NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Variation and Amplitude
From: Glendon
Date: 2007 Jan 08, 21:48 -0800
From: Glendon
Date: 2007 Jan 08, 21:48 -0800
George Huxtable wrote: > Guy Schwarz quoted Tom Cunliffe as saying- - > > "On to Variation. How was it discovered? My guess is that navigators > would take a bearing on the setting/rising sun and their almanacs > had the amplitude tables and determined something was off? Or am I off > base > > Portuguese navigators, setting off deep into the Atlantic around the > time of Columbus, did their navigation from Polaris, and thought that > the compass needle was magically drawn toward Polaris. So when it > started to diverge as they went West, it was a source of great worry > and confusion. > > George My imperfect knowledge of history is that the Portuguese were the first to recognize variation/declination sometime in the middle of the 1400's, and that they began to make charts of variation according to their measurements and their then knowledge of the globe. In accordance with the times, they attempted to hold their information as "commercial in confidence" and not a lot is known about how they did things. (Can anybody correct me on this?) The modern GeoMag visualization of the earth's magnetic field goes back to 1590 and indicates that variation in the Mediterranean and the Bay of Biscay at the time was near enough zero. This suggests to me that as the Portuguese began to move further afield in the late 1400's that they encountered recognizable variation. As George suggests, they may have had Polaris as their reference point. However the first charts of variation made were for the west and southern coasts of Africa where Polaris is not visible. Clearly they had some method for determining north, perhaps from the sun? George mentions the Portuguese venturing deep into the Atlantic, whereas history mostly records their exploration around Africa to India(Goa), and then Melaka. I have read conjecture that the Portuguese/Basque were fishing the cod banks off North America before the recorded voyages of John Cabot and others from the mid 1490's on. Knowledge of variation would have been vital to dead reckoning navigators of the day, as the GeoMag visualization shows variation of 20 degrees at the banks at that time. If anyone has any reliable references to these times and events, I would be most interested. Lee Martin --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---