HORIZON
From Agepedia
HORIZON; the line that seems to link the land or sea and sky; and it is either rational or sensible. The 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. 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 are likewise 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 vertical. These two horizons, though distant from each other by the semidiameter of the earth, will appear to coincide, when continued to the sphere of the fixed stars, because the earth, compared with this sphere, is but a point. The sensible horizon is divided into eastern and western. The eastern or ortive horizon is that part of the horizon wherein the heavenly bodies rise. The western or occidual horizon is that wherein the stars set. By sensible horizon is also frequently meant a circle which determines the segment of the sur face of sthe earth over which the eye can reach; called, also, the physical horizon. In this sense we say, a spacious horizon, a narrow, scanty horizon. It is manifest, that the higher the spectator is raised above the earth, the farther this visible horizon will extend. On account of the refraction of the atmosphere, distant objects on the horizon appear higher than they really are, or appear less depressed below the true horizon, and may be seen at a greater distance, especially on the sea. Legendre says, that, from several experiments, he is induced to allow for refraction a 14th part of the distance of the place observed, expressed in degrees and minutes of a great circle. Thus, if the distance be 14,000 toises, the refraction will be 1000 toises, equal to the 57th part of a degree, or l7 3;/.
