Note:  Do not rely on this information. It is very old.


Gnomon, in Geometry. If a parallelogram is divided into four parts by two lines parallel to its sides intersecting at a point on either diagonal, and if one of these four parts be removed which is cut by this diagonal, the remaining three constitute a gnomon. The term is also used to denote the rod of a sundial that is placed parallel to the axis of the earth, and whose shadow marks the time on the dial.