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 hidden. The word is pure Greek, ἐπίκλισις, which literally signifies bounding or terminating the sight, being formed from ἐπίκλησις, termino, defino, to bound or limit. See Astronomy and Geography.

The horizon is either rational or sensible. The rational, true, or astronomical horizon, which is also called simply and absolutely the horizon, is a great circle of the sphere, the plane of which passes through the centre of the earth, and the poles of which are the zenith and nadir. It divides the sphere into two equal parts or hemispheres. The 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, 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 ortive horizon is that part of the horizon in which the heavenly bodies rise; the western or occidual horizon is that in which 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, and it is also called the physical horizon. In this sense we say, a spacious horizon, a narrow, scanty horizon, and so forth.