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.
To Pythagoras and the Pythagoreans a gnomon was a basic unit from which a number pattern can be generated. This is described in Figurate numbers, in which squares of numbers are generated from the gnomon of an Odd number.