Now that satellite navigation has come down to earth, the magnetic compass seems like a child's toy. But it's a lot more than that, and it's still a valuable tool for backcountry travelers who know how to read the message of its north-pointing needle. This week, Tamia explores one of its secrets.
March 14, 2017
A magnetic compass is a simple instrument. Or so it appears. A needle or card, a graduated housing, maybe a lanyard ring… And that's that. It doesn't beep or chirp, it boasts no colorful map display, and it won't tell you how far it is to your lunch stop. But twist and turn the compass as much as you like, and the needle (or card) continues to point toward the north. Magic? No. Magnetism. And before some physics Ph.D. takes me to task for playing fast and loose with the truth, I should add that the compass needle doesn't really "point" north. Its orientation is determined by the north-south lines of force established by the earth's local magnetic field. Still, the result is the same. The needle…er…points north.
What's that? You're not impressed? You say your GPS can do this, too, plus show you exactly where you are on the map? Right—though unless your GPS also incorporates a fluxgate (electronic) compass, it will lose track of north just as soon as you stop moving. Nonetheless, by comparison with the all-seeing, all-knowing GPS, the magnetic compass is a one-trick pony.
But what a trick! This simple, trembling needle—a Chinese invention, by the way—gave medieval Europe the key that eventually unlocked all the rooms in Gaia's great house. That's no small achievement. And the compass still has a place in paddlers' packs—or better yet, on their decks and in their hands. A compass is self-powered and self-contained. It doesn't depend on satellite coverage or batteries, and it's not subject to sudden, inexplicable crashes. Every electronic device I've owned has failed me sooner or later, almost always without warning. No compass has ever let me down.
As simple and straightforward as a compass appears, however, it holds a dark secret. Its north is not the cartographer's "true" north. Its needle doesn't point the way to the soon-to-be open waters lapping around the North Pole. And therein lies a story: the story of…
THE OTHER NORTH POLE
True north is one end—the north end, obviously—of the axis around which the globe spins. In other words, it's the north geographic pole. But the compass needle is impelled by invisible lines of force to seek the north magnetic pole, the northern point at which those lines of force converge. In a perfect world, the north geographic pole and the north magnetic pole would be one in the same. But ours is not a perfect world.
Having two "north poles" is bad enough in itself. But there's worse to come. While the geographic pole can be taken as fixed, at least until an asteroid collides with earth and knocks our home planet on its beam ends, the magnetic pole is an incurably restless soul, always wandering about in search of pastures new—and traveling some 35 miles a year in the process. (It's now headed toward Russia. Make of this what you will.) It even reverses polarity from time to time. The north magnetic pole then becomes the south magnetic pole. Luckily, this doesn't happen often. Except for a short-lived reversal about the time the Neanderthals were dying out, a north-south flip-flop last occurred some 780,000 years ago. Given the long wait, you might be forgiven for thinking the next flip could happen any day now, but reversals are apparently random events with no fixed period. Santa needn't get change of address cards printed up just yet.
A further aside to that (imaginary) Ph.D. physicist is probably in order at this point: Yes, it's true that the geographic poles also wander. The earth's axis of rotation oscillates, in a more or less predictable fashion—one component of this movement is labeled the Chandler wobble—and now, thanks in no small measure to global warming, the north geographic pole is also drifting eastward. But the movement only amounts to a few centimeters a year. The rate may accelerate as the ice sheets melt and the oceans rise, however. Stay tuned.
Enough of this. Let's look at…
ONE WAY THE COMPASS CAN LEAD PADDLERS ASTRAY
The compass needle knows nothing of the globe's axis of rotation. It responds only to the earth's magnetic field. But cartographers are slaves to convention and convenience. So they draw their maps and charts on grids laid out in relation to true (geographic) north. This doesn't matter if your paddling destination lies along the agonic line, the ever-shifting locus of points where true north and magnetic north coincide, and where north by compass is also north on the map. But most of us would chafe at having to toe this line in perpetuity. Which means we have to adjust our compass bearings to bring them into conformity with the map's grid. And vice versa. This difference—the difference between compass bearing and map bearing, between magnetic north and true north—is known as declination (on quads) or variation (on charts).
