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.
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         On the welkne shoon the sterres lyght.   --Chaucer.
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         The fair welkin foully overcast.         --Spenser.
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         When storms the welkin rend.             --Wordsworth.
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   Note: Used adjectively by Shakespeare in the phase, "Your
         welkin eye," with uncertain meaning.
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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]