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    Re: Back sights.
    From: George Huxtable
    Date: 2010 Mar 21, 22:25 -0000

    Frank Reed commented on my statement-
    "The original purpose of the backsight, in which the horizon was viewed in 
    opposite direction (and was never, as far as I know, used with a telescope) 
    was to allow a lunar, greater than 90º, to be measured with an octant."
    I've never heard that before, and considering the great difficulty in 
    getting an index correction for the back sight, which you described very 
    clearly, it seems unlikely. So do you have a source for this? Do you 
    remember where you encountered this idea that back sights were originally 
    for lunars?"
    Well, "it seems unlikely" is a small step back from Frank's earlier 
    doctrinaire posting that same day, in which he wrote-
    "I think someone, George?, suggested in a post that back sights were 
    originally added to octants "for lunars". Since as George has mentioned 
    it's very hard to get an index correction for back sights, which is true, 
    this was clearly not the case (though it may have been suggested back then 
    by someone who wasn't thinking through the practical aspects). Back sights 
    were not used for lunars."
    Yes, they were.
    Frank may have deduced, via pure reason, that "this was clearly not the 
    case", and therefore "Back sights were not used for lunars". Somehow, then, 
    I doubt if he can have taken in my posting of 18 March which contained 
    "Cook's second circumnavigation, departing 1771, was provided with two 
    15-inch brass sextants, one by Dollond and another by Ramsden, both fitted 
    with backsights, as described on page xx of  Andrew David et al,  "The 
    charts and coastal views of Captain Cook's Voyages", vol 2. Hakluyt 
    Society, 1992.
    And there, I explained some of the difficuties involved in assessing 
    index-error of a backsight, though pointing out-
    "Some index mirrors had a special facet ground exactly 90º from the main 
    surface, to aid such alignment, which could be as good as was the precision 
    of that set angle."
    And indeed, that was the Dollond patent, described in a paper by Maskelyne 
    of c. 1772, of which I attach some relevant pages. As fitted to that 
    Dollond sextant, designed with backsights to allow angles outside the 
    sextant's usual limit of 120º to be observed, which was supplied to the 
    Cook expedition.
    To what purpose? As I explained, David's account  "reports that Wales and 
    Bayley , the astronomers of the expedition, measured Sun lunars of up to 
    Those pages by Maskelyne were taken from an old volume I have, "Selections 
    from the additions that have been occasionally annexed to the Nautical 
    Almanac, from its commencement to the year 1812." It doesn't actually state 
    the Almanac issue to which each addition had been attached, but probably 
    around 1772, to judge by that sextant, and by the date of a paper by 
    Maskelyne in Phil. Trans., "Remarks on the Hadley's quadrant, tending 
    principally to remove the difficulties which have hitherto attended the use 
    of the back-observation", vol LXII, 1772, pp 99-122. (which I don't have).
    That annex to the Almanac contains quite a bit more interesting stuff from 
    Maskelyne about how to sweep a  quadrant, and on suitable telescopes to use 
    with backsights; that sort of thing. If anyone would like to see those 
    other pages, just ask. I always enjoy reading Maskelyne's explanatory 
    style. My guess is that  this text has been reset, as late as 1813, because 
    it has lost all those long-s characters which some readers find so 
    Real astronomers, as were Wales and Bayly, were required, for computing 
    such a Sun lunar distance angles as 155º from scratch, which was way 
    outside the 120º limit that had been set in the Almanac, for use by an 
    ordinary mariner. Why that limit? At a guess, because instruments directly 
    reading above 120º didn't exist, or were very uncommon. Why so? Well, what 
    was the point in providing them, if the Almanac only computed Sun lunars up 
    to 120º? A circular argument.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 




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