Re: Lunar altitudes
Subject: Re: Lunar altitudes
From: HGWorks - Phil Guerra (pguerra@XXX.XXX)
Date: Mon Apr 14 2003 - 00:00:57 EDT
Sorry for the confusion.
----- Original Message -----
From: "George Huxtable" <george@XXX.XXX>
To: <NAVIGATION-L@XXX.XXX>
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@XXX.XXX
> 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@XXX.XXX by phone at
> 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
> Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> ================================================================
