The gnomon is the part of a sundial which casts the shadow. In the northern hemisphere, the shadow-casting edge is normally oriented so that it points north and is parallel to the rotation axis of the Earth. That is, it is inclined to the horizontal at an angle that equals the latitude of the sundial's location. On some sundials, the gnomon is vertical. These were usually used in former times for observing the altitude of the sun, especially when on the meridian. The style is the part of the gnomon that casts the shadow. This can change as the sun moves. For example, the upper west edge of the gnomon might be the style in the morning and the upper east edge might be the style in the afternoon.

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. This is a particular case of the Pythagorean gnomon: a basic unit from which a number pattern can be generated, by adjoining it repeatedly to obtain ever larger figures. This is described in figurate numbers, in which squares of numbers are generated from the gnomon of an odd number.

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

Reference

• Gnomons, from Pharoahs to Fractals, Midhat J. Gazalé Princeton University Press, Princeton, 1999.
• Sundials: Their Theory and Construction, Albert E. Waugh , Dover Publications, Inc., 1973, ISBN 0-486-22947-5.