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Webster 1913 Edition

Horizon

Ho-ri′zon

,
Noun.
[F., fr. L.
horizon
, fr. Gr. [GREEK] (sc. [GREEK]) the bounding line, horizon, fr. [GREEK] to bound, fr. [GREEK] 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.
And when the morning sun shall raise his car
Above the border of this
horizon
.
Shakespeare
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.

Webster 1828 Edition

Horizon

HOR'IZON

,
Noun.
[Gr. to bound, a limit.] The line that terminates the view, when extended on the surface of the earth; or a great circle of the sphere, dividing the world into two parts or hemispheres; the upper hemisphere which is visible, and the lower which is hid. The horizon is sensible,and rational or real. The sensible, apparent, or visible horizon, is a lesser circle of the sphere, which divides the visible part of the sphere from the invisible. It is eastern or western; the eastern is that wherein the sun and stars rise; the western, that wherein they set. The rational, true, or astronomical horizon, is a great circle whose plane passes through the center of the earth, and whose poles are the zenith and nadir. This horizon would bound the sight, if the eye could take in the whole hemisphere.

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