The Collaborative International Dictionary of English v.0.48:

Zenithal \Ze"nith*al\, a.
   Of or pertaining to the zenith. "The deep zenithal blue."
   --Tyndall.
   [1913 Webster]

WordNet (r) 3.0 (2006):

zenithal
    adj 1: relating to or located at or near the zenith