Mike Cowham wrote: > I have yet another question that I am sure that one of you will > be able to answer, or at least put me on the track to finding the answer. > > We all know that Magnetic Declination marked on a compass is an > excellent guide to its date of manufacture. I have the figures for London, > Paris, Rome and New York for the last 400+ years. > > However, I have been studying a compass, actually a portable > dial, that was made for 22.5 degrees SOUTH! I measured this from its > gnomon angle, so there is some possibility of error, perhaps 1 degree. > This latitude passes through Australia, South Africa and South America > plus a few islands like Mauritius. I can virtually rule out the first two > as there are no major towns at this latitude. (Perhaps I should consider > Walfish Bay in S Africa, but the community there would have been really > small). However, Rio de Janeiro (or possible Sao Paulo) sit almost on my > line. It is an English made dial, and I know that Rio had a large British > population. The dial date is between 1800 and 1840, and the magnetic > declination indicated seems to be around zero. > > Therefore I am seeking for records of declination at around this > period to confirm that Rio is the intended location for the dial. If not, > where else had zero declination in 1820? > > One possibility would be to look for old shipping charts, but I > am hoping that someone will have a computer program that will calculate > such things for any point on the Earth for any date? This will be a most > important tool for anyone interested in Portable Dials or Compasses.
Dear Mike Cowham, According to a computer program that I have recently written and which draws world maps of the magnetic declination for any year between 1600 and 2000, the two 'agonic' lines (i.e. the lines of 0 degrees magnetic declination) in 1820 intersected the parallel of 22.5 degrees south very close to the eastern coast of South America and near to the north-western coast of Australia. So your guess that the sundial was made for Rio de Janeiro (or possibly Sao Paulo) would hereby seem to be confirmed although Australia cannot be completely ruled out. The 1820 magnetic declination world map can be downloaded in postscript format (c. 430 kb) from: http://www.fys.ruu.nl/~vgent/magdec/mgdc1820.ps My programs are based on those distributed by the National Geophysical Data Center and uses the models of the geomagnetical field prepared by the British Geological Survey for the period 1600 to 1910 and the International Geomagnetical Reference Field models for the period 1910 to 2000. A very useful introduction to these programs and models (and geomagnetism in general) can be found in: W.H. Campbell, Introduction to Geomagnetic Fields (Cambridge University Press, Cambridge, 1997). Best wishes, ================================================================ * Robert H. van Gent * Tel/Fax: 00-31-30-2720269 * * Zaagmolenkade 50 * E-mail: [EMAIL PROTECTED] * * 3515 AE Utrecht * Home page (under construction): * * The Netherlands * http://www.fys.ruu.nl/~vgent/ * ================================================================
