Re: 1507 text ref of Inventio Fortunata

From: Philip Deitiker (Donevenask_at_worlnet.att.net)
Date: 08/16/04 

Date: Mon, 16 Aug 2004 02:45:02 GMT

Martyn Harrison <nospam@spammers.of.the.world.unite> says in
news:6ujvh09rciv7ue6e75cvj570j4dgitdcv1@4ax.com:

> Apparently on date Sun, 15 Aug 2004 13:45:29 GMT, Philip
> Deitiker <Donevenask@worlnet.att.net> said:
>
>>BTW compasses are not very useful in the far north
>>atlantic, unless you are traveling from boston to the north
>>pole, because the deflection of the pole is so close to
>>greenland. If you approach greenland from the east when you
>>are traveling on the 78 parallel heading true west your
>>magnet point north.
>
> I don't understand this. You can plot "new lands" from a
> nautical point of view by mapping lat and long relative to
> the magnetic north pole, without realising this is
> displaced relative to the true north pole.

how do you establish your magnetic latitude (how big the
circumference of the earth is at that latitude line) without
knowing the position of the north pole?
 
> IOW, if you know you have to sail at 275 degrees magnetic
> to get to where you wanted to go, it doesn't matter if this
> is actually 277 degrees relative to true north, you should
> get there anyway. Course, when you think you are sailing
> true west, it'll be something like a degree or two out but
> then the port you are sailing to is also (in that case)
> fifty or a hundred miles further north then you think that
> it is.

Actually you can't, the reason is that magnetic poles of the
earth are not exactly bipolar with respect to each other, and
the magnetic lines are not exactly circumradial either. The
magnetic poles is generated from the southern hemisphere and it
runs a skew in the north. This is one source of distortion. The
other source of distortion is that you concept of sphericity is
based on the latitude lines and the magnetic counterpart does
not. In addition the north pole is moving at up to 40 km per
year since 1300 and thus the map you make in one century will
confound the explorer in the next. This is not a problem in
india, in canada all hell breaks loose.
 
  See other post for details. Magnetic variation in 1300 was
probably severe in the north, not so bad in sweden, but as one
approached labrador it would be horrendous, the magnetic pole
would be due true west. The current magnetic variation at the
tip of greenland is 30. In houston it is 4 degress, but only
because we are due south of the pole, as one moves east or west
and particularly northeast or northwest variation changes
rapidly. This change will create distortions. It is close to
zero somewhere in Louisiana. There could be very good reasons
the magnet was used in the eastern mediterranean and not the
western regions until later. The best place to map using
compasses is the indian ocean and indonesia. The worst place to
map is in eastern canada. One possible reason is that sailors
who relied on magnets instead of stellar observations would run
off course and smack into something or some enemy they did not
plan on smacking into.

> These errors being corrected once someone realises there is
> a variation of magnetic to true north and goes to the
> trouble of determining true latitudes by astronomy to show
> up the magnetic variation,

Distortion.
1. Imagine that the magnetic north pole is around iceland. Now
imagine you are traveling NW to iceland. But your compass will
read due north as you travel in the direction of greenland you
will be heading due south, and as you head down greenlands coast
heading southeast, and up the Labrador sea southwest finally as
you cross around greenland and start heading to england you
would be heading north.
2. Imagine however that you are marking your map off in days at
knots traveling in a given magnetic direction, when you travel
at the equator, you imagine yourself traveling on a cylinder
because the distortion is not to bad. As you pass objects to
your south and north you note them as being perpindicular to
your craft. Now imagine that you are marking off equidistant
periodic landmarks on boths sides. Each landmarks is equal
distance.
  Next you are traveling around iceland again you are marking
off landmarks that are equidistance from your boat but on the
edge of the horizon and each landmark being a pair which form a
perpendicular line that the boat cross. from the perspective of
the boatman, if he does not know where the pole is but he is
conserving course, the map he makes shows him traveling around a
column up a circum columnar ladder that interconnects itself,
the reality is that he is makine a circle along a radial.
3. Your next argument is that he could correct for this
deviation if he knew his magnetic latitude. How do determine
rotational latitude, using a sextet or astrolabe. How do you
determine magnetic latitude, using a calibration of the dip. Was
it possible for them to know the dip with any precision in 1400,
they did not even know where the north pole is. The magnetic dip
is not single parametric equation, feild lines propogate out
from the magnetic core of the earth in the shape of an apple the
dip is where these lines cross the earths surface for any
distance to a pole. Let us say that you assumed that in england
you assumed that your magnetic latitude = your true latitude.
Thus you were to base measurement distances and latitude line
circumferance on this measure. Now also assume that the north
pole was in canada, your magnetic latitude line its actual
latitude equivelent in the azores, thus you would be assigning
the things a much bigger landmass to the north of you than to
the south.
4. Therefore the only way to know your magnetic latitude is to
go to the north pole, measure its position in true coordinates,
>From this you could then calculate based on the size of the
earth your actual latitude to arrive at your magnetic latitude.
The best calculator would be a globe and a peice of string with
a nail holding the string from the magetic north pole.
5. When you travel mag north you can then take up courses west
and east by compass measuring of the exact distance traveling
north and east or west, doing this you establish your new
latitude and then you use your calculations for that latitude as
well as corrections for distortion you can create a map.
6. But the magnetic pole is not strait, and it is moving.
Therefore before you get to far into mapping you would need to
set an absolute point of reference for each object. For example
for greenland you would go south to the tip, take a latitude,
then you would need to go directly back to port while measuring
speed along the latitude line.

  Another way to think about the problem is as such, when
christopher columbus plotted his course to the new world he
traveled south, not very wise considering it increased
distances, but kind of smart considering that it is easier to
deal with the curvature of the earth. At the equator he would
always be traveling strait along the XY axis but would be
dipping into the Z axis, this direction is of no concern to him
because it is controlled by the sea. So it removes that
variable. For a pilot traveling from Quito ecuador to Nairobi
Kenya the fastest route is along the equator. For a pilot
traveling from Miami to Ankara the route will take him over
iceland. The more one approaches the north pole the more a path
between to positions on the same latitude will deviate north to
find the shortest route, along the trek ones geograhic and
magneti bearings will be constantly changing. It will be a
mapmakers nightmare. In fact the best way to travel in the north
is just how the vikings originally did, from landmark to
landmark and follow landmarks and create new rule for each
stretch of the journey. With respect to latitude ones maps will
reflect a quality of a cylinder at that equator and the quality
of a disk at the pole. Between the equator and the pole
regarding latitude ones map will reflect the quality of a
cylinder and a disk at the same time.

> whereupon everyone can simply
> adjust their compass readings according to place, with a
> chart that tells them by how much.

I understand that in Thule now they ask the airman to check
magnetic variation before flying into the region, I read a
'NOTAM' for thule that now requires the use of radio navaids set
prior to approach as they have tired searching down lost and
wrecked pilots. When you get close to magnetic north your
compass becomes like your ex-wife at your bachelor party. This
could be a major reason that transpolar flights were not real
popular until GPS. A major reasoning for wanting to do so is
that the more recent jets like the 777 can consistently travel
at higher altitudes, but not kill the passengers (lol) take
advantage of that ozone hole and the very cold air at the poles
scraping out a few more miles per lb of fuel = less fuel = less
$s. Hey and in 20 years we may know the true affects of cosmic
radiation on Flight attendants and Pilots. In any case GPS is a
must now, with a pole on the move and disappearing, in 100 years
you could be flying north from Rio and end up at the south pole. 

-- 
Philip
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