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Hi, Douglas -

It's interesting to see this topic being discussed, but I had the idea
the 1.2' correction for irradiation was dropped from the Nautical
Almanac circa 1970?

I just checked my my 2010 NA and it's not mentioned in the explanatory
material at the back. Augmentation is, but not irradiation.

Hewitt

On 2/27/10, Douglas Denny  wrote:
>
>
> Colleagues,
>
> 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.
>
> Douglas Denny.
> Chichester. England.
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