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. "And when the morning sun shall raise his car Above the border of this horizon." (Shak) "All the horizon round Invested with bright rays." (Milton)

2. <astronomy> The unbroken line separating sky and water, as seen by an eye at a given elevation, no land being visible.

3. <geology> 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. 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 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.

<astronomy> See def. 2, above. Visible horizon. See definitions 1 and 2, above.

Origin: F, fr. L. Horizon, fr. Gr. (sc) the bounding line, horizon, fr. To bound, fr. Boundary, limit.

(01 Mar 1998)