The Collaborative International Dictionary of English v.0.48:

Descension \De*scen"sion\, n. [OF. descension, L. descensio. See
   Descent.]
   The act of going downward; descent; falling or sinking;
   declension; degradation.
   [1913 Webster]

   Oblique descension (Astron.), the degree or arc of the
      equator which descends, with a celestial object, below the
      horizon of an oblique sphere.

   Right descension, the degree or arc of the equator which
      descends below the horizon of a right sphere at the same
      time with the object. [Obs.]
      [1913 Webster]