Gnomon

Gnomon is the Greek word for the style of a sundial, or any object, commonly a vertical column, the shadow of which was observed in former times in order to learn the altitude of the sun, especially when on the meridian.

The art of constructing a gnomon sundial is sometimes termed gnomonics.

In geometry, a gnomon is a plane figure formed by removing a parallelogram from a corner of a larger parallelogram

Gnomonic projection is a projection of a sphere in which the centre of sight is the centre of the sphere.