Forty years ago as a Boy Scout, my hometown (Aberdeen WA) was 22 degrees different between true north and magnetic north. If you did not take this into account, you got lost. When I moved to Mississippi ten years ago, I tried to teach the difference between true north and magnetic north only to discover that they matched precisely in Batesville MS. There was no point in those youth knowing there was a difference, unless they moved. Magnetic north in Aberdeen was then 18 degrees from true north. I was relating this as a "those kids will all own GPSes before they ever need to know this" tale when I discovered that today Magnetic north equals true north is now west of Brinkley AR and moving west fast. Magnetic declination in Aberdeen WA is 16 degrees today. I feel old. I had never planned on North moving in my lifetime. Why post this in the environmental fora? All to often I see 'change' associated with 'blame' or more accurately with 'avoiding blame'. No one suspects the change in magnetic north in being man made. Feel free to embrace environmental change without blame. puts more emphasis on field strength, something I have not measured.
A private pilot, it was always important to have a current chart because the earth's magnetic field does change. Today, we have services to download charts . . . into our GPS. Magnetic declination is not 'cast in stone.' <grins> Bob Wilson
Nautical charts have had (for at least 50 years) both a magnetic deviation, and an annual variation, for every compass rose on them. "Feel free to embrace environmental change without blame." Unless, of course, you are doing something that might cause environmental change...
We have local variation in this neck of the woods. That adds a little more complication to using a magnetic compass. Tom
Sectional air charts show areas with 'local magnetic deviations' of sufficient magnitude to get pilots lost. GPS is heralded as the cure to all these woes, no doubt why Glonass and Galileo and Beidou birds have already been launched to supplement. Or to prevent data interruptions. Or prove national technology. Or whaetver. But these days every one with w/sufficiently deep pockets seems to want their own constellation. India cancelled theirs, yes?
The way I understand it the magnetic poles are always in motion due to the molten iron core of the Earth. IIRC, the Discovery channel did a story on this not long ago. DBCassidy
In a related area: When I was designing GPS receivers, one of the error sources that had to be handled was the variations in the earth's gravitational field. It turns out that the satellite's orbits are distorted by the variation of mass concentrations under the surface. I never found out if the molten core would affect those parameters over time (like it affects the magnetic field).
Geosynchronous satellites tend to oscillate over areas of lower gravity in a process called libration. It's counter intuitive if you don't have a good grasp of orbital mechanics - at first blush one would expect the libration points to be over areas of high gravity, not low. Tom
