# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: The development of bubble sextants
From: Gary LaPook
Date: 2009 Aug 15, 02:27 +0200

```Well, of course, coriolis is a fictional acceleration. You calculate it
with the formula:

Z (coriolis correction) = .0262 times the ground speed in knots times
the sine of the latitude.

What you are actually doing is correcting for the change in the
orientation of the latitude- longitude grid as the plane moves over the
surface of the earth which makes it appear that the vertical was
deflected as computed by the formula. You make the adjustment by
shifting the LOP or the AP or the final fix to the right (in the
northern hemisphere) of the track over the ground the amount calculated
by the formula. See attached table.

gl

douglas.denny@btopenworld.com wrote:
> Three points:
>
> 1)The curvature of the upper bubble chamber lens is equal to that of the
focal length of the lens/mirror arrangement so the bubble is in focus at all
times in all positions.
>
> 2) The bubble chamber is circular so the bubble chamber/sextant can have
lateral movement (roll) as well as the important movement in pitch, to allow
some degree of lateral movement not easily possible in a straight (standard)
bubble tube arrangement. This is probably why the straight tube bubble
chamber was abandoned on marine sextants, or at least, not developed further
in favour of other methods.
>
> 3) The argument seems to be - if I have it right: to attempt to eliminate
side accelerations affecting the bubble.  You cannot escape side (horizontal)
accelerations affecting the apparent gravitational vertical whatever you do,
as the 'compensating mechanisms'  will be referenced to the local
gravitational field too. Even if some hysteriesis is present between bubble
and 'compensator' mechanism,  both will have to come to the same conclusion
as to where the gravitational field is pointing. In other words you are back
to square one.
>
> Eary type aircraft artificial horizons are such a mechanism that uses local
gravitational field to provide an horizon (or hence vertical whichever you
prefer) - they have a long time constant errecting mechanism using small
pivoted vanes on four points around the gyro. The vanes 'dangle' downwards
due to gravity and provide precessional forces to bring the gyro to an
horizontal plane.
>
> Coriolis force is such an example of horizontal acceleration that cannot be
eliminated; it has to be allowed for - luckily because it is a constant
feature and thus a known quantity.
>
> Douglas Denny.  Chichester. England.
>
>
> >
>
>

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