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    Re: Lunar distance accuracy
    From: Frank Reed
    Date: 2007 Nov 04, 21:40 -0500

    George H, you wrote:
    "but there can be little certainty about whether an astronomer, taking
    measurements on land,  did or didn't use the term "pillar sextant" to
    abbreviate the phrase "sextant on a pillar stand". That would have been the
    natural instrument for an astronomer to use for measuring lunar distances
    from on land, if it had been available. It remains an open question, in my
    view. Unlike Frank, I would claim no certainty, either way."
    
    Of course, you wouldn't claim any certainty on this point, George. It's
    because you made a mistake. Your error, of course, is completely forgiveable
    since the term "pillar sextant" is obsolete today. But to maintain that you
    may have been right all along --in the single case of E.J. White's article--
    while admitting that you were wrong generally, is just a little strange.
    TODAY, that error of identification is minor. But to suggest that White
    would make the same error back then would mark White as some kind of idiot,
    both as an astronomer and as the owner of a Troughton sextant of which he
    was clearly very proud.
    
    And you wrote:
    "I now accept, from that evidence, that the general use of the term "pillar
    sextant " was, indeed, to describe a double-frame instrument rather than one
    mounted on a pillar. I'm quite happy to leave it at that."
    
    OK, so why did you deny it in the previous paragraph?
    
    I wrote previously:
    "If we take his lunars in sets of four and average them (which I consider
    the best approach with lunars), the results are generally within 0.1 minutes
    of arc. I would note that these results are very similar to my own
    experience."
    
    And now you write:
    "But if you are trying to demonstrate the observational SCATTER, it's absurd
    to conflate readings taken on different dates, with different moon phases,
    wildly differing arc readings. If there are scale errors in the sextant, if
    there are prediction errors in the almanac, deficiencies in the clearing
    process, that procedure will average them out, whereas to demonstrate
    scatter, you need to show them up."
    
    Hmmmm. What are you talking about??? The point of averaging four in a row is
    not to DEMONSTRATE observational scatter. It is to reduce it. IF you have a
    known observing location or locations (a given here), with a late 19th
    century or later almanac (which White obviously had), and an accurate
    clearing process (there are plenty, but White used Chauvenet, which is
    excellent), THEN, apart from arc error (which White reports was small),
    there is no difference in averaging observations made ten minutes apart from
    observations made ten days apart. Note that there IS a small difference when
    the observations are just a minute or two apart since, in the latter case,
    you can average the angles and then do the clearing. If the observations are
    more than a few minutes apart, you have to do the clearing and then average
    the results.
    
    In another post, you wrote:
    "That was completely missing the point. I was referring to SAILS, those big
    white things that are dangled from the spars, blown by the wind to propel
    the vessel along, and obscuring much of the sky, especially under square
    rig. Not "spars and lines and other "features""."
    
    Ahhh, SAILS! Yes, terrible nuisance. When we read the old logbooks and
    navigation manuals, they go on endlessly about the sails interfering with
    lunar distance sights. Hey... Wait a minute. Do they?? :-) Do you know of
    any non-speculative sources that worry about the sails getting in the way?
    Was the navigator (frequently the captain, generally an officer) immobile
    and forbidden from moving to a new spot where he could shoot lunars? Even
    that famous chef aboard Cleopatra's Barge in 1817 seemingly had no trouble
    with those darn sails getting in the way. George, this concern you have over
    the dreaded sails obscuring the sky is a case where you are imagining
    difficulties.
    
    I wrote previously:
    "Large fraction? The standard deviation is 0.25 minutes of arc. Is that a
    large fraction? This is an almost perfect match for my accounts of expected
    lunar distance accuracy."
    
    
    And you wrote:
    "-0.46' ... +0.09' ... +0.25' ... -0.29'
    Differences of a large fraction of an arc minute. I rest my case."
    
    So then what exactly WAS your case?? The observations show a standard
    deviation of 0.25 minutes of arc. You do understand, don't you, that this
    statement is consistent with the small subset of data you listed, right? One
    can always "cherry pick" outliers from any sample.
    
     -FER
    http://www.HistoricalAtlas.com/lunars
    
    
    
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