NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Reliable Index Correction to a Tenth Minute of Arc
From: Hewitt Schlereth
Date: 2010 Feb 27, 10:14 -0400
From: Hewitt Schlereth
Date: 2010 Feb 27, 10:14 -0400
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 Dennywrote: > > > 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. > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ----------------------------------------------------------------