What is "celestial 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.

   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. (Astron.)

   1. 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.

   2. A plane parallel to the sensible horizon of a place, and passing through the earth's center; -- called also rational horizon or celestial horizon.

   3. (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.

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

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.

   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.

n. the great circle on the celestial sphere whose plane passes through the sensible horizon and the center of the Earth [syn: horizon]