Horizon \Ho*ri"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.
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            And when the morning sun shall raise his car
            Above the border of this horizon.     --Shak.
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            All the horizon round
            Invested with bright rays.            --Milton.
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   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.
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   3. (Geol.) The epoch or time during which a deposit was made.
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            The strata all over the earth, which were formed at
            the same time, are said to belong to the same
            geological horizon.                   --Le Conte.
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   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.
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   5. The limit of a person's range of perception, capabilities,
      or experience; as, children raised in the inner city have
      limited horizons.
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   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.
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   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.
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Rational \Ra"tion*al\ (r[a^]sh"[u^]n*al), a. [L. rationalis: cf.
   F. rationnel. See Ratio, Reason, and cf. Rationale.]
   1. Relating to the reason; not physical; mental.
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            Moral philosophy was his chiefest end; for the
            rational, the natural, and mathematics . . . were
            but simple pastimes in comparison of the other.
                                                  --Sir T.
                                                  North.
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   2. Having reason, or the faculty of reasoning; endowed with
      reason or understanding; reasoning.
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            It is our glory and happiness to have a rational
            nature.                               --Law.
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   3. Agreeable to reason; not absurd, preposterous,
      extravagant, foolish, fanciful, or the like; wise;
      judicious; as, rational conduct; a rational man.
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   4. (Chem.) Expressing the type, structure, relations, and
      reactions of a compound; graphic; -- said of formulae. See
      under Formula.
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   Rational horizon. (Astron.) See Horizon, 2
      (b) .

   Rational quantity (Alg.), one that can be expressed without
      the use of a radical sign, or in exact parts of unity; --
      opposed to irrational or radical quantity.

   Rational symptom (Med.), one elicited by the statements of
      the patient himself and not as the result of a physical
      examination.
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   Syn: Sane; sound; intelligent; reasonable; sensible; wise;
        discreet; judicious.

   Usage: Rational, reasonable. Rational has reference to
          reason as a faculty of the mind, and is opposed to
          irrational; as, a rational being, a rational state of
          mind, rational views, etc. In these cases the
          speculative reason is more particularly, referred to.
          Reasonable has reference to the exercise of this
          faculty for practical purposes, and means, governed or
          directed by reason; as, reasonable desires or plans; a
          reasonable charge; a reasonable prospect of success.
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                What higher in her society thou find'st
                Attractive, human, rational, love still.
                                                  --Milton.
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                A law may be reasonable in itself, although a
                man does not allow it, or does not know the
                reason of the lawgivers.          --Swift.
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