HORIZON

, in 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 Horizon is either Rational or Sensible.

, Rational, True, or Astronomical, called also simply and absolutely the Horizon, is a great circle having its plane passing through the centre of the earth, and its poles are the zenith and nadir. Hence all the points of the Horizon, quite around, are at a quadrant distance from the zenith and nadir. Also the meridian and vertical circles cut the Horizon at right angles, and into two equal parts.

, Apparent, Sensible, or Visible, is a lesser circle of the sphere, parallel to the rational Horizon, dividing the visible part of the sphere from the invisible, and whose plane touches the spherical surface of the earth.

The sensible Horizon is divided into Eastern and Western; the Eastern or Ortive being that in which the heavenly bodies rise; and the Western, or Occidual, being that in which they set.

, in Geography, is a circle dividing the visible part of the earth and heavens from that which is invisible. This is peculiarly called Sensible or Apparent Horizon, to distinguish it from the Rational or True, which passes through the centre of the earth; as already observed. These two Horizons, though distant from one another 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.

By Sensible Horizon is also often meant a circle which determines the segment of the surface of the earth, over which the eye can reach; called also the Physical Horizon. And in this sense we say, a spacious Horizon, a narrow or scanty Horizon, &c; depending chiefly on the height the eye is elevated above the earth.

For, it is evident that the higher the eye is placed, the farther is the visible Horizon extended. Thus, if the eye be at A, at the height AD above the earth; draw the two tangents Ah, Ar; and let one of these lines Ah, be moved round the point A, and in its revolution always touch the surface of the earth; then the other point h will describe the visible Horizon hor, &c. But if the eye be placed higher as at B, the tangents BH and BR will reach farther, and the visible Horizon HOR will be larger.

The visible Horizon is most accurately observed at sea, and is therefore sometimes called the Horizon of the sea. In observing this Horizon, the visual rays Ah and Ar will, on account of the curve surface of the sea, always point a little below the true sensible Horizon SS or EF, and consequently below the rational Horizon TT, which is parallel to it.

To find the Depression of the Horizon of the sea below the true Horizon, which varies with the height of the eye, and in a small degree with the variation of the refractive power of the atmosphere, see DEPRESSION.

As to the right-lined distance, or tangent Eh, it may be found thus; as radius : sin. [angle] C : : CA : Ah, or thus; as radius : tan. [angle] C : : Ch : Ah, either of which will be nearly the same as the arc or curved distance Dh. Or, without finding the angle C, thus; the square of Ah is equal to the difference of the squares of CA and Ch, i. e. , and hence , which is also equal to Ah nearly.

The distance on a perfect globe, if the visual rays came to the eye in a straight line, would be as above stated: but by means 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 SS, and may be seen at a greater distance, especially on the sea. M. Legendre, in his Memoir on Measurements of the Earth, in the Mem. Acad. Sci. for the year 1787, 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 14000 toises, the refraction will be 1000 toises, equal to the 57th part of a degree, or 1′ 3″.

Horizon of the Globe, a broad wooden circle. See Globe.