Gnomon.

Gnomon. When a rectangle is divided into four parts by cross lines parallel to its sides, the sum of any three of the parts is called the gnomon. For Gnomonic Projection, see PROJECTION.—Gnomon has also a meaning in dialling (see DIAL); and a gnomon, or style erected at right angles to the horizon, sometimes of great height, was much used by ancient astronomers for finding the altitudes and declinations of sun and stars.