Tuesday, January 14, 2020
True Virgins Make Dull Company, Add Whiskey
For the navigation nerds out there like me, the ever-changing location of the magnetic North Pole is a matter of fascination, and everyday use. I can well remember that when I learned to fly, the long runway in Birmingham, Alabama was Runway 5/23. Adding a zero to the numbers told you that whenever you landed from the west your heading was something close to 50 degrees and from the east, your heading was close to 230 degrees. Years later, the runways were renumbered to 6/24 because the magnetic variation at Birmingham had changed a few degrees, so you were landing at a heading closer to 60 degrees or 240 degrees.
Those of us who have learned to navigate, by plane or by boat, have always had to deal with adjusting compass headings to make them correct. Admittedly, the advent of LORAN and then GPS navigation have decreased the importance of the magnetic compass, by letting us tell the autopilot to take us to a certain latitude and longitude, but the compass is still front and center in airplanes and boats to determine the course we are following. Nowadays, you can purchase a GPS compass that automatically does all of this for you.
Deviation is simply the error in one’s own compass caused by its construction or nearby magnetic fields in the cockpit or pilothouse. In a boat or airplane, a compass is “swung” by mysterious experts who produce a “compass card” of how to adjust your course at various headings to be accurate. So, for example, the card might say that if you want to head due east, or 90 degrees, on your boat or plane, you might need to add two degrees and hold a compass heading of 92 degrees to actually be on a heading of 90 degrees. This part of the calculation is unique to your own vessel, and is really quite simple to understand.
Magnetic variation is an entirely different matter, and is caused by the fact that the earth’s magnetic North Pole is not exactly located at the earth’s geographic North Pole. This adjustment has always varied over time, and in most places around the world magnetic heading is a matter of just a few degrees difference from true heading. But in areas near the North or South Pole, the variation can be enormous. Suppose you are flying between the geographic North Pole and the magnetic North Pole. To get to the geographic north pole you would need to fly in the exact opposite of what your compass says is due north. In more normal locations, to determine what course to fly on your compass, you add together the effects of deviation and variation. So if the two numbers are plus three and minus two, a boat or plane wanting to go on a 90 degree true course would require flying a compass course of 91 (90+3-2). The way I was taught to remember this calculation is the phrase "True virgins make dull company, add whiskey". What it stands for is the calculation to get from true heading to magnetic heading on your own compass is the formula True + Variation = Magnetic + Deviation = Compass Add Westerly (you add westerly variation and subtract easterly variation).
What is interesting though is that the exact location of the magnetic
North Pole is moving more quickly lately, and seems to be totally unpredictable. It has recently been moving east toward Russia at a speed of around 30 miles per year. Its location is updated officially every five years but an interim change had to be issued during the last five-year period to account for errors in the forecast. It has just been officially updated for 2020. "Magnetic north has spent the last 350 years wandering around the same part of Canada," Ciaran Beggan, a scientist from the British Geological Survey (BGS), told Business Insider. "But since the 1980s, the rate it was moving jumped from 10 kilometers [6.2 miles] per year to 50 kilometers [31 miles]." See here. This all has to do with the molten core of the earth that consists of liquid iron sloshing about beneath our feet. Per Wikipedia, "Earth's outer core is a fluid layer about 2,400 km (1,500 mi) thick and composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. Its outer boundary lies 2,890 km (1,800 mi) beneath Earth's surface. The transition between the inner core and outer core is located approximately 5,150 km (3,200 mi) beneath the Earth's surface. Unlike the inner (or solid) core, the outer core is liquid. It is also constructed of iron."
I recommend an article from the New York Times that you can access here. It explains the history of movement of the magnetic North Pole over hundreds of years. Check it out. Meanwhile, these variations make little difference at our latitude. If I want to go north I generally follow the compass north. A few degrees here or there are more accurate than I can steer a boat in the ocean anyway. But we need to keep in mind that geologic evidence shows times in earth's history when the earth's magnetic field was totally reversed, so that following a modern day compass north would actually take you south. If that happens again, we will need to start paying attention.
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