It’s hard to believe that until the second half of the 18th century there were no accurate charts of the world’s oceans. Why not, you may wonder? The answer is simple but perhaps not obvious. Because it was impossible to navigate accurately out of sight of land.
To make a good map, you have to be able to determine exactly where you are – and where everything else is in relation to you. On dry land this could be achieved as early as the 17th century by measuring an accurate baseline and then using the geometrical system known as triangulation to build on it. Eventually a network of triangles gave the mapmaker a grid of fixed points extending across the landscape. But this method didn’t work on the open sea. In fact even important features like the Scilly Islands – which are actually visible from Land’s End – were charted in the wrong place until the end of the 18th century when the Ordnance Survey triangulation system finally embraced them!
Until the 1750s there was simply no reliable way of fixing a ship’s position when out of sight of land. By measuring the height of the sun at midday it was relatively simple to find out how far north or south of the equator you were, but it was impossible to find out how far east or west you had travelled. In other words, finding a ship’s latitude was possible, but finding its longitude with any precision was not.
Only when this problem was solved – in two very different ways, almost simultaneously, did it become possible to map the world’s oceans. And both methods depended on the sextant!