A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Feb 8, 08:31 -0800
Bill you wrote: in The Sextant Book
As you will see when I describe the gyro unit, when the Kreiselsextant is subject to an acceleration, the only connection between the sextant and the gyro is via its low friction, small area bearing. Effectively, the gyro is almost detached from the sextant and retains the direction of its axis of spin in space.
I’m concerned that people might assume that the gyro artificial horizon is the panacea, and that it must be preferable to the bubble or the pendulous reference artificial horizons. Apart from post 1943 U Boats, it clearly isn’t, as the Allies found after WW2, or we would have seen more of them and fewer bubble and pendulous reference sextants. As far as I can see, unless the pivot point and CG coincide, which will lead to other problems, the gyro has to suffer from acceleration errors. If the body of the sextant accelerates, and the gyro is rigid in space, then there must be a side force applied to the pivot by the cup, and the gyro will begin to precess.
Moreover, there might be other problems. You said in your earlier post:
I was able to meaure the index error at 18 minutes.
Might some of this be due to hang-off caused by Earth rotation? If the earth is rotating at 15 minutes of arc per minute of time, the axis of a perfect gyro spinning in space will appear to be toppling at rate of 15 minutes of arc x cos latitude of the observer per minute of time. However, our gyro is at the same time attempting to correct itself to the local vertical, so there’s going to be a hang-off error. The mathematicians ought to be able to tell us whether this error, when observed through the sextant light path, will appear constant or sinusoidal. DaveP