The Agonic Line

posted Nov 25, 2013, 3:03 AM by Professor Katz   [ updated Nov 25, 2013, 11:30 AM ]
Columbus noticed something very odd on his third voyage to the New World (1498-1500): his compass didn't always seem to work very well.  When he passed 'the line from north to south a hundred leagues west of the Azores' in a westerly direction, 'the needle shifted by a quarter north-westwards, and continued to shift farther to the north-west as we sailed on.'  What Columbus discovered was what used to be called the 'Line of No Variation', and is now referred to as the 'Agonic Line', i.e. 'the line having no angle'.

As it happens, your compass needle will point to the actual North Pole in only a few places on the globe, and if you draw a line through those points from north to south, you get an Agonic Line.  West of that line, the magnetic N on your compass will increasingly point to somewhere east of the North Pole.  If you are east of an Agonic Line, then the magnetic N on your compass will increasingly point to somewhere west of the North Pole, and it only gets worse the farther away you get from the Line.  This phenomenon is very dramatic in the United States:


The bigger problem is that the location of 0 degree Agonic Lines
changes over time, since the magnetic North Pole is always on the move, and is never exactly in the right spot, thanks to movement in the molten metallic core of the earth:


In Europe, the same phenomenon exists, of course, but the difference between True North and Magnetic North is not as severe, although it increases dramatically towards the poles.


If you don't have a GPS you will need to take all of this into account when you go hiking, especially in the USA.  If you are in Washington State, the magnetic N on your compass will point up to 20 degrees east of True North ... and if you are in Maine, the magnetic N on your compass will point up to 20 degrees west of True North.  If you don't know this, you could get very badly lost.

In Israel, the difference is only a little over 4 degrees, so it's not such a big deal.  You just need to remember that True North is a little over 4 degrees less than what you are seeing as the direction of the magnetic red N, so just turn your compass a little over 4 degrees in a counter-clockwise direction so that the red N appears 4 degrees to the right of the arrowhead.
If you want to know how to calibrate your compass, wherever you are, click here.  It could save your life ... unless you have a GPS ... or prefer to stay at home studying interesting subjects such as the Agonic Line.
Comments