VARIATION

[Guest]

IS THERE A SOURCE FOR CURRENT DATA ON MAGNETIC VARIATION? MY CHART IS 1991 showing 7 DEG w DECREASINg 10MIN ANNUALLY. Can I assume that in 2001 it is 7deg minus 1deg 50min = 5deg 10min?

 

[Guest]

A chart of current world variation, which can be interpolated to the nearest ¼ degree at most places, can be seen at;

http://geomag.usgs.gov/MagCharts/wmm-gif/WMM-00Is.gif

 

[Guest]

Apologies, this is the correct URL:

http://geomag.usgs.gov/MagCharts/wmm-gif/WMM-00D.gif

 

[Guest]

Had a quick look but these are "declination" charts?

Declination isn't the same as variation (is it) or have I got it all wrong?

Cheers, CRB

 

[Guest]

Same thing ... I think ...

See for example http://www.bartleby.com/61/93/M0029300.html

But I have some vague recollection there is a technical difference. Would appreciate any expert explanation.

 

[Guest]

Right CRB, it seems we yachties should abolish the term \'variation\' ...

... and come into line with the rest of the world by calling it by its "proper" name - declination.

RYA please note: one for the Yachtmaster syllabus.

 

[Guest]

Same thing ... I think ...

See for example http://www.bartleby.com/61/93/M0029300.html

But I have some vague recollection there is a technical difference. Would appreciate any expert explanation.

 

[Guest]

Variation and Declination are the same thing, except when they are not?

From “another” forum:-

"Declination, magnetic - The angular direction, east or west from the north branch of the celestial meridian, of magnetic north as determined by the positive pole of a freely suspended magnetic needle that is not subject to any transient, artificial disturbance. In nautical and aeronautical navigation the term variation is used instead of declination, and the angle is called variation of compass or magnetic variation. Except for usage in navigation, magnetic declination is not synonymous with magnetic variation, which refers to regular or irregular change with time of magnetic declination, dip, or intensity."

Sourced from - http://lists.contesting.com/_towertalk/199811/0507.html

I think the last sentence is the relevant bit - Eric Hiscock talks about “declination”, where the compass magnet end is actually pulled downwards (tilted) by the earth’s magnetic pole. Being reversed for North or South of the equator, there are he found, balance weights fitted to overcome this. He thus needed a new compass card with the opposite balance weight fitted, once he crossed the equator.

Your URL is probably right for answering Jim’s question, and I suspect this is all a bit academic for the average “little ship” compasses fitted to our boats anyway, but it suggests these terms are not interchangeable?

We need a real expert here methinks, any takers please?

Cheers, CRB

 

[Guest]

Thanks Jim ... can you educate me further ...

I thought that the zenith of the ecliptic is exactly due 'true' south (as seen from the northern hemisphere), in which case aren't the two identical?

Or are we into the finer details of precession and nutation. In which case, what is the maximum difference?

 

[Guest]

Re: Right CRB, it seems we yachties should abolish the term \'variation\' ...

NO! Stick with MAGNETIC VARIATION - 'the angular difference EAST or WEST between 'TRUE' & 'MAGNETIC' North and DECLINATION - 'the angular distance NORTH or SOUTH of a celestial body at its ZENITH from the ECLIPTIC.

 

[Guest]

Re: Thanks Jim ... can you educate me further ...

The point I was trying to make - without success it seems - was that the term 'declination' is more usually used and understood in the context of astro navigation and that 'variation' is commonly and most usefully used in relation to the magneitc compass.

 

[Guest]

Thanks for the informsation. Unfortunately the quoted web site was never available to me. But I did find a download at http://www.comanco.com/Magvar/magvar html This is a downloadable applet where you input youir Lat/Lon and it gives the Mag Var to 0.5. Can also show a great circle bearing between two places

 

[Guest]

Thanks Jim, you may have noticed your site used the NDGC Geomag program, which was also used to generate the map in the URL that I posted.

This was an interesting discussion and I finally realised why there is confusion about the word 'variation'. Yachtsmen mean the horizontal difference between north and magnetic north, but in geophysics it refers to the rate of change in declination.