Ho-rĭ'zon, n.

(118), n. [Gr. ὁρίζων (sc. κύκλος), the bounding line, horizon; from ὁρίζειν, to bound, from ὅρος, boundary, limit; Lat. & Fr. horizon, Pr. orizon, Sp. horizonte, It. orizzonte.]

1Sense 1. The circle 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.

In the East the glorious lamp was seen Regent of day, and all the horizon round Invested with bright rays. Milton.

2Sense 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 the rational or celestial horizon. (c.) (Naut.) The line separating sky and water, as seen by an eye at a given elevation.

Apparent horizon. See 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.—Dip of the horizon, the vertical angle contained between the sensible horizon and a line to the visible horizon, the latter always being below the former.