1911 Encyclopædia Britannica/Horizon

HORIZON (Gr. ὁρίζων, dividing), the apparent circle around which the sky and earth seem to meet. At sea this circle is well defined, the line being called the sea horizon, which divides the visible surface of the ocean from the sky. In astronomy the horizon is that great circle of the sphere the plane of which is at right angles to the direction of the plumb line. Sometimes a distinction is made between the rational and the apparent horizon, the former being the horizon as determined by a plane through the centre of the earth, parallel to that through the station of an observer. But on the celestial sphere the great circles of these two planes are coincident, so that this distinction is not necessary (see Astronomy: Spherical). The Dip of the horizon at sea is the angular depression of the apparent sea horizon, or circle bounding the visible ocean, below the apparent celestial horizon as above defined. It is due to the rotundity of the earth, and the height of the observer’s eye above the water. The dip of the horizon and its distance in sea-miles when the height of the observer’s eye above the sea-level is h feet, are approximately given by the formulae: Dip = 0′.97 √h; Distance = 1^m.17 √h. The difference between the coefficients 0.97 and 1.17 arises from the refraction of the ray, but for which they would be equal.