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The gnomon is the triangular blade in this sundial

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

A gnomon as in Euclid book II
  • Anaximander (610–546 BC) is credited with introducing this Babylonian instrument to the Greeks.[1]
  • 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.
  • 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 most common use in this sense is an odd integer especially when seen as a figurate number between square numbers.


The cantilever spar of this cable-stay bridge, the Sundial Bridge at Turtle Bay, forms the gnomon of a large garden sundial
Gnomon situated on the wall of a building facing Tiradentes Square, Curitiba

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.

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 the whole of the character is revealed by a single part.

In a recent book Gazalé, Midhat J. coined the term gnomonicity as a synonym for the contemporary idea of Self-similarity both in the increasing and decreasing scale as in fractals.

In popular culture

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".

A gnomon inside the church of Saint Sulpice in Paris, France, built to assist in determining the date of Easter, was incorrectly identified as a "Rose Line" in the novel The Da Vinci Code.

NASA Astronauts used a gnomon as a photographic tool to indicate local horizon and to display a color chart when they were working on the Moon's surface.



  • Gazalé, Midhat J. Gnomons, from Pharaohs to Fractals, Princeton University Press, Princeton, 1999. ISBN 0-691-00514-1.
  • Heath, Thomas Little (1981), A History of Greek Mathematics, Dover publications, ISBN 0486240738, ISBN 0486240746 (first published 1921).
  • Laertius, Diogenes, The Lives and Opinions of Eminent Philosophers, trans. C.D. Yonge. London: Henry G. Bohn, 1853.
  • Mayall, R. Newton,Mayall, Margaret W., Sundials: Their Construction and Use, Dover Publications, Inc., 1994, ISBN 048641146X
  • Waugh, Albert E., Sundials: Their Theory and Construction, Dover Publications, Inc., 1973, ISBN 0-486-22947-5.

  • [[1]], Apollo 16 Traverse guide

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