Burt's solar compass
Burt's solar compass or astronomical compass is a surveying instrument that makes use of the Sun's direction instead of magnetism. William Austin Burt invented his solar compass in 1835. The solar compass works on the principle that the direction to the Sun at a specified time can be calculated if the position of the observer on the surface of the Earth is known, to a similar precision. The direction can be described in terms of the angle of the Sun relative to the axis of rotation of the planet. This angle is made up of the angle due to latitude, combined with the angle due to the season, and the angle due to the time of day. These angles are set on the compass for a chosen time of day, the compass base is set up level using the spirit levels provided, and then the sights are aligned with the Sun at the specified time, so the image of the Sun is projected onto the cross grating target. At this point the compass base will be aligned true north–south. It is then locked in place in this alignment, after which the sighting arms can be rotated to align with any landmark or beacon, and the direction can be read off the verniers as an angle relative to true north. This device avoided the problems of the normal magnetic compass used by surveyors, which displayed erratic readings when in a locality of high iron ore content and inconsistent and unknown local magnetic variation. The instrument was found to be so accurate that it was the choice of the United States government when surveying public lands, state boundaries, and railroad routes. It won awards from various organizations and was used by surveyors from the nineteenth into the twentieth century. (Wikipedia).
