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Robin Stuart you wrote: David,
          I'm not sure what level of accuracy you are aiming for but your calculated meridian transit time doesn't appear to take account of the Sun's changing declination, currently 0.5' per hour.

Robin

Thankyou for reminding me of that.  I suppose I was spoiled by my Jupiter observations on 22^nd.  Jupiter’s declination is only given to the nearest minute in the Air Almanac, and it was only changing at 0.1’/hr in any case.  The Sun’s declination, as you point out, was changing five times as fast at noon on 23^rd.  Please tell me if this logic is correct.  In two hours, the Sun’s declination would have reduced by 1’, so I should have waited until the Sun’s altitude was approximately 1’ less than that measured on my observation two hours earlier.  This would have given me a longer time between observations, so my time after Mer Pass at Greenwich would have been longer, so I was actually a touch west of my calculated longitude. 

I could have corrected for this at the time by setting the sextant index to 1’ less than the altitude measured two hours earlier.  Alternatively, I can bodge a solution by using Norie’s ‘Change of Altitude in One Minute of Time’ Table (another part of the good book I never knew existed until this week).  15 degrees off Az=360/180 and lat 53 gives a change of altitude of 2.3’/minute.  Therefore, I’ of reduced altitude is gained after 60/2.3 = 26 seconds, so my time between observations would have been increased by 26 seconds.  Therefore, my time of Mer Pass after Greenwich would have been increased by 13 seconds.  13/4 = 3.3’ of longitude.  My revised longitude is now 000.29.5W + 000.03.3 = 000.32.8W compared to my GNSS longitude of 000.32.3W.  That seems too good to be true.  Where’s my mistake? DaveP