HORIZON (Gr. 6pfTwv, 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 Horehound . 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 islt feet, are approximately given by the formulae: Dip =o'.97 ~Ih; Distance = I'"• 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 .