Horizon

Horizon, the circular line formed by the apparent meeting of the earth and sky; this, in astronomy, is sensible horizon. The rational horizon is the circle formed by a plane passing through the centre of the earth, parallel to the sensible horizon, and produced to meet the heavens. The artificial horizon is a small trough containing quicksilver, the surface of which affords a reflection of the celestial bodies. It may be used for calculating the altitude of the stars when the sea-horizon is obscured by fog or otherwise not available to the sailor determining his position. The dip of the horizon is the angle through which the sea-horizon appears depressed in consequence of the elevation of the spectator. The true dip of the horizon, however, is not exactly the same as its apparent depression. The apparent sea-horizon is raised above its true place by refraction through an angle which varies according to the state of the atmosphere and the relative temperatures of the air and water, the variation ranging from one-third to one-twenty-third of the amount of the true dip. The rule commonly employed is to diminish the true dip by about one-fourteenth of its amount to find the apparent dip.