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What's your Magnetic Variation - the compass not you !

Mag1Sitting at home staring out at the rain on a wet and windy Sunday in East Sussex while planning our trip to the Channel Islands this summer, it occurred to me that I dont really pay that much attention to magnetic variation since the variation is so small that it normally doesnt make much of a difference especially when you factor in all the other ways in which either my navigation, my estimation or the various real vagaries of wind, tide and so on affect a course and an eventual landfall. I guess it would make a real difference on an ocean passage but that wont concern me this year – unfortunately:-)

Anyway, I thought I would check what the actual magnetic variation is today, here in Brighton…you might like to use the resources here to check on the real variation at your location.

I think every school child knows that magnetic north is not quite true north – and certainly every sailor knows it! The magnetic pole is located on Prince of Wales Island in northern Canada, this is 1,500 miles away, at about latitude 73° N, longitude 100° W.

If you think of the magnetic field of the earth being generated by a bar magnet running through the earth from north to south. Both ends of the bar would be the north and south magnetic poles. The bar itself would be the axis or, as it is called in geophysics, the Agonic Line. This Agonic Line moves as the magnetic pole shifts, due to many factors in the rotation of the earth. Over time the Agonic Line can be miles from where it was. Actually every 2 months or so a flight is manned and sent to find the magnetic pole. So for example the Agonic Line often blamed for pilot error in the so called “Bermuda Triangle” is not even in the Triangle anymore! It is located in the Gulf of Mexico beyond Key West – completely outside of the “Triangle”. Anyway lets not get distracted with mystery and myth!

Declination: Strictly speaking the figure of interest to sailors is the Magnetic Declination. To correct a magnetic compass reading to true north when west of the 0° Greenwich Meridian, the declination value should be subtracted from the magnetic north reading. When east of the 0° line, you should also subtract the declination from the magnetic north reading but since the value is negative then two minuses make a plus. If you see what I mean:-)

Variation: Magnetic Variation occurs over long periods of time. Due to the dynamic nature of the earth’s interior, the magnetic field is constantly changing. This causes “drift” of magnetic north and oscillations of the Agonic Line. This change in magnetic declination is called annual change (also secular variation or magnetic variation). It follows trends in the short term, but is very unpredictable in the long term.

Deviation: The imaginary magnetic lines that encircle the globe radiating from the Agonic Line are not straight, but bend and arc depending on local magnetic conditions – including the magnetism caused by the structure or equipment of your boat. This bending is called deviation.

Nevertheless – in nearly all the navigation literature and certainly on the RYA courses that I have attended the instructors talk about Magnetic Variation (i.e. Declination) – just in case you were getting a grip…

So , wCompassiconhat can you do to check the magnetic declination where you are?

…well if you want to wonder out tonight, and you have a clear sky to look at, – locate the North Star, look down to the horizon and point your handheld compass at this point. Then you can check out the angle between the needle (which points to magnetic north) and the N on the face of your compass. This will be the magnetic declination E or W at your location.

…alternatively, get your chart out for the area you are in and see if it shows the magnetic declination for your area (many do) then check the date your chart was published – and if it is less than 10years old you could ignore the fact that the declination may have drifted since the chart was published. To give you and idea, here are three locations and the rate of drift at these points…

  • Brighton, England – Declination = 1° 36′ W changing by 0° 8′ E/year
  • Miami, Florida – Declination = 5° 57′ W changing by 0° 5′ W/year
  • Sydney, Australia – Declination = 12° 32′ E changing by 0° 0′ W/year …no drift!

Mag2….finally if you just want to fiddle around on your computer, and dont happen to know your exact location, then go to the excellent (if a bit boring) Getty Thesaurus of Geographic Names Online – type in your location and you will get a reading like the one below for Brighton, England

Brighton (inhabited place)
Lat: 50 50 00 N degrees minutes Lat: 50.8333 decimal degrees
Long: 000 10 00 W degrees minutes Long: -0.1667 decimal degrees

Conveniently this gives the Long/Lat in both degrees and decimal.

Then go to the equally excellent, for this purpose, National Geophysical Datacentre web site and type in your co-ordinates, and press “Compute Declination”. Thats it!

You get an accurate calculation of magnetic declination and also the amount by which it will drift each year (variation). Great!

Experiment with locations around the world, its just fascinating on a cold, windy and wet day…alternatively if you are reading this relaxing in the sunshine, with a cold beer – well, I will decline to say what I think of you:-)


3 comments to What’s your Magnetic Variation – the compass not you !

  • Dave Broughall

    I owned a charter fishing boat for twenty years,based folkestone,and never found a need to to learn about magnetic variation,mainly because of the use of decca then gps,however,now I have taken to the air and speeds are a lot different,so this article was very usefull,thank you.

  • Jerry H

    OK article – thanks; but I don’t think it’s correct to say that Greenwich Meridian has anything to do with this topic.
    The important thing is the angular difference between the direction of the magnetic pole from your position, as indicated by the compass, and the local line of meridian of longitude – which you cannot measure if you don’t know where you are and dont have a map – and which is why you need to know the variation for the local region/country.

  • You confuse Variation and DECLINATION..

    Magnetic Variation is the difference in angle on the surface of the earth between magnetic north and true north.
    I.E. the angle subtended by the two lines drawn between your location and the north magnetic pole and your location and the geographic magnetic pole.

    Magnetic Declination is the angle subtended by the magnetic flux line entering the earth, and the earth, at your location. At the equator this is almost zero as the flux lines are almost parallel to the earth’s surface. At the magnetic poles it is tending towards ninety degrees.

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