A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2015 Nov 24, 12:30 -0800
Francis you wrote: To me the remarkable thing is, he did not use a bubble sextant, therefore relied on horizon with a dubious altimeter, one handed and low above the ocean, and likewise Bygrave, one handed.
and: One question for Brian and Gary which has always bugged me. Why did he not choose a date when he could have got shots of moon and sun for mid flight 2 body fixes? That would have been mighty reassuring?
The problem with bubble sextants is the effect of acceleration associated with speed and heading changes on the liquid in which the bubble is floating. That’s why averaging devices are used. With those great big floats so far below the aircrafts longitudinal axis, the aircraft would have been short on lateral stability once any sideslip set in. At least one of the descriptions in AOtTS suggests that Chichester completed his shot with the aircraft in close to a spiral dive. A bubble sextant, or more likely in those days, a sextant with a spirit level would have been useless in such a situation. Even with a very stable aircraft, several shots have to be taken to get a mean or a median (depending on the sextant) reading. This would mean more time away from controlling the aircraft, and Brian has said he couldn’t have allowed himself more than five seconds before the aircraft began to diverge, and that’s without floats. Therefore, in a way, a marine sextant was the best possible choice for Chichester, especially for beam shots, because I seem to remember a common A level Physics question was to show that a twin reflection sextant is unaffected by tilt in the direction of viewing.
An interesting fact about dip, if you look at the extended table, is that although dip increases with height, the change in dip reduces, so a height error of 10 feet at 500 feet say has nothing like the same effect as a height error of ten feet close to sea level.
W.r.t waiting for a Sun Moon fix, you can bet your life that after waiting all that time in perfect weather, the sky over the Sun Moon period would become obscured. With all the things that Chichester had to fit in to complete the flight successfully, waiting for a suitable Sun Moon day would be very low on his list of priorities. DaveP