horizon

HOR'IZON, n. [Gr. to bound, a limit.] The line that terminates the view, when extended on the surface of the earth; or a great circle of the sphere, dividing the world into two parts or hemispheres; the upper hemisphere which is visible, and the lower which is hid. The horizon is sensible,and rational or real. The sensible, apparent, or visible horizon, is a lesser circle of the sphere, which divides the visible part of the sphere from the invisible. It is eastern or western; the eastern is that wherein the sun and stars rise; the western, that wherein they set. The rational, true, or astronomical horizon, is a great circle whose plane passes through the center of the earth, and whose poles are the zenith and nadir. This horizon would bound the sight, if the eye could take in the whole hemisphere.