A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bygrave and Chichester
From: Gary LaPook
Date: 2009 Aug 03, 19:14 +0200
From: Gary LaPook
Date: 2009 Aug 03, 19:14 +0200
By "single body fix method" I take it you are talking about the recent thread about taking many sights of the sun around noon and finding both latitude and longitude from these sights. I have never seen anything about using this method in flight by anybody. Although it is theoretically possible to do a conventional noon sight for latitude in flight it is extremely difficult due to the speed of the plane which masks the point of actual noon. But, more importantly, there is no particular advantage in attempting to do so. The traditional noon sight only made sense in the olden days when it was onerous to do the trig for a Sumner line LOP and the noon sun sight had much simpler computation, just addition and subtraction. By the time of in fight celnav there was a plethora of tables (Dreisenstock, Ageton, Weems, Hughes) and, of course, the Bygrave slide rule that made the trig trivial so there was no reason to go through the contortions of trying to use the special case of a noon sun sight. I did notice, however, while analyzing Fred Noonan's chart work on the Earhart flight in 1937 that he continued to utilize the special case of the Polaris shot. This also continued to be used by Air Force navigators since it is extremely simple and doesn't suffer the problems inherent with the noon sight. You use the "Q" correction table from the Air Almanac or from H.O 249, not to determine latitude but to do a simplified calculation of computed altitude for Polaris and then treat the Polaris line as any other LOP advancing it as with any other LOP to determine a fix. The "Q" table is a simplified table that accomplishes the same thing as the Polaris tables in the Nautical Almanac to a lower level of precision. Using the "Q" table you enter the table with the local angle of Aires and then apply the correction with sign reversed to calculate computed altitude, compare it with observed altitude to determine intercept. The azimuth is also found from these tables and can vary from 358 � to 002�. An example should help. If Polaris was actually at declination 90 � north then the altitude measured with the sextant would equal your latitude. So using the simple case when Polaris is directly east or west of the pole the altitude measured is also equal to your latitude and the "Q" correction is also zero. So using the mariner's method you measure an altitude of 35 � 30' so you determine that your latitude is also 35 � 30' north. But the way a flight navigator would do it is he would assume a position for finding a fix using Polaris and other stars, say 35 � 00' north and compute an altitude for that AP of 35 � 00'. Then he compares his Ho of 35 � 30' and determines his intercept of 30 nm toward Polaris. This line would also plot at 35 �30' north, assuming that it had not been advanced to cross the other LOPs to find the fix. Since the actual azimuth of Polaris is used this line may be slightly more accurate than when using the mariner's method. gl Christian Scheele wrote: to determine > I recommend this > >> book, Seaplane Solo, to everybody and I can email a copy to anyone who >> is interested. >> > > I would much appreciate a copy, Gary. > > I am not starting a new thread because the subject is related to your > commentary on the difficulties of making celestial observations. Could you > give me a reference to anything on the attempts by Byrd and Weems to use the > single-body fix method by taking sun shots through the open hatch of their > seaplane? > > Christian Scheele > > > ----- Original Message ----- > From: "Gary LaPook"
> To: > Sent: Saturday, August 01, 2009 3:59 AM > Subject: [NavList 9281] Bygrave and Chichester > > > >> My interest in the Bygrave was triggered by my reading Sir Francis >> Chichester's account of flying a Moth, open cockpit single engine >> airplane across the Tasman Sea in 1931 doing celnav on the way to find >> two tiny islands where he could refuel, each leg about 500 nautical >> miles. Today, at Headcorn Aerodrome in England, I had the opportunity to >> fly the same type of aircraft and my admiration for Chichester increased >> ten fold. It is a very light aircraft so it is bounced around a lot my >> even the lightest turbulence. The controls are very sensitive, >> especially in pitch, so it takes a lot of concentration to keep the >> plane flying straight and level. It is also very noisy and the wind >> blows vigorously through the cockpit. I don't know how Chichester >> managed to do it, flying the plane, shooting sun lines with a marine >> sextant, doing the computations with the Bygrave (holding it >> horizontally so it didn't get blown out of the cockpit), estimating >> drift angle, and plotting the LOPs and the drift lines. I recommend this >> book, Seaplane Solo, to everybody and I can email a copy to anyone who >> is interested. >> >> gl >> >> >> > > > > > > --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@googlegroups.com To unsubscribe, email NavListemail@example.com -~----------~----~----~----~------~----~------~--~---