
NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2025 Jan 24, 12:38 -0800
Noell Wilson
You reported: "Lieutenant Commander J. M. Sheehan, U. S. Navy in 1935 says that with much, much practice he could get good sights with a bubble octant."
As far as I’m aware, no bubble sextant had automatic averaging in 1935. The operator pressed a trigger when they thought the body was steadiest for a succession of presses. Then they calculated the mean, median, or modal value depending upon individual taste and type of sextant. However, if you look at an object moving sinusoidally, it appears steadiest when the effect of change of acceleration on the bubble is greatest. I.e. at the top and the bottom of a wave. E. g. For a ship it might be ploughing over the waves at 45 degrees while observing a body ahead. You ride up the side of the wave while slowing down, hover over the top, then accelerate down the other side before struggling up the next wave. In the airborne environment, it was the inaccuracies that the dynamic response of the aircraft was causing despite the navigator thinking they’d done a great job that led to the development of automatic averaging. Legend has it that two-minute averaging was chosen for the Hughes MkIXA sextant to match the long period oscillation (the phugoid) of the Sunderland flying boat. Personally, I suspect ‘slow roll’ might have been more of a problem.
Lack of automatic averaging would have undoubtedly led to the poor impression gained of the bubble sextant at sea. Even with automatic averaging, it’s most important to forget your personal feelings and follow the bubble, or its value is lost. (Trebble Clef emojis "Always let your bub-ble be your friend”). DaveP