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Polaris North vs. Magnetic North


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It occurs to me that I could save time when setting up if I used a compass to align the telescope with Polaris North.  Rather than setting it up on Polaris which I sometimes can't see.  Given that I am about 0"42' East and 51" 41'N how many degrees will I need to set a magnetic compass (marked in standard 360" not in military radians) to in order to account for the difference please, i.e. So many degrees east or west of magnetic North.  I've tried asking Google, but the answers weren't clear and there is bound to be someone on SGL who will know the answer easily. 

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For observing I would have thought just the compass would be good enough. Imaging would be a different story but this post is in observing so I guess that is what you want to do.

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Some types of field compass allow you to adjust for Magnetic Declination, which is the difference between true north and magnetic north for your location, and is important in critical navigation such as mountaineering when you could be navigating in low visibility conditions.  

The difference between True North, Magnetic North and Grid North is shown at the corners of printed Ordnance Survey Maps (1:50,000 and 1:25,000 scales) along with a year so that you can work out how much declination has changed since the map was printed. 

You can also use the British Geological Survey website to calculate it based on a postcode. (http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html)

This estimates that for my work postcode that "Magnetic north is estimated to be 1 deg 23 min west of grid north."

I occasionally use this method for polar aligning when I can't see Polaris clearly, and its good enough for visual, but you do get some drift.  Probably as good as roughly positioning Polaris in the polarscope hole.

Ade 

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Thanks all.  It seemed easiest to click on Andy's links as I know my OS for my area is a few years old.  Magnetic-declination.com says that the setting should currently be -0"14'

If I understand things correctly that is scarcely nothing is it?  1" would be just a small movement 1/360th of the circle, but 14 minutes is hardly worth bothering with for observing purposes or for setting up the Goto from a starting position facing North.  So the position of Polaris is roughly North Pole North - yes?  The direction that, if there was no adjustment, would be the top on an OS map. 

I've never really stopped to think about it too much - I've never walked in conditions I can't see in and always assumed that I'd work it out based on adjusting each reading if I ever needed to - rather than adjusting a compass.  I know the adjustment is necessary, but its always seemed easier to work it out as I go rather than confuse matters further by changing the compass itself.  However, I'd hung a compass onto the telescope and thought that I may as well adjust that one as it will always be in the same place, but if its only 14 minutes I don't think its worth bothering with is it?  I am now also confused by the fact that its a minus figure (I am in the East).  So if I could twist the compass by that small amount does the minus mean that I'd twist the dial clockwise?  That's what I think the second website link says, but the more I read the more I am confused. :help:

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The difference is about 1 degree, if I recall magnetic N is about 1 degree West of true North (Left). Use the Magnetic Dec sile given by Andy

What I guess will be the problem is getting your scope that 1 degree different, it is just too small to do it accurately by eye, I have tried on something else and asically failed miserably. Simply sticking Polaris at the centre of the polar scope is about the same. Think Polaris is just over 1 degree off of true north.

Edited by ronin
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17 minutes ago, ronin said:

Think Polaris is just over 1 degree off of true north.

Exactly, so when using Polaris you are less accurate than using your compass and I know from experience that using Polaris gives excellent GOTO results.

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13 minutes ago, Astro Imp said:

Exactly, so when using Polaris you are less accurate than using your compass and I know from experience that using Polaris gives excellent GOTO results.

Then I do understand it, because that's the conclusion I had come to when I read that.  I'll use the compass the next time and benefit from the added speed of setup.  I've found that I can place the edge of the compass against the edge of the square stand so when it points North I shall know I'm OK.  The horizontal bit is easy as the mount has a scale on it and a pointer on the moveable bit so when it points zero I know its starting off flat.

Thanks all

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The degree of difference between celestial north and magnetic obviously depends on your location. You can't just say it's one degree. If you live in Iceland, close to magnetic north, even a very small movement across the island radically changes the difference between the two. It also changes over time. The position of magnetic north behaves as if, over time, the magnetic pole were describing a circle. The further you are from it the less it matters.

Olly

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1 hour ago, ronin said:

Think Polaris is just over 1 degree off of true north.

Polaris is due north twice a day* ! Just gotta choose your moment :D

It describes a circle, path, round the pole about 1.5deg wide, being as how it has a declination of about 89deg16min (ie 3/4deg from the pole, nerenuf)

* for example when it culminates

 

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There are charts available. Including some free ones on the web such as the 2 below. What I've been looking for is an extremely accurate listing for every city (or larger town) on the planet. Anyone know where such may be available from?

