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    Re: Reliable Index Correction to a Tenth Minute of Arc
    From: Douglas Denny
    Date: 2010 Feb 27, 07:14 -0800

    Quote: George Huxtable:-

    "Douglas Denny, speaking of irradiation, tells us- "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." Well, with or without a working scanner, he could have told us a bit more about those alarmist figures and the conditions under which they were obtained. There's no doubt of irradiation being a real effect, though it might strike different observers to differing extents, but I regard those numbers, taken at face-vale, as quite absurd. They would negate the whole practice of astro-navigation. Is "the bright sun's disc" is to be taken, perhaps, as the unshaded Sun? If so, it has no relevance to any instrument observations, except perhaps, to users of early versions of the cross-staff. Douglas will have to tell us more before we begin to believe what he is saying.
    George. contact George Huxtable, at george---me.uk "

    =============

    I could already hear George's snorts of indignation down his nostrils when I wrote my comments about irradiation.

    Yes George it is an important effect ignored _totally_ in manual navigation. The curious question I would ask is why? - when it can have such a marked effect which could indeed mitigate against or even negate practical accuracy?

    Is it perhaps that at the time of the Ames laboratory experiments (1969), manual navigation was already doomed as a practical means of obtaining positions on the globe by sextant with the extensive use of Loran, Omega and the promise of satellite navigation? and thus no further work was called for after the Ames experiments?

    I suspect too that for centuries the _practical_ use of manual navigation was quite accurate enough in use that there was no _need_ to look further into accuracy improvement by corrections applied to something which is a constant but unknown small error, which, perhaps was being put down to simple variability in obtaining sights for other more well-known reasons.

    There does not appear to be any published scientific work on irradiation and manual navigation before 1960, though there are many references to papers around the time of the Ames experiments in the 1960s with visual work and accuracy of the eye: a lot of it associated with the aerospace developments of the time.
    Looking through the references list, there are only one or two references of the 1940s and 1950s on resolution of the eye and photochemical effects.

    Having stirred-up the hornets nest with George frumping around in bad mood I had better get on with trying to make my scanner work again or get the paper scanned elsewhere and then sent.... Asap! before he explodes. :-)


    Regards,

    Douglas Denny.
    Chichester. England.

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