I was very lost.
I was on the top of a hill somewhere in Dartmoor and the fog had come down. I had an OS map and a compass, and all I had been doing for an hour or so was walking in a straight line up the moor. So why could I not just turn around and follow my reciprocal path? Well, the other piece of information you need is that I was carrying a load of plastic bags and a very large and heavy piece of metal – an augur.
It was about 1986 and I was on a college field trip with Oxford’s Earth Sciences department. I was performing a transect for a soil survey, and using the steel/iron auger to screw into the earth at regular intervals to retrieve soil samples to take back to the lab for analysis.
The perceptive of you will have spotted the fatal flaw in this set-up: a large magnetic object held in close proximity to a compass will divert the compass’ needle. I thought I had been walking in a straight line up the cloudy-covered moor by following a constant compass bearing. But I had probably been swerving all over the vast, open vegetation, depending on where I was holding the augur when I took the next bearing.
And there were no fixed physical references to confirm my direction and position on the map.
I had noticed a sheep that had just laid down (it clearly knew that the weather was about to close in) and it was memorable because it was two tone – black at the front and white at the back. The colour join was an unnatural straight vertical line. But you shouldn’t fix you position on a live animal!
Now that I was lost in fog at the summit, I realised my schoolboy error. I was getting damp, and the light was fading. I couldn’t plot an accurate course back to the car park (where the university minibus would be waiting), simply because I didn’t know my exact position. So, I took a guess at a rough course and followed that downhill.
My luck changed when I espied, in the lowering gloom, the same half-and-half ovine that was still lying – I assumed – where I had past it previously. I guessed that I was close to the right track and eventually made it back to the van, obviously not owning up my near miss to my colleagues!
Swinging the compass
Compass deviation used to be a real headache on boats and ships, and not just on later iron-clads. In fact, it often resulted in fatal shipwrecks. Wooden ships tended to have metal objects not far from the binnacle (where the compass sits, located near the ship’s wheel). Maybe there was a chain plate nearby, or the rudder stock, or an anchor, or a cannonball, or a stanchion. In fact, all the iron nails or rivets – if aligned in the same way during construction – could create a magnetic force strong enough to cause the compass needle to deviate!
To complicate matters, the deviation of the ship’s compass alters, firstly, with the direction of the ship’s travel. And secondly, deviation alters with the ship’s position on the globe, because the earth’s magnetic field varies. This is not a problem for a skipper who restricts his sailing to a local area, but it was big problem for merchant seaman and navies traversing large oceans.
Alan Gurney’s book “Compass” (Norton, 2004) – thank you to Sari and Thorne for the gift – is an excellent yarn, and a thorough account of the creation and evolution of the compass. Gurney relates how the solution to correcting a ship’s compass for deviation was a long, global, trial-and-error work, and a largely British affair. The solution can only be described as a highly-technical botch, attributable to Lord Kelvin (previously Sir William Thompson). He created a binnacle that incorporated several manoeuvrable magnets and other soft and hard iron objects. These were minutely adjustable to totally counter the magnetic effect of the ship on its compass. The reality was that many maritime captains found this binnacle too technical and convoluted and didn’t use it.
When I was teaching Day Skipper practical courses, one of the navigation items to show the students was the yachts ‘deviation card’. This was normally stuck under the lid of the chart table. It showed the degrees of deviation for each of the eight inter-cardinal points (N-NE-E-SE-S-SW-W-NW). The card was created by ‘swinging the compass’, which means turning the boat through 360 degrees and taking regular bearings to a distant, fixed object that is identifiable on the chart and on a known magnetic bearing from you. You record how different your actual compass bearing is from the known magnetic bearing on the chart. When sailing in a heading, you must correct your heading for the yacht’s known compass deviation.
Halley Lines
Of course, deviation is only one of two major factors that has confounded navigators over the years. And I was reminded of the first on a recent trip to Oxford to visit the Ashmolean Museum. Alix and I were walking up Queen’s Lane, from the High Street, and passed Teddy Hall. We continued to wind along New College Lane passing New College, when I spotted a newish blue-plaque on a pillar. It said “Edmond Halley, 1656-1742”. It has replaced a wooden plaque that had read “Edmund Halley, Savilian Professor of Geometry 1703-1742, lived and had his observatory at this house”.
Now, until reading Gurney’s book, I hadn’t realised that Halley was a key player in solving compass ‘variation’. I also had no idea that he was a ship’s captain in the Royal Navy and commanded vessels that would sail huge distances to study and record this phenomenon.
