NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Mercator Sailing
From: Bill Murdoch
Date: 1998 Jun 08, 6:22 PM
From: Bill Murdoch
Date: 1998 Jun 08, 6:22 PM
The best source I have found for information on calculating sailings is the Admiralty Manual of Navigation Volume 1 by HMSO. It does a better job than Bowditch in explaining the methods. It contains formulas for great circle sailing on a sphere and on a spheroid and for mercator sailing on a sphere and a spheroid. Bowditch does its great circle sailing on a sphere and its mercator on a spheroid. After reading the Admiralty Manual of Navigation, I have laughed few times over articles (and a USPS JN example) which compared a distance sailed in the tropics calculated by great circle sailing on a sphere to the same distance calculated by mercator sailing on a sphereoid and pronouncing the spherical sailing which is smaller to be greatly better. In reality the earth is fatter at the equator, a minute of arc there is more than a nautical mile, and the spheroidal mercator method recognizes it. The geat circle sailing on a sphere assumes the earth is a sphere and that a minute of arc is always a nautical mile. The mercator sailing gives a larger result primarily because it is sailed on a different surface not because the rumb line course is longer. If both were calculated on the same surface, the results would be much closer. Of course, it makes little practical difference, but I find it interesting. Bill Murdoch =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= =-= TO UNSUBSCRIBE, send this message to majordomo@ronin.com: =-= =-= navigation =-= =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-=