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    Re: Sun Moon Lunars to 155 degrees
    From: George Huxtable
    Date: 2010 Mar 29, 22:33 +0100

    Kermit and I will discuss some aspects of Bayly's page of lunar
    observations from 5 July to 11 Sept 1773 in a bit more detail
    Kermit asks several relevant questions, and I will offer some "informed
    guesses", and suggestions.
    "1 - We are to assume that all observations are supposed to be made on
    board a non-moving vessel and at the very same time."
    No, they will mostly have been made from a vessel under way, except for
    those taken in harbour at the foot of that page. However, each
    corresponding set, of lunat distance and altitudes,  may well have been
    taken simultaneously, as such vessels were well-manned, with a number of
    competent observers on board, and  provided with two brass sextants, and
    most likely a number of wooden octants, good enough for taking altitudes. I
    imagine the Navy making a bit of a ceremony of the job.
    MT COMMENT HERE : I see what you mean. Obviously HM Ship certainly was
    (fast) moving. My only point of adressing this assumption here was under
    the following approach : if we are to reprocess to-day these historical
    data (through one method or the other), I do not see any other realistic
    option than "pretending" that all measures occured at the same time and
    from a non-moving vessel. From the published data, and for lack of more
    specific details, I cannot take any other "reasonable" assumption to carry
    out my computations, while at the same time I have to appreciate that my
    computation results might be "biased" due to this lack of all relevant
    data. Would you agree here ?
    from George- Although these were Royal Navy vessels, they were no more than
    adapted merchant ships intended for coal-carrying in The North Sea,
    wide-beamed and bluff-bowed.. They seldom exceeded 6 or 7 knots.
    For each set of observations, a lunar distance and two altitudes, we can
    only presume that they were simultaneous. In which case, I don't see why it
    matters whether the ship was moving or stationary. The stated latitude was
    presumably deduced, for that same moment, by DR from the previous (or the
    forthcoming) noon Sun observation; and that DR is the only aspect that
    involves the ship's motion, as I see it. Why can't each line on the page;
    the lunar distance, two altitudes, and deduced latitude be simply taken as
    simultaneous values? Then, why is ship motion a problem, to Kermit?
    Kermit. Regarding the heights ... the only TWO reasonable assumptions here
    seem to be :
    - either they are just corrected for sextant error (for the same reasons as
    here above, no possibility for a more "resonable" assumption) and in such
    case we have to process them as such,
    - or they are unrefracted topocentric because, to the best of my knowldge -
    and thank you for any correction here - , that was the form (i.e. Body
    Center Unrefracted Topocentric) which was then required to carry out most
    conventional Lunar Clearings.
    from George- I'm unsure what Antoine means by the word "unrefracted". Does
    that mean "there's no correction been made for refraction"? Or does he mean
    "that's what the altitude would be if the atmosphere didn't refract it"?
    Sorry if I'm being stupid.
    What is required, as input for finding true Sun altitude to determine LAT,
    is observed altitude, corrected for index error, dip, parallax, and
    refraction. What is required, to input for the lunar distance clearing
    operation, is observed altitude for each body, corrected for index error
    and dip. But without any prior correction for parallax and refraction,
    which are dealt with in the clearing process.
    Kermit. DOES ANYBODY KNOW whether these heights are simply "corrected for
    Sextant error" or whether they have already been 'pre-processed' into "Body
    Center Unrefracted Topocentric" heights ?
    from George- I suggest Kermit tries to find an explanation in the
    corresponding pages relating to the third voyage.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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