Apparent horizon

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. [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]