Gnomon

The gnomon is the part of a sundial Sundial

A sundial measures time [i] by the position of the sun [i]. ...

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 Angle

An angle is the figure formed by two rays [i] sharing a common endpoint [i], called the vertex [i] ...

that equals the latitude Latitude

Latitude, usually denoted symbolically by the Greek letter f [i] , gives the location of a place on ...

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 Meridian

Meridian is: * Meridian [i]: an imaginary circle perpendicular to the horizon. ...

.

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Encyclopedia

The gnomon is the part of a sundial Sundial

A sundial measures time [i] by the position of the sun [i].
...

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 Angle

An angle is the figure formed by two rays [i] sharing a common endpoint [i], called the vertex [i]...

that equals the latitude Latitude

Meridian is:

Meridian [i]: an imaginary circle perpendicular to the horizon.

...

. 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.

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.

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 0-486-44146-X.