# 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 18, 12:06 +0200

```I've never seen one of these for sale on ebay nor have I ever seen a
mention of it in any navigational manual. Just because you can get a
patent for something doesn't mean it will work.

gl

Hanno Ix wrote:
> Gentlemen:
>
>
> US Patent 1,912,358
> V. Bush: Apparatus for establishing an artificial datum
> Filed April 8, 1928
>
> The inventor means an artificial horizon. Please refer to the
> attachments for the drawings.
>
> He has implemented a mechanical lowpass filter. We have discussed LP's
> before. Better yet: This filter simultaneously works in 2 coordinates,
> roll and pitch.
>
> He also points out that the natural oscillating frequency of the
> mirror should be as "low as possible" but he doesn't give any data.
> Since the roll frequency of ships is rather low to begin with (judging
> from you-tube: about. 0.1 Hz = 1 period per 10 sec for an oil-tanker )
> the filter has to have a cut-off frequency of about 0.01Hz  which
> amounts to 1 period per 100 sec. The filter also needs to be
> sufficiently damped. The reason is simply avoiding resonant
> oscillations of the filter/mirror in response to the ship's movements.
> To build an LP of this kind is the challenge!
>
> The inventor has made every effort to decouple the housing of the
> mirror from the body of the sextant. So to speak, he created something
> like a bubble level floating within bubble level.
>
> With a lowpass of this kind, influences from pitch/roll would be
> reduced by a factor of 100, possibly more. So, a 10 degree roll would
> create 0.1 degree (6 arc-min) deflection of the mirror. Smaller ship
> will have higher roll/pitch frequencies than oil-tankers, maybe 0.5
> Hz. Accordingly, on smaller ships 1 arc-min oscillation of the mirror
> might be possible. Is that error sufficiently low given the
> circumstances? It is certainly much less than I saw once in a bubble
> sextant.
>
> Perhaps most importantly, this patent points out how to separate
> accelerations of the sextant which are instantaneous from gravitation
> which is constant in time.
>
> When I made my proposal with the tubular ring the other day I had
> similar ideas in mind, however I had not fully understood the
> interaction between bubble and spirit.
>
> Regards
>
>
>
> >
>
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