In ordinary speech the horizon is line where the earth and the sky seem to meet, or the circle which bounds that part of the earth's surface visible to a spectator from a given point. This is termed the sensible, visible, or apparent horizon, as distinguished from the rational or celestial horizon, an imaginary great circle, parallel to the sensible horizon, whose plane passes through the earth's centre, whose poles are the zenith and the nadir, and which divides the sphere into two equal hemispheres. In observations with the sextant at sea, when the real horizon is invisible a small basin containing mercury may serve as an artificial horizon. The observation that is then made is the
angle between the sun or star and the image of the sun or star in the basin of mercury, and it is easily seen that half this angle is the altitude of the object above the real horizon. In geology, the term is applied to any well-marked formation which suffices as a starting-point from which to study the rest. Research Horizon
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
General Information
Research Horizon