I use a Brunton Military model compass. Very tough and accurate. I've been using Brunton-gear for decades - it's never let me down from tasks simple - find accurate directions for marking areas - to tasks difficult - like mapping newly discovered limestone caves here in New England for publication. Very good equipment.

Dave

 

declination-map.jpg.16f0ccb28dd40e7043d7f475a615e5d8.jpg

 

 

magnetic-fields1.thumb.jpg.274c676385228d6f342b4155cd3956ab.jpg

 

 

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55 minutes ago, Dave In Vermont said:

There are charts available. Including some free ones on the web such as the 2 below. What I've been looking for is an extremely accurate listing for every city (or larger town) on the planet. Anyone know where such may be available from?

Nope sorry,

but dont worry about the odd degree or two, the whole thing my soon flip north to south leaving us briefly without a magnetosphere to protect from the solar wind and interstellar suff such as cosmic radiation,  consolation = Aussie and NZ end up on top :)

https://www.nasa.gov/topics/earth/features/2012-poleReversal.html and others more gloomy

and you thought running out of oil was a big concern LOL !

 

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4 hours ago, SilverAstro said:

Nope sorry,

but dont worry about the odd degree or two, the whole thing my soon flip north to south leaving us briefly without a magnetosphere to protect from the solar wind and interstellar suff such as cosmic radiation,  consolation = Aussie and NZ end up on top :)

https://www.nasa.gov/topics/earth/features/2012-poleReversal.html and others more gloomy

and you thought running out of oil was a big concern LOL !

 

We digress, but another polarity reversal (Earth has had many) does seem to me to be under-worried about!

Olly

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While you're dusting off your THE END IS NEAR! signs, my reasons for wanting such specific readings was simply helping with my project. There is one large block-area near me with a very weird magnetic-declination flux: Around 19° W. instead of the 14° W. or so it should be. My guess is an iron meteorite in the ground around here. It would also account for the inordinate number of lightning-strikes in the area.

I'd like to precisely triangulate it and maybe even dig down to it. Or it may be ET's Flying-Saucer.....

THE SAUCER IS NEAR!

Have fun!

Dave

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The last polar flip happened 800000 years ago. If homo erectus survived it, can't see it harming us, sorry ?. More of inconvenience than Armageddon, if you ask me. 

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2 hours ago, ollypenrice said:

We digress, / under-worried about!

Indeed we do, so I shall try to keep this brief :)

love 'under-worrying', thanks for that, memorable.

22 minutes ago, Erla said:

The last polar flip happened 800000 years ago. If homo erectus survived it, can't see it harming us, sorry ?. More of inconvenience than Armageddon, if you ask me. 

Not entirely happy about the validity of your statistics :happy7::icon_scratch: but we can leave that to another place, just want to say " but how many survived ? " are we not all descended from just a small band of intrepid individuals and one woman ?

back to worrying about where true north is and awaiting the result of  Dave's ( @Dave In Vermont ) big dig (that is an interesting diversion)  >>> :)

 

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1 hour ago, SilverAstro said:

" but how many survived ? "

According to NASA, magnetic flips were numerous and do not correlate to mass extinctions or, for that matter, do not show any substantial impact on life from the fossil records. But I'm far from expert, on almost everything :D. Who said ignorance is a blessing?

Edited by Erla
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I admit to being flippant with the flip and JOC's compass problem, also somewhat amused to have something else to worry about other than oil running out (or maybe not), global warming doom (or,  um lets not go there), ice-age in the offing (or maybe not - see global warming), self destruction by nuclear madness (choose betwix bombs or power stations) and last (?) but by no means least, contrails blighting our astronomy ( not to mention mitigation for a loss of polar ice.)

I will try to be non-flippant and go back to under-worrying and maybe say no more :)

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If I get my compass anywhere near my Skyliner 250PX OTA or actuator boxes, the needle moves by more than 1 degree. I have found a position where the stray fields seem to cancel, and have added a small melamine-coated chipboard shelf to take a hiking compass. I use a bubble level and wedge under one of the feet to get the mount level, so the Alt scale reads true.

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Uh, well, since I'm trying to align to the Stars, I tend to use the stars to do my alignment.

I do use the GPS gizmo on my Smarter-than-me-phone to at least get my North leg forward on my mount.

Then Bazingo on Polaris with my guide scope and telescope. (Because so far, I've always set up where I could find Polaris. And use the Big Dipper to aid me.)

As for Polar shifts, really, what does anybody expect to do about it except to hold on for the ride? Or die trying.

 

But then... I am just visiting this planet.

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