or HORIZON, in Geography and Astronomy, a great circle of the sphere, dividing the world into two parts or hemispheres; the one upper and visible, the other lower and hid. The word is pure Greek, ὁρίζων, which literally signifies "bounding or terminating the light;" being formed of ὁρίζω, termino, defino, "I bound, I limit;" whence it is also called finitor, "finisher." See ASTRONOMY and GEOGRAPHY.

The horizon is either rational or sensible.

Rational, true, or astronomical HORIZON, which is also called simply and absolutely the horizon, is a great circle, whose plane passes through the centre of the earth, and whose poles are the zenith and nadir. It divides the sphere into two equal parts or hemispheres.

Sensible, visible, or apparent HORIZON, is a lesser circle of the sphere, which divides the visible part of the sphere from the invisible. Its poles, too, are the zenith and nadir: and consequently the sensible horizon is parallel to the rational; and it is cut at right angles, and into two equal parts, by the verticals.—The sensible horizon is divided into eastern and western. The eastern or oritive horizon, is that part of the horizon wherein the heavenly bodies rise. The western or occidental horizon, is that wherein the stars set. The altitude or elevation of any point of the sphere, is an arch of a vertical circle intercepted between it and the sensible horizon.

By sensible horizon is also frequently meant a circle, which determines the segment of the surface of the earth, over which the eye can reach; called also the physical horizon. In this sense we say, a spacious horizon, a narrow scanty horizon.