A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Douglas Denny
Date: 2010 Feb 27, 04:44 -0800
I note there is considerable discussion on this forum about the necessity of obtaining as 'good' an estimation of index error as possible in a sextant - and quite rightly so of course - in the interests of obtaining practical accuracy for what is, or rather was, a working instrument in daily use for thousands of people throughout the world in their everyday business of navigating ships around the world. Now it is more of an academic issue with interested parties such as ourselves.
I have made the point before, however, there is a danger of becoming besotted with the fine technicalities and detailed physics of instumentation and astronomy at the expense of _practical_ navigation, as it would have been actually used by sailors. They were only interested in results and simple practical methods. We might be looking too minutely at the wood with a microscope and loosing sight of the forest.
I am not suggesting there is anything wrong in our modern dissection and analysis of techniques and instruments to the nth degree, as it is a fascinating business and interests us a lot (I know because it has been a passion of mine for a lifetime), but I warn of loosing sight of the _practical_ side of our passion, and a more overly view of the subject at the same time.
To discuss measuring down to a tenth of a minute with a sextant seems to me to be attempting to 'Guild the Lilly' as there are so many factors which preclude making this a practical, though much desired, observation goal to attain. One such factor I mention below:-
Bill Morris has the ability with sensitive autocollimators for observing angular deviation of optical elements down to a tenth of a second of arc; and I note others have discussed endlessly the ability to take sights to a tenth of a minute of arc; ....but there is an aspect of practical navigation which has not been discussed here before as far as I know which negates a lot of this, and that is a natural phenomenon of the eye itself which is the spreading of a bright image laterally across the retina called 'irradiation'.
It has the effect of displacing the apparent edge between a bright area and a darker area towards the darker.
There is little published information about this phenomenon, and little practical means of dealing with it, but the effect can be in the order of fifteen minutes of arc for the bright sun's disc, which appears larger than it really is; and in the order of three to ten minutes of arc between a point source and horizon.
Standard practice in terrestrial navigation incorporates only one correction for irradiation of 1.2 minutes of arc in the table for the Sun's upper limb in The Nautical almanac.
The only paper published on the subject that I know of is a scientific appraisal made by Richard F. Haines and William H. Allen of NASA at the Ames laboratory, Moffett Field, California in 1968/9.
See Journal of the Institute of Navigation Volume 15. No 4. Winter 1968/9.
I presume this investigation was towards better accuracy for space navigation as the astronauts used sextant measurements to determine accurate positions when in orbit, or transit to and from the Moon. Although radio telemetry and radar methods could do this too, I believe the use of manual sextant observations was considered important and was certainly used by astronauts as far as I know.
Haines and Allen used a criterion of minimal angle of resolution (MAR) for various sources including: two point sources; a point and extended Circular source; Point source and simulated horizon.
There is a comprehensive reference listing too at the end.
I would have scanned the copy I have to make it available here as it is a very important research document for navigation, but unfortunately my dinosaur computer has gone Awol with the scanner, and won't scan anything. So I cannot. Sorry. If I get it working again I shall do so unless someone else can look up the article and post it here.
Around the time I graduated forty years ago in Cardiff, S.Wales, I knew Captain Cotter very well and he encouraged me to look into this phenomenon, as he was fully aware of the scarcity of information on irradiation in practical navigation. Accordingly, with the blessing of the staff, I made some experiments in the optical laboratory of the university.
I set-up a metal plate at the end the longest lab I could find with accurately cut holes of different diameters, and used a bright lamp source behind with a diffuser. Measuring light intensity at the source plate with an accurate photometer, I then measured fifty settings each of the apparent discs just touching (on both sides) with a standard marine sextant at gradually increasing light levels.
The idea was to plot a graph of irradiation effect of increased MAR between the two extended light sources Vs. light intensity of source.
To my eternal shame and intense irritation I lost the results many years ago when moving from Cardiff. I could still kick myself heartily when I think of it.
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