A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A-10 Sextant Manual
From: Gary LaPook
Date: 2009 Jun 12, 09:42 -0700
From: Gary LaPook
Date: 2009 Jun 12, 09:42 -0700
It's really not a problem since the navigator always has to compute the azimuth for every shot. This means that he always knows what the azimuth is (and thereby the direction the the LOP runs) since he computes it based on the time of the sights and the location of his AP Since he chooses an AP near his DR (presumably not for from his actual position) is accurate enough to plot the LOP and find the destination. You can compute some trial cases and see for yourself how much he azimuth varies. An easy way to see this is to just glance at the various entries in precomputed tables such as H.O. 214, H.O. 229, H.O. 249 etc. gl gl On Jun 12, 3:52�am, "Christian Scheele"
wrote: > "The bigger problem is that the azimuth of the sun changes as it moves > across the sky, not that the LOP is slightly curved. " > > I think I get it, and stand to be corrected in my previous email. The > movement of the sun, in relation to changing declination and the AP, �leads > to a non-uniform rate of change of azimuth. That's what makes it difficult > to "keep on" the LOP, which swings around at this non-uniform rate of > change. Thanks again. > > Christian Scheele > > ----- Original Message ----- > From: "Gary LaPook" > To: > Sent: Friday, June 12, 2009 10:14 AM > Subject: [NavList 8635] Re: A-10 Sextant Manual > > > The bigger problem is that the azimuth of the sun changes as it moves > > across the sky, not that the LOP is slightly curved. Look at Weems > > writings on this at > > >http://www.geocities.com/fredienoonan/landfall.html > > > Chichester describes the same procedure in the Observers' Book. > > > The problem of shifting LOPs and curved LOPs is handled by using the > > destination as the AP and this was the method being used by Chichester. > > Done this way, as long he manovered the plane to keep reading the same > > altitude from his sextant as that which would be measured at the same > > instant of time at the destination, then he can be sure he is staying on > > the LOP through destination, whatever the azimuth happens to be. > > Chichester did this and took an additional sight to confirm that he was > > staying on the LOP to Lord Howe. > > > gl > > > Christian Scheele wrote: > >> I didn't misunderstand you. > > >> "Pointing out his > >> plane's engine problems you say he was reckless." > > >> I then went on to discuss Chichester's navigation method on my own > >> account: > > >> "I don't really think I'm > >> qualified enough to judge his decision to navigate using a (his?) method > >> iof > >> ....precomputed D.R. ...I mean, his sun shot of course gave him only a > >> position line, or rather, an assumed position line" > > >> I was, however, really unclear. Gary's response alerted me to this. I did > >> not intend to argue against the concept of using LOPs, but it sure looks > >> like it, so my apologies. In my next email, therefore, I rephrased my > >> comment, which, in fact, concerned LOPs which are so long that they are > >> really lines with changing gradients for plotting purposes. (Assuming > >> nobody > >> would bother to plot curves themselves on a Mercator chart.) If you then > >> fly > >> along one course because you assume from D.R. that you know which > >> gradient > >> or bearing applies to the part of the LOP you are on, when in fact you > >> are > >> on a part of �the LOP that has another bearing, then would you not fly > >> off > >> the LOP? If so, the real question becomes: Would you be able to use a > >> sort > >> of running fix to work out your position and thus rectify the problem of > >> determining which part of the LOP you are on? I think so, what do you > >> think? > >> That must be quite a manoeuvre for one flying single-handed. It > >> demonstrates > >> how important good D.R. is. > > >> Perusing "The Lonely Sea and the Sky", I confirmed that Chichester > >> estimated > >> his wind speed on his flight, rather than use on a device. He also didn't > >> want to check his compass before the flight "except against Norfolk > >> Island". > >> Ignoring the problem concerning long LOPs for a moment, it would seem > >> that > >> these navigational decisions of Chichester compounded the risk already > >> posed > >> by the questionable state of his plane. > > >> Christian Scheele > > >> ----- Original Message ----- > >> From: > >> To: > >> Sent: Thursday, June 11, 2009 12:17 PM > >> Subject: [NavList 8615] Re: A-10 Sextant Manual > > >> Regarding �Sir Francis Chichester: > > >> Dr. Kolbe has it right. > > >> I was referring not his astonishing navigation for which I have the very > >> highest regard indeed, but to the fact he was willing to put his life on > >> the > >> line with an aeroplane that he describes in his book already had various > >> problems such as having fitting floats without checking if they leaked or > >> not, and an engine that had already given him problems, and in the run-up > >> check before leaving for Norfolk Island he says:- > > >> "I could only get 1780 revs, forty less than I expected, and my spirits > >> sank. I should never get off with a full load with a motor like that, but > >> said nothing to the CO about it. �The seaplane was launched. I faced her > >> into the wind, and opened the throttle; to my surprise she left the water > >> as > >> easily as a sea bird..... ". > > >> The man must have been mad or very determined, or both. > > >> To cross the Tasman Sea - a nasty stretch of water notorious for bad > >> weather, two thirds the width of the Atlantic, in a single engined float > >> plane with engine in dubious condition is more than reckless. � But got > >> away > >> with it. > > >> My impression from reading his books is he was what we would call a > >> "loose > >> cannon" and contemptuous of any advice or authority. �From reading about > >> him > >> it seems he must have been a truly remarkable person but not very > >> 'likeable'. > > >> Douglas Denny. > > >> Chichester. 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