The Collaborative International Dictionary of English v.0.48:

Observatory \Ob*serv"a*to*ry\, n.; pl. Observatories. [Cf. F.
   observatoire.]
   1. A place or building for making observations on the
      heavenly bodies.
      [1913 Webster]

            The new observatory in Greenwich Park. --Evelyn.
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   2. A building fitted with instruments for making systematic
      observations of any particular class or series of natural
      phenomena.
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   3. A place, as an elevated chamber, from which a view may be
      observed or commanded.
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   4. (Mil.) A lookout on a flank of a battery whence an officer
      can note the range and effect of the fire; usually
      referred to as an observation post. --Farrow.
      [1913 Webster +PJC]