gnomon: see 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."

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.

Gnomon may also imply the design paradigm relationship between an indicator and a dial or other reference, as with a speedometer and needle. In this case, the needle functions as a gnomon against the incremented speedometer background.

Gnomon is also a mathematical term that describes the part of a parallelogram that remains when a similar parallelogram is removed from one of its corners.

Also, 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.

Anaximander (610546 BC) is credited with introducing this Babylonian instrument to the Greeks. 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 popular culture

In the book The Tower at the End of the World, written by Brad Strickland, an island with a giant tower and thin stairs is found on a lake. The tower and stairs, along with the odd sculptures surrounding it, are later discovered to be a giant sundial, with the tower and stairs being the gnomon. 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 Dan Brown's novel The Da Vinci Code.


  • Gazalé, Midhat J. Gnomons, from Pharaohs to Fractals, Princeton University Press, Princeton, 1999. ISBN 0-691-00514-1.
  • Waugh, Albert E., Sundials: Their Theory and Construction, Dover Publications, Inc., 1973, ISBN 0-486-22947-5.
  • Mayall, R. Newton,Mayall, Margaret W., Sundials: Their Construction and Use, Dover Publications, Inc., 1994, ISBN 048641146X

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