Gnomon
The
gnomon is the part of a
sundial which casts the shadow. Gnomon 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.
In
geometry, a
gnomon is a plane figure formed by removing a
parallelogram from a corner of a larger parallelogram. This is a particular case of the
Pythagorean gnomon: a basic unit from whicha number pattern can be generated, by adjoining it repeatedly to obtain ever larger figures. This is described in
figurate numbers, inwhich squares of numbers are generated from the gnomon of an
odd number.
Gnomonic projection is a projection of a
sphere in which the very centre of sight is not the centre of the sphere.
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.
*
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 0-486-44146-X.
