The Collaborative International Dictionary of English v.0.48:

Astronomer \As*tron"o*mer\, n. [See Astronomy.]
   1. An astrologer. [Obs.] --Shak.
      [1913 Webster]

   2. One who is versed in astronomy; one who has a knowledge of
      the laws of the heavenly orbs, or the principles by which
      their motions are regulated, with their various phenomena.
      [1913 Webster]

            An undevout astronomer is mad.        --Young.
      [1913 Webster]

WordNet (r) 3.0 (2006):

astronomer
    n 1: a physicist who studies astronomy [syn: astronomer,
         uranologist, stargazer]