Horizon

(Gr. orizo , I bound or terminate). In astronomy and geography, is the plane of the great circle of the sphere, dividing the visible from the invisible hemisphere. The horizon is either sensible or rational . The sensible horizon is a plane which is a tangent to the earth’s surface at the place of the spectator, extended on all sides till it is bounded by the sky; the rational horizon is a plane parallel to the former, but pa ing through the centre of the earth. Both the sensible and rational horizon are relative terms, and change with every change of the spectator’s position on the surface of the earth; in all cases they are perpendicular to the direction of gravity.