The Collaborative International Dictionary of English v.0.48:

Planisphere \Plan"i*sphere\, n. [Plani- + sphere: cf. F.
   planisph[`e]re. See Plain, and Sphere.]
   The representation of the circles of the sphere upon a plane;
   especially, a representation of the celestial sphere upon a
   plane with adjustable circles, or other appendages, for
   showing the position of the heavens, the time of rising and
   setting of stars, etc., for any given date or hour.
   [1913 Webster]