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The Collaborative International Dictionary of English v.0.48 (gcide)

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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