\Ho*ri"zon\, n. [F., fr. L. horizon, fr. Gr. ? (sc. ?)
the bounding line, horizon, fr. ? to bound, fr. ? boundary,
limit.]
1. The circle 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.
         And when the morning sun shall raise his car Above
         the border of this horizon.           --Shak.
         All the horizon round Invested with bright rays.
                                               --Milton.
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 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.
3. (Geol.) The epoch or time during which a deposit was made.
         The strata all over the earth, which were formed at
         the same time, are said to belong to the same
         geological horizon.                   --Le Conte.
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.
{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.