A new instrument for solving spherical triangles

A simple calculator for solving the equation of a spherical triangle is described. This was designed for use in celestial navigation by amateur yachtsmen. It is much more rapid to use than the tables commonly employed. It can be made to be accurate to ± 1/2´of arc in calculating zenith distance, and so is suitable for any navigation to the usual standards of accuracy. Computation is based on Ohm’s law and effected by two potentiometers, each having two moving contacts. These are set by dials marked in angle to make resistances represent the appropriate derivations of latitude, declination, hour angle and zenith distance. When one of these is unknown, if the potentiometers are balanced the fourth resistance gives the solution. Sources of error and their avoidance in practical construction are discussed. It can solve any spherical triangle for a body above the observer’s horizon for declinations and latitudes up to about 80°, but for zenith distances less than 30° its accuracy is limited. The instrument is robust, portable and about the same weight as a book of nautical fivefigure tables.