hello mike, I'm a new sundial's fan and i don't really understand what you write: " We all know that Magnetic Declination marked on a compass is an excellent guide to its date of manufacture." That's mean, sundials are not dated but on these, we only read the Magnetic Declination (MD), longitude and latitude ?!? And with the MD, you can find its date of manufacture. Thank you for more informations.
Best regards, laurent Münich, 48°10' N, 11°34' E [EMAIL PROTECTED] schrieb: > > Dear Expert Friends, > 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. > > Any help from any source will be welcomed. > Regards, > Mike Cowham. > <[EMAIL PROTECTED]> > Cambridge, UK.
