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    Classic Trig Log Sight Reduction Revisited
    From: Greg Rudzinski
    Date: 2014 May 27, 09:34 -0700

    Francis Chichester had to resort to classic sight reduction on his 1966/1967 circumnavigation because his modern tables were sadly left behind. This inspired me to reduce a few observations using the classic trig log method to see what Chichester had to cope with. Three tables are needed (Log of Numbers, Natural Trig Functions, Log of Trig Functions). It took 8 minutes to reduce the calculated altitude Hc and 4 minutes to reduce Z for azimuth (not counting Nautical Almanac look-up time or plotting). The good news is that few mistakes were made after several dozen trials. This may be the result of good table legibility (1981 Bowditch vol.2). Sight reduction time was improved by 3 minutes by using a slide rule to calculate Z for azimuth. This works well because an exact azimuth is not critical due to the short intercepts working from a DR assumed position. I have now grown to liking this old method :)

    Find the needed tables 1,2,&3 in Bowditch 2002 here for anyone wishing to give the old way a try:


    Greg Rudzinski

    P.S. Lots of Trig Log tables available on ebay and amazon in both hardcover and paperback.
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