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    Re: Practice Horizon Bubble
    From: Bill B
    Date: 2016 Jul 26, 18:54 -0400

    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.
    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)
    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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