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    Re: Star-star distances for arc error
    From: George Huxtable
    Date: 2009 Jun 24, 16:44 +0100

    Douglas Denny ended his last posting with a farewell, writing-
    "I object for instance to being dismissed out of hand with an offahnd and 
    glib:  "clear indication I have not tried it" - when I have.  So I think it 
    time to leave."
    Well, it's a pity to lose a new member who is clearly interested in the 
    topic, although reluctant to consider views other than his own. I sympathise 
    with his objection to the phrasing of the words he complains about.
    But just in case he hasn't quite closed his eyes just yet, and to put the 
    record straight, I will have a go at addressing some aspects of that 
    departing posting that seem clearly wrong to me.
    It's helpful that he has identified his Hughes sextant as a micrometer 
    instrument, in which case the  "platinum divided scale", indicating just 
    whole degrees, serves not much more than cosmetic purposes. The provision of 
    a calibration certificate becomes useful in correcting for any scale errors, 
    and provided those corrections are applied and not ignored (and they vary 
    smoothly over the arc) imply that any such scale errors do not degrade the 
    overall accuracy of the instrument. The extent of the maximum correction, of 
    1' 30", does not of itself prevent the instrument being read and used to 
    Although my old Vernier sextant has a Kew certificate of zero measurable 
    error (which I think implies no more than 10",) at all calibration points, I 
    would be just as happy to use Douglas's Hughes sextant, after making the 
    appropriate corrections. A corrected error isn't an error at all.
    We were informed that the attempted tests, which failed, were tried using a 
    sextant that was clamped in place, not hand-held, and I suggest that was one 
    reason for the lack of success. It prevents the sextant being "swung" about 
    , to achieve proper coincidence. Indeed, it's hard to see how a user would 
    achieve that alignment in the first place, before tightening a clamp. And 
    than, that plane will swing around the Pole, at 15� per hour. No the sextant 
    should have been used held in the hand, just as it's designed for. Mariners 
    measuring lunars at sea, for whom such clamping was impossible, had to put 
    up with the bodies in view "dancing around" far more than did Douglas on 
    land, who complains "Some form of rest at the very least is needed, or a 
    clamp with some degree of freedom. The stars dance about too much for 
    accuracy otherwise." That's exactly the motion that a sextant is designed to 
    cope with. I have tried to make my view clear, that measuring star-star 
    lunars is tricky, calling for skill and practice, and a clamped instrument 
    provides no easy answer.
    Douglas is clearly wrong in suggesting that - " The resolving power of the 
    telscope is not the issue here,  but the ability to superimpose star 
    images - accurately. In other words: eye acuity is the issue.".
    And elsewhere, he goes into different aspects of human-eye acuity in some 
    detail. But all that is relevant only if he is using a peep-tube rather than 
    a telescope. That's the purpose of using the telescope, to enhance the 
    acuity of observation, by a factor of approximatly the magnification of the 
    telescope. So those acuity values we were given should have been improved by 
    the factor of magnification, 3 or 5, or whatever. I am surprised that an 
    optics professional has failed to take such matters into account.
    Let's turn to Douglas' views on lunars, about which he seems less than 
    expert. He wrote
    "Lunar distances were very poor indeed." (whatever that means) and quotes 
    Dyson, in 1922 stating that mariners using lunars couldn't do better than 
    about 20 miles. Which is in my view, is a fair assessment, but of course to 
    measure a lunar to 20 miles or 20' at the equator requires measuring an 
    angle to within 40 seconds, under at-sea conditions. Which already 
    undermines Douglas' claim that one can't do better than 1', even from on 
    He continues- "And this means with use of the reflecting circle too - more 
    accurate than a sextant - eliminating some of the inherent errors of the 
    sextant.". Nonsense! Dyson was writing about sextants. The circle was a 
    proposed solution hardly ever used by navigators at sea, though 
    occasionally, until the early 19th century, by surveyors and hydrographers. 
    It was doomed when machine division of sextants arrived around 1780.
    He adds "It took at least an hour for the astronomers on board ship when the 
    method was tested to 'clear the distance'  to find longitud", Also nonsense! 
    The clearing process wasn't what took the hour's calculation. That was the 
    time it took to calculate the position of the Moon in its orbit, and the 
    need for doing that disappeared when precalculated lunars were published in 
    the first Nautical Almanac of 1767.c
    ontact 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. 
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
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