# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Portuguese shipwreck question
From: George Huxtable
Date: 2009 Nov 2, 09:08 -0000

```Thanks to John Huth for providing details of the paper his information on
ancient lats and longs came from, as follows-

"It's actually a journal article written in Isis.   Isis does have an online
archive, so you can dig it up free of charge, but I believe you have to
register to get it:

John K. Wright, �Notes on the knowledge of latitudes and longitudes in the
middle ages,�  Isis, Vol. 5, No. 1 (1923) pp. 75-98"

I've contacted the website of the journal, Isis, at
http://www.journals.uchicago.edu/loi/isis, which lists a lot of
back-numbers, but volume 5 is missing from that list. It also seems, as far
as I can tell, that where such back numbers are available, on-line access
isn't free, except to members of certain subscribing institutions. Not to
the ordinary-Joe, such as I am.

The full run of Isis is available at the Bodleian Library in Oxford, so next
visit, I'll take a look, and maybe a copy.

================

Mercator mapping didn't exist at the time of the Toledan tables, but if John
wishes to display the positions on a Mercator map background (which seems
sensible), he should first convert the latitudes to a Mercator scale of
meridional parts, and plot those numbers against degrees of longitude. This
can be done, to quite sufficient accuracy (it ignores the Earth's
ellipticity) by applying an Excel function such as-
=131.9*LOG10(TAN(RADIANS(45+lat/2)))

This would provide an x-y plot on top of which to fit a suitably-scaled
Mercator map of that part of the mediaeval world.

On the other hand, if John considers that modern mapping provides a suitable
background on which to overlay these locations, he could plot the points
onto a map such as that available from OMC (online map creation), at-
http://www.aquarius.ifm-geomar.de/
This will provide a modern map of any area he wishes, with a choice of
projections (though Mercator may well be most appropriate). It will provide
coastal outlines and other features, with a grid, as requested, in a
vector-graphics format. On that it will plot his labelled points, defined by
longitude and latitude, which can be pasted-in from a spreadsheet.
Longitudes would, of course, have to be adjusted from their zero in the
Canaries to be based on Greenwich instead.

That program has some awkwardnesses, but I have some experience in getting
round these, and may be able to help.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Apache Runner"
To:
Sent: Sunday, November 01, 2009 10:18 PM
Subject: [NavList 10375] Re: Portuguese shipwreck question

It's actually a journal article written in Isis.   Isis does have an online
archive, so you can dig it up free of charge, but I believe you have to
register to get it:

John K. Wright, �Notes on the knowledge of latitudes and longitudes in the
middle ages,�  Isis, Vol. 5, No. 1 (1923) pp. 75-98

I just copied his table out of that article.   The scatter plots were my
first crude attempts to see if I could figure out what these places
corresponded to.   I didn't get as far as actually putting onto a Mercator
projection, however.   If you know of some software were one can input these
points onto a Mercator projection, that would be nice, although I can
probably cobble something together using Mathematica.

Attached is an attempt to draw a map on a rectilinear grid (not a Mercator)
and my guesses as to some of the place names - modernized.

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