The Collaborative International Dictionary of English v.0.48:

Welkin \Wel"kin\, n. [OE. welken, welkene, welkne, wolcne,
   weolcne, AS. wolcen, pl. wolcnu, a cloud; akin to D. wolk,
   OFries. wolken, OS. wolkan, G. wolke, OHG. wolchan, and
   probably to G. welk withered, OHG. welc moist, Russ. & OSlav.
   vlaga moisture, Lith. vilgyti to moisten.]
   The visible regions of the air; the vault of heaven; the sky.
   [1913 Webster]

         On the welkne shoon the sterres lyght.   --Chaucer.
   [1913 Webster]

         The fair welkin foully overcast.         --Spenser.
   [1913 Webster]

         When storms the welkin rend.             --Wordsworth.
   [1913 Webster]

   Note: Used adjectively by Shakespeare in the phase, "Your
         welkin eye," with uncertain meaning.
         [1913 Webster]

WordNet (r) 3.0 (2006):

welkin
    n 1: the apparent surface of the imaginary sphere on which
         celestial bodies appear to be projected [syn: celestial
         sphere, sphere, empyrean, firmament, heavens,
         vault of heaven, welkin]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "welkin":
   Caelus, aerosphere, air, atmosphere, azure, biosphere, blue sky,
   caelum, canopy, canopy of heaven, cerulean, cope, ecosphere,
   empyrean, ether, firmament, gaseous envelope, heaven, heavens,
   hyaline, lift, lifts, noosphere, sky, starry heaven, the blue,
   the blue serene, vault, vault of heaven