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The gnomon is the part of a sundial that casts the shadow. Gnomon (γνώμων) is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals."

It has come to be used for a variety of purposes in mathematics and other fields.


History of the term

  • Oenopides used the phrase drawn gnomon-wise to describe a line drawn perpendicular to another.[2]
  • Later, the term was used for an L-shaped instrument like a steel square used to draw right angles.
  • This shape may explain its use to describe a shape formed by cutting a smaller square from a larger one.
  • Euclid extended the term to the plane figure formed by removing a similar parallelogram from a corner of a larger parallelogram.
  • 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.


The Chinese also used the gnomon, mentioned in the 2nd century Nine Chapters on the Mathematical Art as being used much earlier by the Duke of Zhou (11th century BC).

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.

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