# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Time of meridian passage accuracy**

**From:**Douglas Denny

**Date:**2009 Sep 26, 11:39 -0700

George. No need for retractions I think. You know full well the reference to change of Dec of 0.8 minutes per hour mentioned as being 24 was a simple error - and corrected within minutes of my posting it and re-reading it. Mentioning that _is_ pedantry. Nor is there any implied 'disparagement' in the words: "There is a vast difference between practical navigation and theoretical astronomy. That is the point which you have academically and pedantically missed." You are too sensitive. (I believe I was accused of that once). --------- There is indeed a huge difference between the two - practical and theoretical navigation - you are focussing however, on the finer academic aspects yet the question was about a practical point of one minute in time leeway at Mer Passage observation. Practical sea navigators have been calculating exactly when Mer Passage of the Sun is for centuries, and taking their sight at that time - or rather within one minute - and is quite accurate enough for practical use at sea. In post 9948. You continue to miss the practical aspects of the point. The question was about a one minute time limit on Mer Passage observation. I take it this means taking an observation within one minute of the _calculated Mer Passage_ for the Sun, not that is necessarly observing the Maximum altitude (which you seem to think I am implying, probably because I gave a series of observations showing that) - and in which case the observation is well within reasonable limits of accuracy for practical use. If stationary - as I was with my observations (and which you ignored) - then culmination of the apparent (true) Sun is by definition Local Apparent Time of the (true) Sun. The Equation of Time and correction for longitude gives this time with respect to UTC exactly enough alright. In my graph example given, I calculated the former accurately and already know the latter accurately. In a moving observer this is different. Then it is agreed culmination, or 'dip' of the Sun, max altitude whatever you wish to call it, and true meridian passage are not necessarily the same... "subtly different" is how you put it, and it is indeed of greater importance in a North / South moving observer. It remains nevertheless that the original request was for opinon as to the accuracy of the Bowditch declaration of taking a sight within one minute of Mer passage, and I have no hesitation in continuing to suggest taking a sight within one minute of time of calculated Mer Passage of the Sun is quite satisfactory in practical navigation. --------- You have also ignored my observations and graph; the conclusions regarding that graph with respect to the question posed, (especially that it takes nearly 20 minutes of time to change within a minute of altitude at culmination); the fact it was made as a stationary observer; and made with exact calculation of when exact Mer Passage of the Apparent Sun actually is at my location - (which is why it is symetrical about the calculated time). Curves: For a stationary observer, theoretically very slight distortion would be present as declination is changing in the 30 minutes or so of the observation. Negligable though. Max altitude can still give accurate Latitude with culmination, which is what I was doing. The change in Declination as a parameter in Meridian Passage is of more significance if the observer is moving North/South, but is not dramatic - a few minutes of arc. The graph of the curve will also then be more significantly distorted in symetry. A stationary observer or one moving East or West will have practically no distortion of symetry of the curve, but an observer moving East/West will have error in time of apparent culmination - a shift of the curve. This is all treated with rigor in Charles H. Cotter's book 'The Complete Nautical Astronomer' with full formulae for calculation of these effects and full derivations. Douglas Denny. Chichester. England. --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---