Pretty cool stuff
This has nothing to do with the Vikings´s method but I believe the Arabs sailed north and south up the coast of India and Africa using the AlKemal (sp) plate. This was a square metal plate with a knotted string at the center. When in sight of land the plate held with the base on the horizon and the moved forward and backward until the North star touched the top of the plate. A knot was tied in the string at the point where it touched the nose to denote that specific headland.
Thereafter, in bad weather or out of sight of land they knew their latitude with reference to the coasts of Africa or India. Of course the seasonal monsoons drove them in the right direction for open ocean trade.
Apparently the Polynesians had a similar method when sailing the huge Pacific. They used the magic gourd. A gourd with two holes in it that represented the latitude of the of the islands in relation to the north star.
Sail south, and on the return find the exact latitude of your islands by using the magic gourd and turn right keeping the north star aligned at arms length with the base on the horizon. Those two holes in your water filled gourd to maintain it level in a moving environment. This way you would know when you moved either north or south of your intended track.
The height of the magician in the boat and the water level also made an artificial horizon. Navigation was one of the dark arts.
I read about these things in a navigation book about 60 years ago which simply explained the principles before all the complexities were discussed. Of course knowledge of Longitude came much later.