Trelleborg, a Danish observatory made by the Vikings.
The constructions of Trelleborg in Denmark and the bearing-dial from Greenland are constructed based on the same geometrical pattern. Click the image and pull a line when holding down the buttom. Make your own experiments with Viking geometry.
Bearing the height of the sun at noon gives us the N-S and E-W directions and the Vikings made a bearing of the sun at sunrise when the sun was just visible over the horizon. Follow the orbit of the sun over Trelleborg here or investigate a nomogram for sun heights on different latitudes incl. Trelleborg here.
See how the Vikings constructed Trelleborg here.
More info about Trelleborg and Viking geometry by Preben M Henriksen here.
More info about "modern" geometry used for astronomical purposes here.