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    Re: On lunars generally
    From: Douglas Denny
    Date: 2009 Jul 7, 01:07 -0700

    Not so. I have been discussing this for years and years. The normal resolution 
    (for standard resolving tasks) of the human eye is around 1 minute of arc, 
    somewhat better under excellent conditions. Do you recognize now that when 
    you look through a telescope (a GOOD telescope) that angles on the sky are 
    magnified? And therefore if the eye can normally see details of one minute of 
    arc without magnification then using a 3x or a 7x telescope, the observer can 
    see details of 0.33 and 0.14 minutes of arc respectively? ***I should emphasize 
    that the standard deviation for my lunar observations with a 7x telescope is 
    about 0.25 minutes of arc which implies that there are other sources of error 
    in addition to the limiting resolution of the eye-telescope system.***
    Quite so.  I contend the main limit is the resolution of the eye itself and 
    you are already down to normal limits with a sextant with the eye alone.  A 
    telescope imroves slightly - but only slightly and with no direct correlation 
    with telescope magnification. 
    A x7 telescope does NOT improve sextant accuracy of results by seven times.
    "As for lunars, they were made practical at a 
    time when there was only ONE functioning chronometer in the world. They were 
    actively employed by real navigators at sea for 75 years after they were 
    first practically introduced, and they were also used in land exploration and 
    mapping for another 50 years after that. The idea that lunars were dropped as 
    soon as Harrison demonstrated his chronometer H4 is incorrect."
    Of course 'lunars' were being used when there was only one functioning 
    (sea-going) chronometer in the world - there was nothing else available for 
    longitude - and astronomers had been struggling for centuries to obtain 
    almanac data sufficiently accurate to use them.  Lunars were only developed 
    after centuries to a (just) reasonable working method when the Harrison 
    chronometer was developed.  And, don't forget Harrison spent his whole 
    lifetime developing them and was in his 70s/eighties when H4 came onto the 
    It was not the Harrison chronometer that became the 'standard'. It was the 
    Arnold and Earnshaw movement developed shortly afterwards. 
    It is not surprising that lunars therefore were in use for an interim period 
    of say fifty years until large-scale production made costs low enough for 
    shipping companies to adopt them.  
    The point you cannot escape however is that as soon as chronometers were a 
    viable financial proposition - lunars were ditched like a hot potato.  Why? 
    if lunars were so 'easily used' with "calculations done in ten minutes"  etc? 
     Answer - because they were difficult to take under practical conditions, 
    inaccurate despite great care,  and calculations were a pain.
    I do not understand why you shoud want to dispute this. It is the received wisdom from many sources.
    You also confuse the issue by making land use (surveying) as a supportive 
    reason for using them. The resaon this persisted is because chronometers are 
    still liable to unknown rate changes especially when carried overland, and 
    making accurate sights with a theodolite (much better than at sea) made 
    reasonable accuracy attainable with plenty of time to to do all the 
    calculations and careful observations.  It is not a reasonable comparison.  
    Douglas Denny.
    Chichester. England.
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