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On 7/26/2016 1:23 PM, Charles McElhill wrote:
> In the Nautical Almanac, in the table for moon altitude corrections,
> noticed a small paragraph in the lower right, stating that if you use a
> bubble horizon, you should neglect the dip correction and subtract 15'
> for your reading.  My queston is,would this apply to this particular
> practice bubble horizon, or is this for a more sophisticated bubble type
> horizon.

Charles

I see the dip portion of your questions has been answered.

Regarding the subtract 15' portion:

The upper moon altitude correction table assume a lower limb (LL)
observation with a standard sextant and natural horizon. It also assumes
a 30' moon diameter, hence subtracting 30' from an upper limb (UL)
observation.

I have only used an aircraft bubble sextant once, but my recollection is
I viewed a circle, a bubble, and a body. The goal was to center the
bubble and the body in the circle. Therefore, I was observing the center
of the body (moon in your case) and would need to subtract the semi
diameter to adjust it to the LL observation assumed in the altitude
correction table. This is also why the NA instructs you to use the
midpoint of the UL & LL HP corrections.

The Celestaire practice bubble sextant is a somewhat different beast. It
has a spirit level with a horizontal line and a body in view. If you
center the bubble on the line the sight tube should, in theory, be
level. If you make the body's LL tangent to the line you have (again in
theory) a LL observation so no 15' adjustment is needed. One can also
attempt to center the body and the bubble on the line (my preferred
method), in which case the semi diameter of the sun or moon would be
subtracted. Both methods are difficult with with a zero magnification
moon or sun, a relatively thick line, and a "bouncing" bubble.

Either way you need not only the standard IE correction but a bubble
correction (BC) as well, as the chances of the sight tube being
manufactured precisely enough so it is exactly level with the bubble
centered on the line are small indeed. I found my bubble correction by
tripod mounting the sextant, presetting the sextant to HC Plus
refraction and IE at LAN, centering the bubble and noting how far off
the preset altitude my observations were.

There may be other ways to observe, for example by bringing the bubble
tangent to the line and calculating a BC based on that. Whatever works
for you.

   

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