Fortunately, declination and variation aren't closely guarded secrets. You'll find a declination diagram on every topographic map, and the compass rose that's a fixture on all nautical charts allows you to read off the variation directly. Life is good.
Here are examples of declination diagrams that I've reproduced from two older topographic maps:
Magnetic north is indicated by the letters MN, true north by a star or (occasionally) the letters TN. (The star represents Polaris, the pole star, which lies almost directly above the earth's north geographic pole.) As the labels suggest, declination can be either easterly or westerly, depending on whether magnetic north lies to the east or west of true at the mapped location. In the examples above, the left-hand image (A), reproduced from a Washington State quad, illustrates easterly (+) declination; the right-hand image (B), from a New York quad, westerly (-) declination.
But what about the arrow labeled GN? This indicates grid north. It reflects the fact that a flat map can't be superimposed on a globe without a little cutting and fitting. The upshot? The map grid doesn't coincide perfectly with the converging lines of longitude, and grid north doesn't agree exactly with true north. This is something that most paddlers can safely ignore. The difference between grid north and true north is usually so small that only surveyors and missile-aimers need take it into account. (The practical limit of accuracy for a hand-held compass is around two degrees, and a kayaker in a choppy seaway will be doing well to hold a course to within one point, or a little more than 11 degrees. A difference of a degree or two won't matter much in either case.)
OK. Now that we've brought our kayaker on stage, we'll move from quads to charts. The declination diagram on a quad is a simple schematic, but the compass rose on a typical nautical chart is a much more elaborate affair, and bearings can be read directly from it. On the other hand, you can't even assume that the angles shown on a quad's declination diagram are accurately drawn. For instance, the angle labeled 13½° in the right-hand diagram (B) reproduced above might actually measure 13½ degrees. But then again, it might not. Caveat navigator.
With these preliminaries behind us, let's put to sea. In the illustrations below, I've reproduced two compass roses from nautical charts. The outer scales give true bearings; the inner scales, magnetic bearings. And both roses combine beauty with function—though I feel duty bound to add that, in practice, a chart's compass rose is of little use to any navigator who doesn't have access to a yacht's chart table, not to mention a parallel rule. No matter. The compass rose stills make the meaning and measure of variation (we're all at sea now, remember?) clear at a glance, something that the quad's schematic declination diagram fails to do. The first rose (C), reproduced from a chart of southern Californian waters, illustrates easterly variation; the second (D), from an Atlantic chart showing the waters off Tampa, Florida, depicts westerly variation.
In each case, the offset between the inner and outer scales reflects the variation, and to drive the point home, the figure (in degrees and minutes of angle) is printed just above the center of the rose. But hold hard, there! Why does the printed variation include a date? (If you look back at the declination diagrams from the two quads, you'll find that they, too, are dated.) Because variation changes over time, that's why. Remember what I said about the wandering magnetic pole? As it moves, it takes the earth's magnetic field with it. The annual changes are small—barring a reversal, that is—but over the years they add up. So if you're using an old quad (not a bad idea sometimes; the new digital quads often conceal as much as they reveal) or an old chart (never a good idea: the sea is a fluid medium, the foreshore flats are remodeled every time there's a storm, and aids to navigation are occasionally shifted), you'll have to bring the variation (declination) up to date. You'll find annual corrections printed on charts—but not on most quads—and you can also get correction tables from a number of sources.
Does this sound too much like bookwork? Then determine the current correction for your home waters on your own. To do the job properly, you'll need a pocket transit or other highly accurate compass, but the process is pretty straightforward, at least in the midlatitudes of the northern hemisphere. (There's no readily visible counterpart to the pole star south of the equator.) And it's a valuable exercise in its own right, as well as a useful reminder that the compass is still a state-of-the-art tool for anyone who likes to venture off the beaten track—as it has been for a thousand years or more. Sort of like canoes and kayaks, come to think of it.
Questions? Comments? Got something to add? Just e-mail Tamia.
Copyright © 2017 by Verloren Hoop. The moral rights of the author have been asserted.