From The Collaborative International Dictionary of English v.0.48:

Gnomon \Gno"mon\, n. [L. gnomon, Gr. ? one that knows, the index
   of a sundial. See Gnome.]
   [1913 Webster]
   1. (Dialing) The style or pin, which by its shadow, shows the
      hour of the day. It is usually set parallel to the earth's
      [1913 Webster]

   2. (Astron.) A style or column erected perpendicularly to the
      horizon, formerly used in astronomocal observations. Its
      principal use was to find the altitude of the sun by
      measuring the length of its shadow.
      [1913 Webster]

   3. (Geom.) The space included between the boundary lines of
      two similar parallelograms, the one within the other, with
      an angle in common; as, the gnomon bcdefg of the
      parallelograms ac and af. The parallelogram bf is the
      complement of the parallelogram df.
      [1913 Webster]

   4. The index of the hour circle of a globe. Gnomonic
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