This is in response to Cathy Spude with the NPS in Colorado, trying to
"calculate the declination of Polaris from true north in Colorado in 1864"
 
Cathy
        There's no reason to attempt to find the change in declination of Polaris
as Polaris doesn't seem to have changed its position relative to the earth
(and vica-versa) since the really olden days.  I'm far from an expert on
this, but I'll pass on what I know.  Somwhere there is a fantastic table,
published by NOAA, or some other Gov. entity, that gives changes in
declination through time (from about 1600 to present) for every few minutes
of Longitude and Latitude and most major communities across the country -
unfortunately I just moved to Richmond VA and can't find a thing!  Sorry.
Maybe someone else can find the reference.  It is out there and any good
historical geographer shouldn't be without it (I used it for analysis of
burial alignments as a factor of magnetic north at the time the cemetery
was opened).
        There are highly detailed tables and maps documenting changes in the
magnetic fields and since these changes are continuing they are updated
every 5 years.
        For more info I suggest you contact the National geophysical Data Center,
NOAA Code E/GC1, 325 Broadway, Boulder, CO, 80303, tel. 303-497-6521, or
e-mail at [log in to unmask]
        BUT --- before you go on, note that rather than a change in Polaris, it is
clear that you mean the change in magnetic declination through time -
secular variation of the direction in which the compass would point in a
particular time period.  Because the earth's magnetic field is appartently
generated by something deep within the earth's interior, which is
presumably liquid or otherwise viscous, there is continuous but slight
change in the orientation of the magnetic declination.
        Fortunately for the early explorers (and boyscouts) the present magentic
and geographical poles are relatively close to each other (and have been
for some time).  This has not always been the case.  There is considerable
evidence from deep sea cores that the magnetic poles have changed
considerably and dramatically over the last few millions of years.  Not
only do the 'North" and "south" ends of the earth's magnetic field wander
around (5-10 minutes in a year on average).  Not only has the magntic poles
wnadered but I understand that they also switch to the opposite every
10,000 years or so.
        Unfortunately there is more to the story than the global scale of the
magnetic field.  Several factors effect the patterns of the earth's
magnetic field.  Since iron is one of the most common minerals of the
earth's crust, there are regional, and often very local, variations which
can send that old hand-held compass off several dozen degrees.  Serious
hikers and geographers know this and there are special maps showing these
anomalous places and the degree of interference.  Other factors such as
solar flares and the moon are suspected to have minute (and in you case
uneventful) effect on the magnetic declination of a particular place.
        As for Colorado in 1864, I think it isn't just a matter of figuring out
where you are in relationship to where the magnetic north pole was at the
time, but you have to figure that you are in one of the continents most
massive geological formations where you may encounter incredible variation
from peak to valley.
        All I can say is good luck.
 
Dan W. (unemployed historian and archaeologist, Richmond, VA - formerly of
upstate NY)