Re: [APML]: Great RA Alignment tool

[Wil Milan <wmilan@airdigital.com>]

mark.kaye@sympatico.ca wrote:

> >Magnetic variation is the offset of magnetic north from true north due to
> >one's location on the Earth. The variation lines are plotted on many maps
> >(such as aviation and marine charts). Magnetic variation is strictly a
> >geometric effect due to the magnetic pole being some distance from the
> >geographic pole.
>
>    Our topographical maps call this Mean Declination.  Funny how this word
> pops up in several places in our field.  Where I live, the mean Declination
> is about 10 degrees 35 minutes west of true north.  What I would like to know
> is why Grid North is different from True North, being east by 52 minutes.

I'm not sure, but I can thing of at least two reasons why the grid lines may be
off of true north:

- Incorrect surveying when the lines were originally laid out, sometimes
centuries ago.

- Precession of the Earth's axis over that same period.

Or maybe someone else has an explanation which is more than theories. :-)

Wil M.
--
"When I consider your heavens, the work of your fingers, the moon and the stars
which You have set in place, what is Man that you are
mindful of him, or the son of Man that you care for him?"  -- Psalm 8