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    Re: Lunar distance accuracy
    From: George Huxtable
    Date: 2007 Nov 5, 11:19 -0000

    I had written-
    
    | "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."
    
    about which Frank made some disparaging comments
    
    and I continued-
    
    | "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."
    
    To which Frank replied-
    
    | OK, so why did you deny it in the previous paragraph?
    
    
    I didn't, as he will discover if he reads it again.
    
    ==========================
    
    I had argued against Frank's procedure, which was-
    
    | "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 I replied-
    
    | "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."
    
    Frank responded-
    
    | 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.
    
    Frank's averaging procedure works for determining longitude. But not for
    demonstrating scatter, which is what we are all trying to examine. Frank's
    arguments are valid only if there are no systematic or other differences
    that cause lunar distance observations, taken under very different
    conditions, to yield different answers. But that is the very question that
    we are investigating, and seeking evidence for, isn't it? That certainly
    seems to be Alex's aim, in a long and systematic series of observations,
    closely parallel to White's, which are worth taking very seriously, and
    perhaps publishing.
    
    ============================
    
    I had pointed out that an observation taken on land "bears little relation
    to
    the difficulties that would have been faced by a real mariner, in a rolling,
    pitching, vessel, often doused with salt spray, with much of the sky
    obscured by square sails"
    
    For some reason, Frank took exception to that, pointing out how useful in
    locating the Moon in the sky "spars and lines and other "features" could be;
    a dubious claim at sea.
    
    and I replied-
    
    | "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.
    
    Very scathing. I wonder, when Frank walks in the woods and treads in some
    bearshit, whether he seeks documentary reference that that is what bears do,
    before wiping it off his shoe. To Frank, it seems that if he can't find it
    in Google, then it didn't happen. Nobody but Frank could ask for documentary
    evidence that the sails got in the way of the sky. Of course nobody
    mentioned that in their journals. It was a fact of life, that went with
    trying to do celestial navigation on a square-rigged vessel.
    
    Normally, navigation was done from the quarterdeck, where the motion and the
    spray were least, where the ship's clock was within call, as was the cabin
    lamp to read the sextant. But from there, the visibility was badly effected
    by the sails, depending on whereabouts in the sky you needed to be looking.
    It was worse on a ship-rigged vessel than on a bark. It was worse when the
    sky-objects happened to be to leeward, or forward, rather than to windward,
    or aft. It was worse when observing a lunar distance, requiring a clear view
    of two objects at once (the entire space between those objects didn't need
    to be clear), than when observing an altitude up from the horizon.
    
    Yes, of course, the navigator had to find a spot wherever he could on deck,
    where he could get a view, which might mean leaving the poop to go forward,
    and accepting more motion and spray, sometimes water swilling around the
    deck, animals and fowls in their pens, deckwork going on. And even then, to
    take a lunar might require asking permission of the captain for a change of
    ship's course, or the raising of a corner of the forecourse, or taking in a
    foretopmast staysail, to give him the view that he needed. Depending on the
    captain, and the conditions at the time, that's a permission he might ot
    might not get.
    
    Obscuring of sky by sails is just one of factors I listed, that make lunar
    distances harder for mariners at sea, and relatively easy for observers on
    land.
    
    To my sorrow, I've never travelled on a square-rig vessel under sail; and I
    wonder whether Frank Reed has.
    
    :==========================
    
    In Navlist 3547, I had referred to a professional astronomer
    "from one observation, to the next on another day, he records differences of
    a large fraction of an arc-minute."
    
    and listed the first four, out of the sequence of 42, which were-
    -0.46' ... +0.09' ... +0.25' ... -0.29'
    
    to which Frank quibbled-
    
    | "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 when I restated those numbers, with
    "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.
    
    I didn't mention, or question, any standard deviation. I said what I said,
    and Frank objected to those words. He may put some other interpretation on
    those numbers, but the rest of us can see that they clearly show differences
    of a large fraction of an arc minute, exactly as I said. And they were not
    "cherry-picked outliers". They just happened to be the first numbers in
    White's list, out of 42.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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