to the earth's axis.
2. (Astron.) A style or column erected
perpendicularly to the horizon, formerly used in astronomocal
observations. Its principal use was to find the altitude of the sun
by measuring the length of its shadow.
3. (Geom.) The space included between
the boundary lines of two similar parallelograms, the one within the
other, with an angle in common; as, the gnomon bcdefg
of the parallelograms ac and af. The parallelogram
bf is the complement of the parallelogram df.
4. The index of the hour circle of a