Ra·tion·al a.
 1. Relating to the reason; not physical; mental.
    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.
 2. Having reason, or the faculty of reasoning; endowed with reason or understanding; reasoning.
    It is our glory and happiness to have a rational nature.   --Law.
 3. Agreeable to reason; not absurd, preposterous, extravagant, foolish, fanciful, or the like; wise; judicious; as, rational conduct; a rational man.
 4. Chem. Expressing the type, structure, relations, and reactions of a compound; graphic; -- said of formulae. See under Formula.
 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.
 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.
 What higher in her society thou find'st
 Attractive, human, rational, love still.   --Milton.
    A law may be reasonable in itself, although a man does not allow it, or does not know the reason of the lawgivers.   --Swift.


 Ho·ri·zon n.
 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.
 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 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.
 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.

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