Dictionary.net

The Rational horizon of a station is the plane drawn through the centre of the great sphere at right angles to the direction of the plumb-line at the station. If the radius of the earth is taken to have sensible magnitude, there is a Sensible H. parallel to the former, and passing through the station. The circle in which these planes cut the great sphere is the Celestial H., or the Horizon. The circle which bounds the visible part of the earth or ocean is the Visible or Apparent H., and is sometimes called the Sensible H. (For Dip of the H., vide Dip.) An Artificial H. is a little trough of mercury. An observer measures the angle between a star and its image formed by reflexion in the mercury, and thus obtains the double altitude of the star.