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    Re: Old style lunar (in expert hands)
    From: Alexandre Eremenko
    Date: 2004 Dec 10, 10:59 -0500

    Question about old observations on land:
    why did they use sextants rather than theodolites?
    As I understand, theodolite is a much more precise instrument,
    because of the stable platform.
    And you can certainly measure lunar or any other distances with it.
    Its only disadvantage on land is that it is somewhat larger and
    heavier than a sextant.
    In the book "Divided circle" by J. A. Bennett, I see many
    fine (and portable) theodolites of XVIII century and earlier.
    Alex.
    
    
    On Fri, 24 Dec 2004, Kieran Kelly wrote:
    
    > Fred Hebard wrote in relation to the Australian explorer Augustus Gregory
    >
    > "Also, perhaps Thompson was not the best of lunarians.  Kieran Kelly
    > last year or so posted a series of lunar observations taken by an
    > Australian explorer that were exquisite.  I can't imagine that the
    > distance cleared from those would have been in error by more than 0.1
    > or 0.2' of arc, given an accurate sextant."
    > And
    > "Augustus Gregory. This was about 50-60 years after Thompson. I don't think
    > sextants improved much in the intervening period but the quality of lunar
    > predictions probably did. It could be that Gregory was just that much
    > better than Thompson. Thompson was blind in one eye but I don't know if
    > the vision in his other eye was impaired in any way."
    > I agree with Fred's comments and have been following this topic with
    > interest. If any other list members would like to examine the paper he
    > mentioned it can be found at:
    > http://www.ld-DEADLINK-com/
    > As I have pointed out many times before on this list, land navigators were
    > sometimes expert lunarians and developed their own methodologies and
    > techniques. I have led four expeditions in outback Australia relying on maps
    > produced by Gregory 150 years ago using longitudes derived from lunars. He
    > shot both the lunar distance and the altitudes of the bodies in the same
    > process. Unlike L & C he sat down and worked lunars out in the field.
    > To anyone interested in 19c technique I urge you to read the Gregory paper.
    > Also on this topic Frank  Reed wrote: "But something that might not be so
    > obvious is that dead reckoning is simpler at sea since it consisted of long
    > legs on constant headings. At least there aren't any currents on land!"
    > I have used both horses and camels in desert conditions without landmarks
    > following a compass bearing. Dead reckoning is easy under these conditions
    > because:
    > 1) There is no set or drift - it is easy to steer the animals in a straight
    > line.
    > 2) All pack and riding animals when organised in a "string" walk single file
    > in a straight line behind each other
    > 3) All walk at a remarkably constant speed. Camels 4km/5km per hour
    > depending on ground, heat and scrub ditto for horses except they walk about
    > 6km per hour. Well handled the animals will maintain these paces/speeds day
    > in and day out.
    > After a 30 km march in desert country using a compass, I am very
    > disappointed if I am out by more than 1km (6/10 mile) at the evening camp
    > (checked using GPS).
    > Kieran Kelly
    > Sydney
    > Australia
    >
    
    
    

       
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