A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2018 Jun 5, 08:12 -0700
Paul you wrote: I’m curious why a pendulous reference is considered easier to use than a bubble.
I only have serious experience in the air with Hughes MkIXBM Bubble and Smith’s/Kelvin Hughes Mk 2 Bubble and Pendulous Reference Sextants. With British bubble sextants since about 1939 (Fig 1), you made your own bubble each time you used one. Depending upon luck and the individual bubble chamber, you might get your ideal bubble first try, or you might have to try again. When you did make a satisfactory bubble, it might or might not stop the ideal size for the whole trip. With a pendulous reference sextant, the manufacturer provided a constant sized graticule which never altered (Fig2). Most people stuck with centre shots, but edge shots are possible with a graticule.
The Vulcan needed two sextant mounts but only carried one sextant, which might be either a bubble or a pendulous reference model. We had both. However, we were allowed to draw a second sextant, so we could have one on each side of the aircraft simultaneously making two star sandwich fixing much easier. In that case, crews invariably asked for a pendulous reference version if possible. DaveP