The ancients knew the location of the earth’s axis (and so ‘true north’) by observing the sun’s path across the sky – known as the ecliptic. This apparent motion is a manifestation of the earth’s daily rotation on its annual path around the sun. Because of the tilt of the earth, the sun rises due east at only two times each year; the spring and autumn equinoxes. This is when the sun is directly above the earth’s equator. And 90 degrees from true east is ‘true north’.
The ancients also knew that the earth rotates on its axis which points (roughly) towards the north star, Polaris. (All the other stars seem to rotate around the sky as the earth spins, except for the ‘stationary’ Polaris. [DN – Let’s ignore progression for now].
From the middle-ages, Europeans and Chinese independently realised that a sliver of lodestone would rotate to point roughly northwards towards Polaris. They used lodestones to magnetise other metal objects that would also turn to the north. As China was not a great maritime nation at that time, the Chinese used this phenomenon mainly as a party trick - to magnetise metal fish to point south - or in Feng Shui [Alix: if they were raw fish, would that make it feng sushi?].
But the sailors of Europe around the end of the 12th century, especially the maritime republics such as Amalfi and Venice, realised the value of a compass needle on board their ships.
Interestingly for me (living Cirencester) it was a man called Alexander Neckam (made Abbot of Cirencester in 1213), who is the first European to record the existence of a compass. He was born in St Albans in the same year as King Richard I (1157), and spend some time in Paris. In his textbook 'De utensilibus' (“On Instruments”) he describes how a ship, among its other stores, must have a pivoted needle placed above a magnet, which would revolve until its point looked north, and guide sailors in murky weather or on starless nights.
These European merchant seamen normally avoided winter passages, because clouds obscured the sun and stars that they used for orientation. However, a north-pointing compass needle meant that they could navigate during the six months of October to March, this doubling their trade capacity.
Except that the needles didn’t point exactly to the Polaris or ‘true north’!
As we all now know, compass needles point to the earth’s magnetic pole. The amount by which the compass needle varies from ‘true north’ is called variation. Variation can be a large amount, but even small degrees of variation could make a fatal difference. And because the earth’s core is effectively a big ball of flowing liquid iron/nickel, variation varies depending on where you are on the globe.
Many merchant and navy captains who didn’t understand variation ended up nowhere near their estimated position. When I taught Coastal Skipper candidates, I would describe a cross channel passage from Portsmouth to Cherbourg, which is a about 60NM distant. If they sailed one degree off course, they would end up 1 nautical mile (NM) from their destination. Five degrees off and they would end up 5 miles away. A captain on a 600NM voyage and not correcting for five degrees of variation would be 50NM adrift and probably not even within visible range of his destination! Many mariners piled their ships onto rocks, and many lives were lost. This was why Halley’s voyages work was so important.
Halley sailed around the north and south Atlantic Ocean recording compass variation. His innovation was to join up all the points of equal variation (a bit like isobars do for barometric pressure) and these were published and became known as Halley lines.
Unfortunately, the fluidity of the earth’s metallic core means that variation changes over time, and it’s quite difficult to predict changes. (By pure coincidence, in 2019 there was zero degrees variation at Greenwich.)
Still, a slightly out-of-date figure was better than none at all!
So How Does Missy Bear cope with all this?
Missy Bear has a magnetic compass on the binnacle. But we don’t really use that, because we use digital bearings: she has an electromagnetic compass – called a ‘flux gate’ compass – down below. This gives a digital output of magnetic north. This figure is fed into the chart plotter software which automatically converts it to ‘true north’. All the displays on the system will be ‘true’ figures, e.g., your course-to-steer.
Of course, the systems are all hooked up to GPS so we always know where we are, give or take 10m.
The fluxgate compass will still be affected by metallic objects nearby, so we bear this in mind when stowing gear. And it is good practice to ‘swing the compass’ (calibrate) each year, when you are back on board with all the kit stowed.
Of course, now we are in Greece, much of our sailing will be ‘line of sight’, i.e., we will cover short distances and will be normally able to see our next port of call. So, the odd degree of variation or deviation is not that critical.
In summary, I’m glad I wasn’t a mariner in charge of a Genoese or Venetian ship in cloudy weather taking wealthy pilgrims to or from the levant on Crusade. In the Ashmolean Museum in Oxford, Alix and I saw maps of the routes that the crusaders took from western Europe towards the holy lands. Without modern electronics it would have been so easy – using a crude, uncorrected, dry-card compasses - to discover that I was nowhere near to my estimated position (based on the speed and direction we had sailed/rowed). I wouldn’t have found it much fun explaining that to an angry, tooled-up knight! [Alix: or your wife].
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