NavList:

I wonder if this is the cause of the problem many people have getting
good altitudes with bubble sextants.

As George and Robert both agree, the optics in a bubble sextant are
arranged such that the image of the celestial body is superimposed on
the image of a bubble in a bubble chamber attached to the sextant.

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.

In such sextants, the bubble is round. The object is to center the
celestial object in the middle of the round bubble.

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

The bubble size can be altered by adjusting the amount of fluid in
the bubble chamber. A number of sextants have bubble chambers which
allow the bubble size to be externally adjusted. Sextants such as my
Link A-12 come with two bubbles, one for sun (or moon) sightings and
the other for star sightings.

However, the sight picture of centering the celestial object inside a
round ring is very similar to a target shooter centering the round
aiming mark of a target inside the ring of the foresight.

Do experienced target shooters choose the diameter of their foresight
rings so that there is just a small annulus, "a couple of hairs",
between the outside of the aiming mark and the inside of the
foresight ring? Interestingly enough, no. All target shooters go
through this phase, but in the end the overwhelming experience of
target shooters is that it is better to have a large foresight ring
so that there is a wide and generous annulus between the aiming mark
and the foresight ring.

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.

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

Geoffrey Kolbe


>George wrote:
>
>When observing the Sun, as I understand
> > it, the aim is to match the disc of the Sun with the disc image of the
> > bubble, and try to keep it there.
>
>Robert responds:
>
>That is pretty much the case, although it is better to have a bubble which
>is slightly larger than the disc of the sun. Not too much larger but just a
>couple of "hairs".


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