Magnetic North made an unusual and historic shift. For the first time in more than 220 years of map making, Ordnance Survey has noted that North lies East, and not West, of Grid north for parts of Southern Britain. But how does this shift in magnetic field affect map reading in Scotland’s hills?
Scientists have known for a long time that the Earth’s magnetic field is fluctuating, even fading… What is more uncertain is whether the weakened field is on its way to complete a collapse and reversal that would essentially “flip” the North and South magnetic poles, causing compasses pointing North to point South instead…
The geomagnetic field is the magnetic sphere that extends from the depths of the Earth’s interior to the point where it meets the solar wind – a stream of solar particles, emanating from the Sun’s violent activity. Above the ionosphere, the magnetosphere region extends several kilometres into space. The field magnitude at the Earth’s surface ranges from 25 to 65 micro-Tesla. The Earth’s field is comparable to that of a magnetic dipole tilted at a 10° angle with respect to the rotational axis, as if a bar magnet was placed at the centre.
Unlike a bar magnet, though, the Earth’s magnetic field changes over time. Both geomagnetic poles move over time because the magnetic field of Earth is produced by the motion of molten iron alloys within the outer core. The dipole reversal pattern is erratic, and it is hard to pinpoint when it would happen… But it has happened in the past…
A dipole reversal is a rare event. The last time one occurred was 780,000 years ago. The time between dipole reversals varies from a thousand years to millions of years. Seeing how the Earth’s magnetic field has fluctuated over the geological time scale is even possible.
Over the past 150 years, the poles have moved westward at a rate of 0.05° to 0.1° per year, with little net North or South motion.
Over the last century and a half, since monitoring began, scientists have measured a 10% decline in the dipole.
Current Location for the Magnetic ‘North’
The north magnetic pole moves over time due to magnetic changes in the Earth’s core.
So, the North Magnetic Pole wanders, but it occurs slowly enough that an ordinary magnetic compass does remain useful for navigation.
How does it actually affect hill-walkers in Scotland?
Those who navigate the Great British landscape by map and compass will know there are three Norths.
Hillwalkers may well have broken into a cold sweat when reading recent attention-grabbing headlines about magnetic north moving east. Not having this up-to-date knowledge may conjure up visions of walking innocently over a cornice and tumbling to your doom in white-out conditions.
But it is not quite as dramatic as that.
Current Magnetic Variation
Maps are aligned to ‘Grid’ North. Compasses indicate the direction of Magnetic North. There is a difference between the two, which is the result of trying to reproduce a spherical world on a flat map. This difference varies with time and depending on where you are in the World. For example, the difference between grid and magnetic north in Scotland is very different to that in South America.
For the geeks, British Geological Survey has a website where you can enter your location details and receive the current magnetic variation for your chosen location.
If the result appears to be west of Grid North, the user needs to “add on” to the bearing that has been taken from the map. For example, a bearing from the map in the Cairngorm area was 48 degrees. The user should add on a further two degrees and walk on a 50 degree bearing.
If the result bears east of Grid North – as is now the case in deepest Cornwall – then this figure should be deducted.
Each tiny black line on the compass housing represents two degrees, so it is impossible to differentiate the tiny adjustments required for 100% accuracy. When factoring in the human element of walking along the bearing, this degree of accuracy is probably irrelevant. Although in winter white-out conditions, near corniced edges, the safest thing to do is probably adding those two degrees.
Over the next few years, here in Scotland, this variation ought to decrease gradually to zero. One less thing for Scottish mountaineers to worry about… For a time…