Re: Finding True North

From: Chris L Peterson (clp_at_alumni.caltech.edu)
Date: 09/22/04 

Date: Wed, 22 Sep 2004 23:05:01 GMT

On Wed, 22 Sep 2004 15:58:37 -0400, John Beaderstadt <beadnot@pivot.nyet> wrote:

>Which brings us back to my point that longitude has nothing to do with
>magnetic north.
>
>>If you use a compass correctly, it will take you directly north.
>
>Only if you constantly adjust for local variations, which inherently
>means that you have to know what those local variations are.

Exactly. And you find out what the local variation is by plugging your latitude
and longitude into a mathematical model that describes the shape of the Earth's
magnetic field.

In my book, "magnetic north" is essentially synonymous with magnetic
declination", and that is how the term is normally used. If you are on the
ground with a compass, magnetic north is the direction your compass is pointing.
You stick your geographic coordinates into the formula, and out pops the
declination- the amount you need to adjust the bearing by to get true north. In
practical terms, your longitude is a crucial to determining your magnetic
declination, and therefore to using a compass to identify true north.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

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