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Subject: Re: Lunar altitudes
From: George Huxtable (george@XXX.XXX)
Date: Sun Apr 13 2003 - 07:00:59 EDT
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
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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.
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contact George Huxtable by email at george@XXX.XXX by phone at
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