The Collaborative International Dictionary of English v.0.48:

Astronomy \As*tron"o*my\, n. [OE. astronomie, F. astronomie, L.
   astronomia, fr. Gr. ?, fr. ? astronomer; 'asth`r star + ? to
   distribute, regulate. See Star, and Nomad.]
   1. Astrology. [Obs.]
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            Not from the stars do I my judgment pluck;
            And yet methinks I have astronomy.    --Shak.
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   2. The science which treats of the celestial bodies, of their
      magnitudes, motions, distances, periods of revolution,
      eclipses, constitution, physical condition, and of the
      causes of their various phenomena.
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   3. A treatise on, or text-book of, the science.
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   Physical astronomy. See under Physical.
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WordNet (r) 3.0 (2006):

astronomy
    n 1: the branch of physics that studies celestial bodies and the
         universe as a whole [syn: astronomy, uranology]