NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar altitudes
From: Phil Guerra
Date: 2003 Apr 13, 23:00 -0500
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. > ================================================================