"Romano, Judith" wrote:
>
> We all know that "True North" (Polaris) is not the same as "Magnetic North"
> when orienting a sundial. My grandfather's Boy Scout pocket "Sunwatch" is a
> true gem for our family -- loaded with information for any enthusiastic
> sundial traveler (complete with a compass, dial, and table of corrections
> for the major US cities). One of the corrections included was for "Degree
> Variance" (East or West) of magnetic north in which to point the dial to
> "True North" for an accurate reading. For example, Boston, MA is 14 degrees
> West; Seattle, WA is 23 degrees East; and ironically, Cincinnati, OH has no
> variance and is the same as magnetic north.
>
> Is anyone in the mailing list aware of the mathematical formula used to
> compute this variance for True North?
>
> Judith Romano
> mailto:[EMAIL PROTECTED]
Judith,
Yes, there are geomagnetic models for computing magnetic declination, a
good resource is the National Geophysical Data Center:
http://www.ngdc.noaa.gov/seg/potfld/magmodel.shtml
There is online software available to compute local magnetic
declination. Also, You can get the software, GEOMAG, for free (source
code included. I actually debugged some of the code when the IGRF model
data for 2000 came out. The format of the data set had changed and an
array was undersized resulting in some bogus values.
ftp://www.ngdc.noaa.gov/Solid_Earth/Mainfld_Mag/Models/unixsoftware/
Lastly, I use this software (GEOMAG) in my online Solar Calculator to
report the local magnetic declination, a useful point of info for
solar/sundial data, feel free to use it.
http://www.gcstudio.com/suncalc.html
Regards,
Luke Coletti
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