NavList:

Hewitt Schlereth you wrote:

Bubble sextants seem to give about the same results from a small plane as plastic marine ones do from small sailboats.

That’s true, but it’s not just small planes.  The bigger, faster, and higher flying aircraft became, the more chance there was of seeing the Sun, because you had more chance of being clear of cloud, but because you were travelling faster, acceleration errors, suffered by all aircraft because of their dynamic response, became greater.  This was why averaging systems were introduced.  However, these depended initially upon making a mark when the celestial body appeared steady.  Few people realised until around the late 1930s that this might not be the best way of going about things.  Bubble displacement depends upon acceleration whereas unsteadiness depends upon rate of change of acceleration.  Dynamic instability in aircraft generally follows sinusoids, and differentiating a sinusoid shows that rate of change is least when acceleration is maximum and greatest when acceleration slices through zero.  This is why automatic averaging devices started to be introduced around the start of WW2. For automatic averaging to work, it was necessary to follow the bubble at all times, but this tends to go against the observers intuition.  DaveP