Monday, March 7, 2011

Magnetic North?


Two weeks ago, a little birdie sent this article our way. It circulated around the upstairs office at APS, and we thought something about it just didn’t jive with our understanding of magnetic north and how it affects sailors. Luckily, my fiancé might be considered an expert when it comes to such things – and I clicked it over to him for an explanation.

Here’s what Ocean Engineer, Sailor, and Guest Writer Karl McLetchie has to say in response to all the hype about pilots and boaters adjusting to shifts in magnetic north. Pardon while we get a little science-heavy, and read on for clarification…


Magnetic north is not the point at the top of the earth that determines compass headings. It’s the local component of the earth's magnetic field, combined with local anomalies (think - the island in Lost), to which a magnetic compass aligns.

True north is the direction to the north geographical pole. Basically, it’s the standard "up" reference used on most paper charts and the electronic charts in your GPS navigation system. The difference between local magnetic north and true north is known as declination or variation. Variation fluctuates in a complex pattern around the world, but it is generally low, 0-10 degrees near the equator and increasing to over 90 degrees at the north and south magnetic poles.

The magnetic poles are slowly moving. As they move, the local variation all over the earth changes. Mariners and aviators must keep track of this changing variation both in time and in location so they can match true bearings and courses from their charts with magnetic bearings and courses from their compasses.

You can calculate your local magnetic declination/variation, according to NOAA, here. If you enter coordinates or zip code (21401) for Annapolis, you can see our fine sailing town has a variation of 11° 9' W, changing by 0° 0' W per year.

Luckily, for sailors reading this blog, you don't have to refer to that website every time you leave the dock. You can look at the center of the compass rose on any nautical chart and read the local variation and how it changes per year. If you zoom in on a compass rose here, you can see again the variation near Annapolis is 11 deg W and has no annual change. So with updated charts and simple addition and subtraction, the shifting magnetic poles shouldn't cause any problems (or any great expense) for sailors.

Thanks for clearing that up for us, Karl. Upon doing a little more research, we found the best compass companies out there already have you covered. For example, if you have a Tacktick compass, you don’t have to worry. The compass can automatically get magnetic variation from your onboard electronics system, or if that's not available, you can manually input the variation.

We’ll just point out you should steer clear of the Bermuda Triangle. Though, we know how difficult that can be with the race circuit off the East Coast ;) …

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