The invention of the compass was arguably one of the most important milestones in the modern history of exploration and navigation – allowing unprecedentedly simple navigation, in ways that are still used today. Although low-cost GPS devices are beginning to displace traditional compasses, the compass would still work in the event that one’s GPS device malfunctions, or the satellite system itself becomes unavailable – for this reason, the compass is still valuable despite being seemingly obsolete.
The invention of the compass rested upon the realization which informed several previous but more awkward navigational devices: the Earth possesses a relatively strong magnetic field, which is almost (though not quite) aligned with the geographic north and south poles. The use of an iron magnet can allow one to detect this field, since the magnet, left to rotate and move freely, will align its own north-south axis with the north-south axis of the Earth. For this reason, the first compasses were simply small magnetic rods left to float in a container. Contemporary devices are, of course, much smaller and more sophisticated.
The mechanism by which this works is relatively easy to explain. At the centre of any working compass is a small magnet, which is left to balance or float in such a way as to minimize friction as much as possible. (This was why early compasses floated: floating on water involves minimal friction). One end of this magnet will be marked as “north” – there is no reason to mark “south” on the other end, since north and south are always opposite.
All magnets, and the Earth itself, possess two poles, referred to as “north” and “south”; at all times, the north poles of two magnets will repel each other, as will two south poles, but a north and a south pole, when facing each other, will be attracted, pulling themselves together. The Earth itself is actually an enormous magnet, with one pole – the north – almost exactly aligned with the geographic North Pole, and the other aligned almost exactly with the South Pole. The “north” marker on the compass actually is the south pole of that magnet – so it is drawn towards the north pole of the Earth.
This is essentially all it takes to understand how the compass works physically. However, all this will do is let you know, at all times, where the geographic north is. The rest of the skill set of using a compass, known as orienteering, is about how to read and interpret maps and locate one’s position relative to north. Effectively, once one knows where north is, one can infer from that where west, east, south, and all other specific directions are.
One complicating factor is that as one travels in the far north or far south, compasses become unreliable due to their proximity to the magnetic poles and the differences between the true north pole of the Earth (which is what we really intend to navigate by) and the magnetic north pole (which is what the compass measures, and is roughly a thousand miles away from the geographic north pole). People travelling in this regions must compensate by adjusting their compass measurements accordingly.