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    Re: Lunar altitudes
    From: Phil Guerra
    Date: 2003 Apr 13, 23:00 -0500

    Sorry for the confusion.
    ----- Original Message -----
    From: "George Huxtable" 
    To: 
    Sent: Sunday, April 13, 2003 6:00 AM
    Subject: Re: Lunar altitudes
    
    
    > Dan Allen asked-
    >
    > >> On Saturday, April 12, 2003, at 02:54 AM, Wolfgang Koeberer wrote:
    > >>
    > >> > Chichester,F., Longitude without time, in: Journal of the Institute
    of
    > >> > Navigation, Vol. 19 (1966), 106 -107 (with comments by D.H. Sadler,
    one
    > >> > time superintendent of HM Nautical Almanac Office, p. 107 - 109).
    > >>
    > >> Does anyone know how to get reprints of articles like this?
    > >>
    > >> I've always wanted to read Sir Francis' article, but I've never known
    > >> how to get ahold of it.
    >
    > and Phil Guerra replied-
    >
    > Dan,
    >
    > Here's a link to their publicatins web page, I think you'll find info
    > there...
    >
    > http://www.ion.org/shopping/order_list.cfm
    >
    > ==================
    >
    > Comment from George Huxtable-
    >
    > I think there is a bit of confusion here. There are separate institutes of
    > navigation, issuing journals with rather similar names, on either side of
    > the Atlantic.
    >
    > In Washington there's the Institute of Navigation, producing a journal
    > "Navigation", and I think the website referred to by Phil Guerra pointed
    > there. Wolfgang carefully distinguished publications in that journal by
    > appending "(Washington)" in his list. However, the Chichester publication
    > was not in that journal.
    >
    > In London there is what's now named the "Royal Institute of Navigation",
    > though once it was the plain unvarnished Institute of Navigation, and this
    > produces a quarterly "Journal of Navigation". At some time in its history
    > this journal may have been named "Journal of the Institute of Navigation,
    > or later "Journal of the Royal Institute of Navigation" (JRIN), and it may
    > conceivably have been filed in some maritime libraries under these
    > headings, perhaps only for some part of its print run.
    >
    > The RIN has a website at
    >   http://www.rin.org.uk
    > and as I recall, if you poke around in there you can find a complete index
    > to publications in the Journal. But only an index: not access to the
    papers
    > themselves.
    >
    > The RIN is usually friendly and helpful to non-members, and Heather Leary
    > may be prepared to help with copies or scans of older papers: you might
    ask
    > her anyway, at-
    >   editor@rin.org.uk
    > or by phone at +44 207591 3133.
    >
    > The correspondence Wolfgang refers to predates (by a long way) my own
    > membership of the RIN, so I don't have my own copies of these papers to
    > send around.
    >
    > However, thanks to list member Clive Sutherland, I do have a copy of the
    > 1978 Sadler paper which put rather an authoritative conclusion to the
    > argument, which Wolfgang referred to as-
    >
    > Sadler, D.H., Lunar Methods For "Longitude Without Time", in: Journal of
    > the Institute of Navigation, Vol.31 (1978), 244 - 249 (with a historical
    > note pointing out that the Board of Longitude in 1802 resolved  that it
    > "will not in future take into their consideration  any methods of
    > ascertaining the Longitude founded on the Moon`s Altitude...).
    >
    > I have made a scan of a photocopy of this paper on my own rather primitive
    > equipment: it is everywhere legible (but not much more than that). This
    > could be sent out as an attachment, in TIFF encoding. The paper has 6
    pages
    > and each scanned image covers two of those pages.
    >
    > Because of the Nav-L list's request (which I understand, but regret) for
    > "no attachments, please", this won't be available on-list, but I will
    > happily send a copy off-list to any list member who asks for it in the
    next
    > few days.
    >
    > Intending to illuminate his readers, Sadler included a diagram of such
    > devilish complexity that I can't understand it, so if you enjoy a puzzle
    > you will find an interesting one there. If you do work it out, please
    > explain it to the rest of us...
    >
    > In addition to Wolfgang's list of references, Sadler includes two more,
    > which I have not followed up-
    >
    > Ortlepp, B (1969), Longitude without time, Nautical Magazine, vol 210,
    276.
    > Ortlepp, B. (1977) Improved plotting solution to longitude without time,
    > Nautical Magazine, vol 218, 334.
    >
    > I am not familiar with all the arguments in all that correspondence, but
    my
    > own simplistic view is this-
    >
    > Measuring altitudes up from the horizon was a familiar task to a
    navigator.
    > Howeve, any measurements of altitude, measured up from the horizon, are
    > degraded by the unknown errors in the angle between the observed horizon
    > and the true horizontal; particularly variation in the dip from its
    assumed
    > value. Determining time from the relative altitudes of two bodies would
    > involve those horizon uncertaincies, twice over.
    >
    > Measuring the lunar distance, the angle between the Moon and another body
    > up in the sky, though a tricky oparation which required much skill,
    avoided
    > involvement of the horizon. It allowed a precision of a fraction of a
    > minute to be achieved in the lunar distance. As each minute of error in
    the
    > lunar distance gives rise (in low latitudes) to a 30-mile error in
    > position, it was crucial that any avoidable errors should indeed be
    > avoided.
    >
    > This matter was well understood back in the mid-1700s, and was the reason
    > why the lunar distance method was settled on. This judgment of a our
    > navigational ancestors stood the test of time, until the whole method
    > superseded by the chronometer. It's only right for their reasoning to be
    > re-examined from time to time, however.
    >
    > John S Letcher, jr, in "Self-contained celestial navigation with H.O. 208"
    > (1977), devotes a whole 10-page chapter (chap 17, "Time by lunar lines of
    > position") to this matter. He concludes-
    >
    > "Although it is fundamentally slightly inferior to lunar distances in
    > accuracy, the lunar altitude method is far easier to work out, and it can
    > be applied easily by anyone who knows how to work ordinary sights...."
    >
    > I have two comments about this.
    >
    > 1. In my opinion, Letcher makes light of the inferiority, which is more
    > serious than he allows.
    >
    > 2. Letcher was writing before on-board computers or calculators were
    > generally available. For those who are prepared to use them, the
    mathematic
    > difficulties in clearing the lunar distance have largely disappeared,
    > though the difficulties in the observation remain.
    >
    > George Huxtable.
    >
    >
    > ================================================================
    > contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > ================================================================
    
    
    

       
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