[1] : The Collaborative International Dictionary of English v.0.48

horizon \hori"zon\, n. f., fr. l. horizon, fr. gr. ? sc. ?
   the bounding line, horizon, fr. ? to bound, fr. ? boundary,
   limit.
   1. the line which bounds that part of the earth's surface
      visible to a spectator from a given point; the apparent
      junction of the earth and sky.
      1913 webster

            and when the morning sun shall raise his car
            above the border of this horizon.     --shak.
      1913 webster

            all the horizon round
            invested with bright rays.            --milton.
      1913 webster

   2. astron.
      a a plane passing through the eye of the spectator and
          at right angles to the vertical at a given place; a
          plane tangent to the earth's surface at that place;
          called distinctively the sensible horizon.
      b a plane parallel to the sensible horizon of a place,
          and passing through the earth's center; -- called also
          rational horizon or celestial horizon.
      c naut. the unbroken line separating sky and water, as
          seen by an eye at a given elevation, no land being
          visible.
          1913 webster

   3. geol. the epoch or time during which a deposit was made.
      1913 webster

            the strata all over the earth, which were formed at
            the same time, are said to belong to the same
            geological horizon.                   --le conte.
      1913 webster

   4. painting the chief horizontal line in a picture of any
      sort, which determines in the picture the height of the
      eye of the spectator; in an extended landscape, the
      representation of the natural horizon corresponds with
      this line.
      1913 webster

   5. the limit of a person's range of perception, capabilities,
      or experience; as, children raised in the inner city have
      limited horizons.
      pjc

   6. fig. a boundary point or line, or a time point, beyond
      which new knowledge or experiences may be found; as, more
      powerful computers are just over the horizon.
      pjc

   apparent horizon. see under apparent.

   artificial horizon, a level mirror, as the surface of
      mercury in a shallow vessel, or a plane reflector adjusted
      to the true level artificially; -- used chiefly with the
      sextant for observing the double altitude of a celestial
      body.

   celestial horizon. astron. see def. 2, above.

   dip of the horizon astron., the vertical angle between
      the sensible horizon and a line to the visible horizon,
      the latter always being below the former.

   rational horizon, and sensible horizon. astron. see
      def. 2, above.

   visible horizon. see definitions 1 and 2, above.
      1913 webster
see also:
rational horizon celestial horizon apparent horizon apparent artificial horizon celestial horizon 
dip of the horizon rational horizon sensible horizon visible horizon 

[2] : WordNet (r) 2.0

celestial horizon
     n : the great circle on the celestial sphere whose plane passes
         through the sensible horizon and the center of the earth
         syn: horizon
see also:
horizon 

Celestial >> C Words
Celestial, definition of term: Celestial
celestial_pag1.html

Horizon >> H Words
Horizon, definition of term: Horizon
horizon_pag1.html

Celestial Spher >> C Words
Celestial Spher, definition of term: Celestial Spher
celestial+spher_pag1.html

Celestial Pole >> C Words
Celestial Pole, definition of term: Celestial Pole
celestial+pole_pag1.html

Celestial Equat >> C Words
Celestial Equat, definition of term: Celestial Equat
celestial+equat_pag1.html

Celestial Poles >> C Words
Celestial Poles, definition of term: Celestial Poles
celestial+poles_pag1.html