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    Calibrating the Practice Bubble Horizon with a sea horizon
    From: Bill B
    Date: 2014 Mar 16, 16:42 -0700

    GOAL: Calibrate the Practice Bubble Horizon (PBH) sold by Celestaire using a sea horizon.

    What I think I know that is relevant to the task is as follows:

    The sensible horizon (as seen from an observer on the face of the Earth without atmosphere) is parallel to great circle of the celestial (rational) horizon, and perpendicular to a line between the viewer’s zenith and nadir.

    “Sensible horizon” definitions drive me crazy. In my readings there are confused and/or confusing definitions, dating from at least 19th century texts (URLs for examples below). A current text, Dutton’s Nautical Navigation, 15th edition, page 206, ‘Horizon System Coordinates’ states, “The celestial horizon is parallel to the plane of the observer’s visible horizon at sea. The visible horizon (also called the sea horizon, and sometimes, the natural horizon) is the line where, to an observer, sea and sky appear to meet.” Other diagrams show the sensible horizon tangent to Earth at the viewer’s feet or eye level (therefore parallel with his/her celestial horizon). Bowditch appears to support an eye-level sensible horizon parallel to the celestial horizon as bodies can be observed below the celestial horizon yet above the visible horizon.

    The visible (sea) horizon is not parallel to the sensible or celestial horizon. It is lower than the sensible horizon due to curvature of the Earth, refraction, and height of eye.

    Parallax due to the distance between celestial horizon and sensible horizon planes is negligible for cel nav, with the exception of bodies in our solar system.

    I’m standing on a pier in Lake Michigan. Due to the construction of the PBH I can only view the horizon through the index-mirror side of the contraption. The bubble occupies the left-hand side. (Please see attached.)

    Traditional mirror
    Height of eye: 13 ft.
    Dip: 3!5
    IE: 0
    I center the bubble against the reticle line, adjust the index mirror until the reticle line is superimposed on the visible horizon and read the sextant scale. It reads 5’ on the arc.

    If my reasoning is on track, the reading I got is not the final bubble correction. If I were to use this correction to make an observation of a celestial body, I would still need to include dip height-of-eye in the sight reduction as my initial 5’ correction was for the visible horizon.

    What I believe I require is a bubble correction with the bubble centered when the reticle line on the index-mirror side is viewing the theoretical sensible horizon and parallel to the celestial horizon.

    Therefore, if I factor 3!5 dip out of the 5’ raw bubble correction it should yield a correction that is independent of height-of-eye and requires no dip correction in subsequent reductions regardless of any change in height-of-eye.

    I would very much appreciate the group’s input on “sensible horizon” and my PBH-correction straw man.


    19th century texts



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