NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bowditch long term almanac
From: Geoffrey Kolbe
Date: 2012 May 20, 07:22 +0100
From: Geoffrey Kolbe
Date: 2012 May 20, 07:22 +0100
Hello Hewitt >I'd have to dig my copies out of storage, but my recollection is >that Bowditch differed from you mainly around the equinoxes. My >recollection is that the largest Bowditch correction was 0.75' in 4 >years for Declination. Whereas yours are about half that. Which >means Bowditch's max deviation would be 3.75' in 40 years. Actually, from a quick scan, I have Dec quadrennial corrections as high as 0.74' and the 1972 Bowditch LTA goes up to 0.71'. >In the context of small-boat navigation, I'd say the minuscule 1972 >Bowditch table would still be useful. Hmm. The problem is that the Bowditch LTA corrections are reasonably good in places but quite wrong in other places. So one can be deceived into thinking the Bowditch is useful and reasonably accurate for any date, which is not the case. It might actually be better (safer) just to use the GHA and Dec for the four base years without any quadrennial correction than use the Bowditch corrections and think you are getting the claimed accuracy. The perception seems to be that a LTA of this sort cannot be accurate, and errors of a few minutes here and there are quite acceptable. My point is that it is possible to create a LTA with really quite good accuracy. The expected error of GHAs and Decs calculated using my LTA are around 0.25' across fifty years, which is only a little over twice that claimed for the NA for one year! Geoffrey Kolbe