The Collaborative International Dictionary of English v.0.48:

Astrologer \As*trol"o*ger\, n. [See Astrology.]
   1. One who studies the stars; an astronomer. [Obs.]
      [1913 Webster]

   2. One who practices astrology; one who professes to foretell
      events by the aspects and situation of the stars.
      [1913 Webster]

WordNet (r) 3.0 (2006):

astrologer
    n 1: someone who predicts the future by the positions of the
         planets and sun and Moon [syn: astrologer, astrologist]