Agonic Lines by Simon Phillips
Agonic Changes – Where true north and your local magnetic north lines coincide.
Soon, the agonic line will be passing through my back garden here in sunny Kent. The gradual movement of this line from east to west has been happening for over 350 years in the British Isles. We have had a westerly declination, which means that magnetic north is west of true north.
As a professional yacht skipper, we must be aware of the declination in the areas we are sailing in and how this will affect our compass – we could end up sailing to a different island otherwise!
In order to understand declination, you must first know that there are two North Poles – a True Geographic North Pole at the top of the world, and a Magnetic North Pole where we think our compass points to. The Magnetic north pole moves, significantly. It has been more than 1200 miles away from the true north pole. In 2005, it was around 500 miles away from the true north pole and is now at around 86.5N 172.7W.
Well, where does my compass point then? True North, Magnetic North or somewhere else? You’ve guessed it – somewhere else! It cannot point to true north as nothing is ‘pulling it’ in that direction. The compass does not point to a single point, but it will point along the lines of the magnetic component of where the compass is located. The difference between true north and the horizontal trace and the magnetic field for your area is called declination.
The variation of declination is between 0 and 30 degrees, dependent upon your location. Bearing in mind that 1 degree off course over a 60 mile distance, you will have an error of 1 mile, this is pretty important stuff! Trinidad could soon become St.Lucia then!
The actual declination and its rate of change in an easterly or westerly direction will be shown on your chart.
What causes the movement of magnetic north and what are the effects this will have on compass readings?
The reason magnetic north changes so much is due to the physical structure of the Earth. The different layers of the Earth all play a part in this continual fluidity of the magnetic pole. The inner core of our planet is solid Iron, surrounded by a molten outer core. The layers are rotating at different speeds on planet Earth which gives rise to convections and currents within the core – a dynamo effect. These interactions between the iron rich core and the magnetic field generate electrical current which create new magnetic fields – like a giant electromagnet.
Below is an actual declination chart from 1990 and another from 2015 – it is obvious to see what is happening and the differences between them.
In this illustration, the Agonic line is on green – almost clipping Kent and East Anglia.
How this affects the navigator.
Our work here at Seaway Deliveries is to ensure the smooth passage of any yacht on transit for our clients. As navigators, we have certain calculations to do between reading the compass and plotting on the chart. One is true north – chart, and the other is the compass reading and then the declination difference.
From plotting a course on a chart to give the helm a course to steer, we must do the following; if declination is East, we must subtract it. If declination is West, we must add this to the measured bearing on the chart before telling the helm which course to steer. This is the same the world over on any map / chart and compasses.