The Collaborative International Dictionary of English v.0.48:

Gnomonic \Gno*mon"ic\, Gnomonical \Gno*mon"ic*al\, a. [L.
   gnomonicus, Gr. ?: cf. F. gnomonique. See Gnomon.]
   Of or pertaining to the gnomon, or the art of dialing.
   [1913 Webster]

   Gnomonic projection, a projection of the circles of the
      sphere, in which the point of sight is taken at the center
      of the sphere, and the principal plane is tangent to the
      surface of the sphere. "The gnomonic projection derives
      its name from the connection between the methods of
      describing it and those for the construction of a gnomon
      or dial." --Cyc. of Arts & Sciences.
      [1913 Webster]