HORIZON. In its most familiar sense the horizon is the line or circle around which earth and sky seem to meet. On the ocean this circle is smooth and easily visible, and is then called the sea horizon.
In astronomy the horizon is defined by a plane at right angles to the direction of grav ity, extending out indefinitely on all sides, and called the plane of the horizon. The circle in which this plane cuts the celestial sphere is called the astronomical horizon. All points of it are apparently on a level with the eye of the observer. Owing to the rotundity of the earth the sea horizon is lower than this astronomical horizon — a narrow strip of sky separating the two. The angular distance between them, A E H, is called the dip of the horizon. The higher the observer is above the ocean, the greater is the dip. To an eye on the surface of the water, the sea horizon and the astronomical horizon coincide, so that there is no dip. The geometri cal principle which determines both the dip and the distance of the visible horizon are seen in the figure. The circular arc is here the surface of the ocean. The eye of the observer is situ ated at the point E, a short distance above the surface of the water. A tangent drawn from the eye to the surface meets the latter at the visible horizon, H. Let a horizontal line E A be drawn from the eye, the angle A E H is then the geometric dip of the horizon. The geometer will readily see that this is equal to the angle subtended at the centre of the earth be tween the observer, at and the point H. Since one minute of arc in the curvature of the earth's surface corresponds to one nautical mile, it follows that, geometrically, the dip of the horizon in minutes is equal to its distance in nautical miles. But, in the actual case, the line of sight is curved in consequence of the refraction of the air. The result of this is that the actual horizon is further than given by the geometric theory, and the dip somewhat smaller. The following table shows the relation between the apparent dip and the height of the eye above the water and the distance of the sea horizon.
On board a steamship the eye of an observer on the promenade deck is generally from 15 to 20 feet above the water. It follows that the distance of the horizon is about five miles. A ship farther away than this will have more or less of her hull below the horizon. At double
the distance the entire hull will be below the horizon, and only smokepipe and masts visible. As she goes yet further, these also will dis appear, as if sinking below the water.
On land the plane of the true horizon is frequently determined by a so-called artificial horizon. This may be either a small rectangu lar basin containing metallic mercury or else an accurately surfaced plate which is mounted upon three leveling screws and adjusted to the horizontal plane by means of a small level which may be placed in various positions upon it. The first form of artificial horizon is shown in the figure. In use the basin is placed upon an approximately level portion of the ground or other firm support, the mercury is poured into it, and the glass roof is placed over the whole. If a sheet of paper is first laid upon the ground below the basin, this will prevent moisture from accumulating on the inside of the glass plates and so blurring the reflected image. It is evident that if the observer measures the angle between any object in the sky and its image as seen reflected in either form of artificial horizon, the result thus ob tained will be twice the altitude.' When alti tudes are measured at sea, the ray of light from the distant sea horizon passes immediately above the surface of the ocean through many miles of dense and wet air before it enters the telescope. Its change in direction by refrac tion is therefore very great and very uncertain in amount, so that however carefully the ob servations may be made, the resulting positions will be uncertain by at least from three to five miles and often by much more. With the arti ficial horizon, however, both the rays of light which fall on the mercury and those which enter the telescope directly pass through our atmosphere at an altitude equal to that of the body observed; the effects of refraction can here usually be determined with a high accuracy. Thus on voyages of exploration, when the ship approaches sufficiently near any island or coast, a sextant party is usually set on shore by boat to determine the position by a land observation. The position of the horizon plane as a fixed plane of reference for large, solidly mounted instruments of astronomy is usually determined by means of the spirit level. ASTRONOMY, PRtCrICAL.