Well, that's what I thought. But it wasn't until 1492 and later, even, that the world was thought, by Europeans, to be flat.
Looks like the Vikings, as long-distance sea travelers in general, had based their navigational procedures upon the assumption of sailing a globe-shaped surface. I should read more about this, since I'm studying for my Captain's license, and navigation is a big part of that.
I believe the “World is Flat” was pretty much relegated to churchmen and people who had never been to sea.
People weren’t stupid. If you are near the ocean stand and look at it from the shore. You will see a noticeable curve to the horizon. Board a boat and head out to sea. First the shoreline will drop below your horizon, than the higher land and finally the hills.
Watch an approaching boat. The first thing you se is the mast, then the hull and finally the waterline.
Head back toward shore. The first thing you see will be the tops of the hills, followed by the lower hills and finally the shoreline.
The only explanation is that you are on a curved surface. The Greek Erastothenes measured the circumference of a spherical Earth over 2,000 years ago
This has nothing to do with the Vikings but I believe the Arabs sailed north and south up the coast of India and Africa using the Al Kemal (sp) plate, 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 coast of Africa or India.
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 position 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 always 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.