Cecil’s column is from 1989, when National Geographic used Robinson projection for maps of the world. Wiki says NG switched in 1998 to Winkel Tripel projection.
“Declination” is an angle referenced to the celestial equator; a star’s “latitude.” What Cecil meant was “deviation”, which is the the difference between what you read on a magnetic compass, and the actual direction of Magnetic North. It’s caused by metal in the vicinity of the compass, including that of a steel ship, and changes with your course-- the Earth’s magnetic field induces magnetism in metal, and varying the angle of the metal across the lines of magnetism result in varying strengths.
“Variation” is indeed the difference in angle between True North (the North Pole) and Magnetic North. Because the Magnetic North Pole is some distance from the North Pole, the angle changes depending on where you happen to be. Because the MNP itself moves over time, the Variation will also change over time. A nautical chart will show, in various locations, what the local Variation is in a particular year, and the annual rate of change, and its direction (East or West). Just to make things more complicated, the lines of magnetic force between MNP and MSP are not straight and even, and so that is also accounted for when printed on the chart.
So, Variation is the same for everyone in a particular location, and Deviation is specific to a vessel, and even a distinct compass within that vessel.
The thumb rules for applying corrections:
“Can Dead Men Vote Twice At Elections” =
Compass (the actual reading on a magnetic compass)
Deviation (correction for the particular compass based on course)
Magnetic (magnetic course, which is the same for all in the vicinity)
Variation (correction to convert the direction from magnetic north to true north)
True (direction based on true north; the same direction you’d get from a gyrocompass)
Add East (going from Compass to True, corrections labeled East are added)
So, on a Compass heading of 065 degrees, look up the Deviation for that compass at a course of 065 (there’s a handy chart for each compass). It reads, 1.5E. Add east, and find your heading is 066.5 degrees Magnetic. You can pass that to your buddy in the next boat, and he can apply his own Deviation with a reversed sign so you will both be going the same direction. Your other buddy has a gyrocompass, so he applies the local variation of 2.5W (after correcting for any annual drift since the chart was printed) and steers 064 degrees True.
The saying for the other direction is: “True Virgins Make Dull Companions At Weddings”
True
Variation
Magnetic
Deviation
Compass
Add West
At last, a topic on which I am actually knowledgable! www.qmss.com
Is there a question in this thread, or a point being made?
Well, I was just updating Cecil’s column by noting what NG now uses. You’re free to post something else in the thread.
For years American schoolchildren have grown up looking at those large Mercator maps in the classroom wondering why Australia, seemingly only a third the size of Greenland in front of them was a continent, and humongous Greenland a mere island. Trying to place a globe on a flat piece of paper basically stretches the North Pole, zero miles wide and stretches it 24,000 nautical miles wide. Some feat, eh? Not only making Denmark’s sparsely populated colony look huge, but also much larger at top, which it really isn’t so much. If you were Danish though, it looked pretty good.
I was a navigator flying KC-135’s in the old Strategic Air Command (SAC) days before GPS and Inertial Navigation. I’ve flown up to the Pole and back a number of times either taking or meeting returning reconnaissance aircraft from missions closer to the Soviet Union. Flying and navigating with a standard magnetic compass in jet aircraft where lines of longitude are converging sharply to the poles means the chart heading is constantly changing. At the same time magnetic variation is often changing at ranges around fifteen degrees per one hundred miles and sometimes in the opposite direction of the convergence angle.
So we would fly on a Lambert Conformal Chart that had what was called a Polar Grid Overlay. The Polar Grid Overlay was a Mercator Overlay that covered the entire chart and was based on 60 nautical mile squares. The Grid was in green and we labeled the grid with a false latitude from between 0 to 10 degrees and used a longitude near our own and set the compass in directional gyro mode using a false heading. Why, because with Mercator Charts when you draw a line from point A to Point B you get what is called a Rhumb Line. Along that line you have one and only one heading while on a Great Circle Route which you obtain on a globe or a Lambert Conformal, the heading constantly changes but you have the shortest distance.
Thus by laying a Mercator Grid on top of a Lambert Conformal we were able to slew the compass to a false heading and fly that heading. This Grid heading is checked and verified by the Navigator with an aircraft sextant via the sun, moon, planet or star and verified throughout the flight correcting and rating the compass. I still trust the sun and stars more than anything else, yeah old habits die hard, but you can’t jam the sun.
Conquering the Poles, even for commercial navigation would not have been possible without “Grid Navigation.” When a Korean Air 707 was shot down in the middle of the Soviet Union and landed on a frozen lake it happened because the crew failed to do their Grid Entry checklist properly. They were playing cards when the Soviet aircraft intercepted them. If they just noticed that the sun was on the wrong side of the airplane after entering grid they would have been OK. They also forgot the prime rule of Grid Entry. “You turn the compass, not the aircraft.”
Great to hear about polar nav-- I was in submarines, and so of course that featured large in our training, though I never got to actually do an under-ice run. Handy access to the sun and stars being limited, subs play games with the gyros. Not flying around at hundreds of knots also gives a little time to nail down a solution.
In my later years, we had inertial navigation systems with gyros slaved not to the pole, but to a point in space. Conventional gyrocompasses use the rotation of the Earth to automatically align themselves, but in the polar latitudes that won’t work. Nowadays, there are ring laser gyros, and of course GPS has taken the art of navigation and reduced it to paint-by-numbers; but we old salts labor on.
Of course the Mercator projection isn’t necessarily the most distorted world map.
Sorry, but “magnetic declination” as Cecil uses it is perfectly ordinary terminology, though some prefer “variation”. You’ll find both listed in the OED.
At first, given the context of this column’s original question, that this was a reference to Chicago.
Powers &8^]