NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Portuguese shipwreck question
From: George Huxtable
Date: 2009 Nov 2, 09:08 -0000
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. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com -~----------~----~----~----~------~----~------~--~---