The Collaborative International Dictionary of English v.0.48:

Astrolabe \As"tro*labe\ ([a^]s"tr[-o]*l[=a]b), n. [OE.
   astrolabie, astrilabe, OF. astrelabe, F. astrolabe, LL.
   astrolabium, fr. Gr. 'astrola`bon; 'a`stron star + ?, ?, to
   take.]
   1. (Astron.) An instrument for observing or showing the
      positions of the stars. It is now disused.
      [1913 Webster]

   Note: Among the ancients, it was essentially the armillary
         sphere. A graduated circle with sights, for taking
         altitudes at sea, was called an astrolabe in the 18th
         century. It is now superseded by the quadrant and
         sextant.
         [1913 Webster]

   2. A stereographic projection of the sphere on the plane of a
      great circle, as the equator, or a meridian; a
      planisphere. --Whewell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

astrolabe
    n 1: an early form of sextant