Much of what you read about celestial navigation is focused on ocean navigation where you have a reasonably unrestricted line of sight to the horizon. Terrestial navigators and surveyors often do not have this and must make adjustments in their technique. One very common approach is to use an ARTIFICIAL HORIZON.
An “artificial horizon” is fairly simple device consisting of a reflecting plane that is parallel to the natural horizon. Light from the sun or other celestial object are reflected off this surface and can then be observed in the sextant. The angle between the real object, and its reflected image is double the altitude of the object above the horizontal plane. This device allows the observer to take measurements in valleys, forest, and in weather that might obscure the natural horizon. The Artificial Horizon is particularly useful when high precision is required, such as for setting chronometers.
Probably the earliest reference to an artificial horizon appeared in the manual on navigation entitled Arte de Navegar by Pedro de Medina in 1545. Medina described a device a device to be used by an observer using a cross-staff consisting of a horizontal bar to be attached on the observer’s ship to serve as a substitute for the horizon when obscured. In about 1730, John Hadley devised a simple spirit level that he attached to his octant. Two years later the London instrument maker John Elton published a description of an improved artificial horizon in Philosophical Transactions. This consisted of two fitted spirit levels positioned at right angles to each other to the quadrant’s frame. The difficulty of holding the instrument steady and keeping it perfectly vertical while making an observation made each of these early forms impracticable. In about 1738, George Adams, a noted London maker of mathematical instruments, created an instrument which consisted of a trough of mercury which reflected the celestial objects allowing indirect observation of their anbles. In addition to the mercury and trough, Adams added a glass roof inserted gable-wise in such a manner to prevent the wind from disturbing the mercury surface. Based upon the first law of optics, that the angle of reflection from a mirror is equal to the angle of incidence. This new artificial horizon allowed the user to employe a sextant is used to measure the angle between the sun and its image on the mercury surface, this angle being equal to twice the sun’s apparent altitude. The apparent altitude is the arc of a vertical circle between the apparent direction of the observed object and the plane of the sensible horizon or horizontal plane upon which the observer’s eye stands.
When an artificial horizon is used, the observer must place themself at such a distance that they may see the reflected object as well as the real one; then having the sextant properly adjusted, the upper or lower limb of the image reflected from the index-glass, must be brought into contact with the opposite limb of the image reflected from the artificial horizon. The angle shown on the instrument, when corrected for the index error, will be double the altitude above the horizontal plane.