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Tony Oz you wrote: Which type of artificial horizon (gyroscopic or bubble level or other) emulates the "tilt-invatiantness" better? The golden standard is - of-course - the Newtonian scheme with the natural horizon, which is absolutely inertia-free.

Tony

Don’t mix up tilt with acceleration error.  They’re different things.  You can tilt any sextant while standing stationary on an unaccelerated platform.  A marine sextant is unaffected by fore and aft tilt.  Proving it used to be a UK A Level Physics school exam question.  A bubble sextant will be more or less tilt invariant for small amounts of tilt depending upon its quality of design and manufacture.

Acceleration error only occurs with a bubble or pendulous reference sextant when it’s subject to acceleration. If the liquid moves one way because of its inertia, the bubble must move the other way. A pendulum is similarly affected.  You’ll be unlikely to find a gyro sextant for sale, but if you do, you’ll find it’s bigger than you’d imagined from the pictures.  We’ve argued here if a gyro is affected by accelerations.  It depends so much upon the position of its centre of gravity.  In my opinion, if you build in a bit of pendulousness to ensure it erects with its flat top horizontal, then it must suffer from acceleration errors, but others have disagreed.

In all honesty, if you want small, go for a box sextant, but you won’t get an AH, or an averaging mechanism, and you’ll only get a Vernier to read.  DaveP