NavList:

Geoffrey wrote:

> Most aircraft type bubble sextants have spherical bubble chambers
> fashioned such that the sextant is tilt invariant to a certain
> extent. If the sextant is tilted slightly in any direction, the
> bubble follows the image of the celestial body.

Robert responds:

Correct, at least inasmuch as the bubble units that I have in my collection. Spherical bubble chambers are the way to go because they are more "forgiving" of a fore and aft tilt, but not of a lateral tilt.

Geoffrey wrote:

> So, the question is, how big should the bubble be? The intuitive
> answer is - just slightly larger than the celestial object of
> interest.

Robert responds:

In my experience, this is the case.

Geoffrey continues:
> Given the similarity between the sight picture in a bubble sextant
> and that seen by the target shooter, I think it is reasonable to
> carry over this experience and say that better results will be
> obtained with a fairly large bubble. The bubble in my sextant is
> about a degree in diameter, so the sun is about half the diameter
> of
> the bubble. I find this to be fine for star sightings as well.

Robert responds:

Here is where I have to disagree, based on my own experience with bubble horizons.  The larger the bubble diameter with respect to the diameter of the object being viewed, the less accurate the sight will be. This has been my experience anyway and I have been using bubble horizons for a very long time.

Let me offer an alternate analogy: Let us say you have a particular geographic location, the centre of which is located within a defined circle with a diameter of 20 feet. Now let us say you are given a basketball and instructed to place this ball as close to the centre of that circle as you can. For purposes of this analogy, I will give you a jet pack so you can hover above the circle. How precise do you think your estimate will be? Now let us say that the diameter of the circle is reduced to 3 feet. I venture to guess your precision in determining the centre of this circle would be far more precise.

There is, however a qualifier to add: many bubble sextants use a rather viscous fluid, in which case, if the bubble is too small, it will "lag" and result in inaccurate observations. I replace the viscous fluid in my Plath, with hexane, which has a much lower freezing point and results in a more active bubble. It is not so good for taking observations the deck of a boat even in calm seas, but perfect for practice on land and, if you are like me, for practical land navigation too.

Geoffrey wrote:

> I recommend you try a larger bubble size, you may be surprised at
> the
> improvement in your altitude sights.

Robert responds,

Thank you Geoffrey but I have tried all combinations and I have found that it is best to have a bubble whose diameter is slightly larger than that of the sun. For stars, it is a fine balance for if the bubble is too small, the "lag" factor comes into play.

I am not saying you are wrong. A larger bubble may work for you but it does not work well for me.

Robert


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