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    Re: Star-star distances for arc error
    From: George Huxtable
    Date: 2009 Jun 25, 10:07 +0100

    Henry raises an interesting question, about handling the weight of early 
    instruments for lunar observations.
    
    The first sextant, made by Bird in 1758, was so big (over 18 inches radius, 
    compared with a more modern instruments 7-inches or less) and therefore so 
    heavy, it required to be supported on a pole, with a ball-joint, as can be 
    seen in the photo from the National Maritime Museum in Greenwich. That must 
    have made it fiendishly difficult to operate on the deck of a ship at sea. 
    Details from their website at
    http://www.nmm.ac.uk/collections/explore/object.cfm?ID=NAV1177
    
    It would have been interesting if its weight were quoted, but it isn't.
    
    It had been preceded by another instrument made by Bird, a working version 
    in brass of a wooden mockup, supplied by the German astronomer Tobias Mayer, 
    of his navigator's circle, which I attach. This was an engraving made for 
    Rees' Cyclopedia in about 1817, from the actual instrument, of which no 
    trace presently survives as far as I know. The support pole fitted into a 
    socket in a belt worn by the navigator. The instrument as a whole was 
    dismissed by Campbell, to whom it was submitted for sea trials, as 
    impractical to use at sea.
    
    I must admit to having great difficuty in holding up even a standard 
    brass-framed 7-inch sextant of the early 20th century for a useful length of 
    time. That's one of the reasons why I still prefer an Ebbco plastic job as 
    more practical for me, in measuring altitudes from a small craft at sea, in 
    spite of all the prejudices against it..
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    ----- Original Message ----- 
    From: "hch" 
    To: 
    Sent: Thursday, June 25, 2009 5:06 AM
    Subject: [NavList 8787] Re: Star-star distances for arc error
    
    
    
    
    
    
    I have followed this discussion with a great degree of
    interest and am inclined to agree basically with George’s moderate and 
    measured
    approach. To criticize by assumptions not in evidence is not, in my opinion,
    the way to go.
    
    
    
    In the pursuit of practical navigation, the need for
    measurement of stellar distances seldom, if ever, arises and, although 
    familiar
    with the principles involved, I have never seriously sought to perfect the 
    technique.
    The matter of a support or stand for the sextant has, however, been a matter 
    of
    interest to me: the hydrographical surveyor has long used a rather
    sophisticated stand ashore in conjunction with the artificial horizon and
    altitudes measured thereby, however, to the best of my knowledge, no such
    device has ever been in general use at sea, or for the measurement of
    non-vertical angles at sea or ashore.
    
    
    
    For what it’s worth, I throw out the following quotation
    from Andrew MacKay’s “ The Theory and Practice of finding the Longitude at 
    Sea
    and on Land” c. 1809. .....”If the sextant is provided with a ball and 
    socket,
    and a staff, one of whose ends is attached thereto, and the other rests in a
    belt fitted round the waist of the observer, the greater part of the weight 
    of
    the instrument may by this means be supported by his body.” .... Assuming 
    the
    friction between the ball and socket can be regulated, there just might be
    something worth trying here.
    
    
    
    Any comments.
    
    
    
    Regards,
    
    
    
    Henry
    
    
    
    
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