Fig. 1. Deriving true compass directions with a Viking sun-compass, a shadow-stick and sunstones. A Under clear skies the gnomon casts a clear sharp shadow on the horizontal dial of a levelled sun-compass. Navigation is possible without auxiliary tools. B A cast shadow cannot be seen when the sun is occluded. The Viking navigator must estimate the elevation and azimuth angles of the sun. These data can be gained also by estimating the position of the antisolar point. C A shadow-stick is a small item provided with a series of sockets representing various solar elevation angles. Since the sockets must not overlap, the smallest resolution of elevation angles is determined by the dimensions of the shadow-stick and the diameter of the sockets. D To derive true compass directions with a shadow-stick and a sun-compass, the socket on the shadow-stick corresponding to a given solar elevation is applied on the gnomon tip, and then the end of the stick is turned to point toward the solar meridian. The shadow-stick now replaces the missing or poorly visible cast shadow. To find true compass directions, the navigator must rotate the sun-compass while keeping the shadow-stick still until the gnomonic line fits to the tip of the shadow-stick. E A marked replica Viking round-shield with a diameter of 80 cm was used as a crude sextant with satisfying precision to provide a secondary estimation of the solar elevation as suggested by Captain Jensen [4]. F The estimation of elevation angles of celestial bodies or celestial points with fists and extended arms is a practice frequently used by amateur astronomers. The observer counts the numbers of fists and fingers needed to subtend the arc in question.
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