A-Horizon

n. [ F., fr. L. horizon, fr. Gr. &unr_; (sc. &unr_;) the bounding line, horizon, fr. &unr_; to bound, fr. &unr_; 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. [ 1913 Webster ]

And when the morning sun shall raise his car
Above the border of this horizon. Shak. [ 1913 Webster ]

All the horizon round
Invested with bright rays. Milton. [ 1913 Webster ]

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. [ 1913 Webster ]

3. (Geol.) The epoch or time during which a deposit was made. [ 1913 Webster ]

The strata all over the earth, which were formed at the same time, are said to belong to the same geological horizon. Le Conte. [ 1913 Webster ]

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. [ 1913 Webster ]

5. The limit of a person's range of perception, capabilities, or experience; as, children raised in the inner city have limited horizons. [ PJC ]

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. [ PJC ]

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. [ 1913 Webster ]