HORIZON, or HORISON, 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, ἰσχύς, which literally signifies "bounding or terminating the sight;" being formed of ἰσχύς, termino, definio, "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