The Collaborative International Dictionary of English v.0.48:

Nonagesimal \Non`a*ges"i*mal\, a. [L. nonagesimus the ninetieth.
   See Nonagenarian.] (Astron.)
   Of or pertaining to the ninetieth degree or to a nonagesimal.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nonagesimal \Non`a*ges"i*mal\, n. (Astron.)
   The middle or highest point of the part of the ecliptic which
   is at any given moment above the horizon. It is the ninetieth
   degree of the ecliptic, reckoned from the points in which it
   is intersected by the horizon.
   [1913 Webster]