A three dimensional gnomon is commonly used in CAD and computer graphics as an aid to positioning objects in the virtual world. By convention, the X axis direction is colored red, the Y axis green and the Z axis blue.
Hero defined a gnomon as that which, added to an entity (number or shape), makes a new entity similar to the starting entity.
In this sense Theon of Smyrna used it to describe a number which added to a polygonal number produces the next one of the same type.
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. At present, such a gnomon should thus point almost precisely at Polaris, as this is within a degree of the North celestial pole.
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. One so skilled would be referred to as a gnomonist.
Other uses of the term
Gnomon is the name given to an aesthetic process utilized by James Joyce in his set of short stories Dubliners, whereby there is some absence, removal, or avoidance in each story.
In the book The Tower at the End of the World by Brad Strickland, a giant tower and thin stairs turn out to be the gnomon of a giant sundial. The island the tower is found on is often called "Gnomon Island".