Gnomon

For other uses, see Gnomon (disambiguation).

The cantilever spar of this cable-stay bridge, the Sundial Bridge at Turtle Bay, forms the gnomon of a large garden sundial

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

The gnomon is the triangular blade in this sundial.

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 the name given to an aesthetic process utilised by James Joyce in his set of short stories Dubliners, whereby the whole of the character is revealed by a part, the part that reveals.

Anaximander (610 BC-546 BC) is credited to introduce 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).

References

• 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