Magnetic North Pole

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DaveWalsh
Posts: 109
Joined: Sat Oct 26, 2013 9:37 am

Magnetic North Pole

Post by DaveWalsh » Wed Feb 06, 2019 9:20 pm


davidglasgow
Posts: 28
Joined: Sat Oct 26, 2013 9:37 am

Re: Magnetic North Pole

Post by davidglasgow » Mon Feb 11, 2019 9:36 pm

While rather alarming, this is for me, also reassuring, how can my navigation be other than crap when magnetic north keeps rushing all about the place...

john.ruston
Posts: 37
Joined: Thu Aug 28, 2014 11:51 pm

Re: Magnetic North Pole

Post by john.ruston » Wed Feb 13, 2019 10:25 am

Changes in the field are normal.
Sudden and rapid change is noteworthy.
I'm not sure they make much difference to the practical user of map and compass.
I checked my home location with NOAA's NCEI service on December 4th and the calculator returned a value of 3.12°W (WMM)
I checked again this morning and got a WMM value of 2.99°W (shift 0.22°E.p.a.) and the enhanced model gave 3.04°W (0.21°E.p.a.).
That's quite a change in 2 months but we can work OK with it.
The calculators seem to be working just fine.
Before all this satellite technology we had our local landmarks to check and set a compass. I think these shifts might matter more to someone on a weeks long passage but then there would be several other methods to hand.

DaveWalsh
Posts: 109
Joined: Sat Oct 26, 2013 9:37 am

Re: Magnetic North Pole

Post by DaveWalsh » Mon Feb 18, 2019 10:41 am

When I were a lad, by gum, magnetic variation was almost 10 degrees. There was a warning it was decreasing by so much every year and that sort of thing but not enough for anyone to anything other than ignore. Not more than three years ago I gave a navigation class to a Symposium and I gave the variation as 10 degrees. variation is a confusing thing, easily got back to front so I always advised that 90 degrees east on the ground was 100 degrees on the compass.
3 degrees !
Who'd have thought.
My theory is that kayakers (leave satnav and all that out of this) cam manage to steer a course made good, even crossing and allowing for a modest current, to within 5 degrees either way (the old fashioned seat of the pants way, gymbal compass at the very most). That is why Inisheer from Doolin is such a great teaching example. 5 degrees means hitting a target of size 1/12th the distance away, with confidence.
It used to be 90 goes 100, but what is the rule now, I ask? Just round up to the nearest 5 degrees? 66 goes 70, but so does 69?
You'd need to be Knotman to answer the next conundrum - namely, given that its position is on the "other" side of the planet, somewhere off towards Siberia, will the red needle in my Silva stick when it tries to revolve because it is rubbing off the undercarriage of the compass - same way as there are southern hemisphere Silvas to compensate for the same thing ?
Life used to be a lot simpler.
DW

martinguilfoyle
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Joined: Sat Oct 26, 2013 9:37 am

Re: Magnetic North Pole

Post by martinguilfoyle » Mon Feb 18, 2019 7:45 pm

My head hoits.

john.ruston
Posts: 37
Joined: Thu Aug 28, 2014 11:51 pm

Re: Magnetic North Pole

Post by john.ruston » Tue Feb 19, 2019 9:46 pm

Will it scrape the bottom of the case? I suppose It depends if you've a high Latitudes model which can accommodate extreme Dip. How to explain. The compass needle doesn't point at magnetic north. It follows lines of local flux. Luckily for us this generally averages out somewhere predictably near magnetic north. Thing is, the flux also has a vertical component which will tip the needle downwards. This is the angle of magnetic Inclination or Dip.
The effect is generally more marked in high Latitudes (there are spots where Dip is near vertical) so manufacturers balance needles to suit the zone of use.
Some really posh compasses like Frances-Barker offer instruments balanced for global use.
Dip is usually predicted on the calculators.
Hope this saves a headache.

john.ruston
Posts: 37
Joined: Thu Aug 28, 2014 11:51 pm

Re: Magnetic North Pole

Post by john.ruston » Tue Feb 19, 2019 9:56 pm

North Antrim Coast current Dip is given as 69.19°. That's a tad over 55° North.

DaveWalsh
Posts: 109
Joined: Sat Oct 26, 2013 9:37 am

Re: Magnetic North Pole

Post by DaveWalsh » Wed Feb 20, 2019 10:43 am

John
Is that good or bad? Is your Silva acting more like a water divining rod than a compass?
DW

john.ruston
Posts: 37
Joined: Thu Aug 28, 2014 11:51 pm

Re: Magnetic North Pole

Post by john.ruston » Wed Feb 20, 2019 11:03 pm

David sir, neither good nor bad. The needles and the cards are balanced to avoid grounding. Go too far north and they won't work any more. Field is vertical. I guess you'd use an electronic compass or old school astrocompass. Mostly we don't go to these places.
The value for Dip I gave for Fairhead is from a calculator running the September 2018 revision. Two different calculators gave minutely different values.
Divination - - Compasses so often have water nearby, you have a good idea - perhaps we can put the Hazel rods aside.

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