page_title: Uunartoq Disc
section_title: Method of Use
hierarchical_section_title: Uunartoq Disc / Method of Use
caption_attribution_description: English: Modern speculative replica of the Uunartoq disc, showing gnomonic lines and gnomonic pin shadow used to plot a course of Northwest by North.
context_page_description: The Uunartoq disc is an artifact discovered in the ruins of a Norse Greenland homestead. It is widely believed to be an early Norse sun compass.
context_section_description: Like a sundial, a sun compass features a vertical pin. The sun's rays cause the pin to cast a shadow which is longer in the morning and late afternoon and shortest at solar high-noon, and this shadow is used to create gnomonic lines. The disc is placed in a fixed level position and an observer regularly marks the shifting position of the pin tip's shadow across the disc during the course of the day. When these position marks are connected, the result is a west-to-east gnomonic line that comes closest to the vertical pin at high-noon. In the northern hemisphere, a straight line drawn from the base of the pin to that closest high-noon position will point directly to true north, which will then serve as the compass' north index mark. The gnomonic line will be essentially straight during the vernal and autumnal equinoxes, and downward concave at the summer solstice.
   Once these lines have been inscribed on the face of the compass, it can be used during travel. The user holds the device level and rotates it until the pin tip's shadow touches the appropriate seasonal gnomonic line, and the index mark will point to true north.
  Sølver noted that the Uunartoq disc appeared to have gnomonic lines consistent with those produced during the summer solstice and the equinoxes (which in 1990 were microscopically shown to be deliberately double traced) and also noted that the disc had 8 triangular dial increments per quadrant, for a total of 32, corresponding with the traditional mariner's compass. In addition to its gnomonic lines, it also had a round dimple on the 9th increment that corresponds with east, where the lines end with the setting of the sun in the west.
  In 1984, Norwegian author and explorer Ragnar Thorseth led an international expedition in a replica of a Norse merchant ship, the Saga Siglar. For the passage between Iceland and Greenland, the crew had been given replicas of the Uunartoq disc to test against the ship's modern magnetic compass, and the deviation between the two was described as "negligible" and "...results were far better than the navigators had expected..."
caption: Modern replica showing how a course of Northwest by North may have been plotted by the Uunartoq disc's user around the time of the summer solstice. The traveller rotates the disc while keeping it level until the vertical gnomonic pin tip's shadow touches the summer solstice gnomonic line (1), which reveals the direction of true north (2). The traveller then turns the horizontal directional pin five compass points to the west for NWbN, and adjusts course to match the direction of the pin (3).
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