Columbus and the mystery of the two “Norths”
The Spanish and Portuguese were probably the greatest sea-faring nations in the 1400’s. This would not have been possible without that greatest and yet simplest of magnetic instruments: the compass. It was the compass that enabled them to trade with (and take slaves from) Africa. Magellan and Vasco de Gama could not have set sail on their epic voyages without the magnetic compass. And when Christopher Columbus set off on what was supposed to be an attempt to discover an alternative route to the East Indies.
This last was particularly interesting because it gave a hint of a strange phenomenon. For as Columbus sailed, he kept a log and in it he noticed that as he sailed west, the compass gradually went out of alignment with Polaris the North Star. This made no sense because it was widely assumed that the compass pointed north and the North Star was in the – yes – north. Yet the further westerly he sailed, the more out of alignment they became. How could this be?
The reason – we now know – is because magnetic north is not the same as true north. True north, and its counterpart true south, are defined by the axis of earth’s spin. But although it is this spin of the earth’s crust and mantle (and the slower spin of earth’s molten iron core) that is responsible for earth’s magnetic polarity, the magnetic poles move around the true poles very gradually in a kind of secondary spin.
At the moment, and in Columbus’s day, magnetic north is west of true north. Now when the ships were sailing around the African coast, and mostly sailing north or south rather than east or west, this didn’t really matter. The difference was small and it didn’t change much during the journey. But now that he was sailing west and from a more northerly position, this difference between true north (as indicated by the North Star) and magnetic north (as shown by the compass) became more noticeable.
In fact, initially the gap between the two lines diminished, as Columbus approached the Agonic line, where magnetic and truth north appeared to align perfectly, but then as he sailed further they slipped out of alignment again and the further west he sailed, the more out of alignment they became. This became a seriously problem on the voyage not just because it went against prevailing landfall but because it caused discontent among his men.
Remember that Columbus was looking for a fast route to the east Indies and the reason for the voyage was that he thought the earth was smaller than his contemporaries believed. In other words he believed that sailing to the east Indies by sailing west was the quicker way to go. When they failed to hit land in the time frame that Columbus expected (because he was in fact wrong about the size of the earth), the men naturally became concerned.
Fortunately he did find land: not the Indies, but a land that he called Indios. This was the New World that we now call America.