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    Bubble Horizon Altitude Corrections
    From: Bill B
    Date: 2004 Jul 4, 18:09 -0500

    BACKGROUND
    
    After attending an excellent seminar by Ken of Celestaire at Chicago's
    Strictly Sail show this winter, I decided it was time to quit toying with
    the idea and actually add celestial navigation to my bag of tricks.
    
    My first step was to peruse astronomy books and sights and get a feeling for
    how SHA/RA, declination, GHA etc. fit together while learning the celestial
    bodies.  My assumption--if I can't identify it, I can't shoot it.  A 2102-D
    star finder is on my wish list when I take the show on the water, but for
    backyard practice David Chandler's planisphere and the Navigator Star Finder
    on Omar Reis's superb web site are more than adequate.
    
    The next step in getting my feet wet was to acquire HO229 sight reduction
    tables, a Nautical Almanac, Susan Howell's Practical Celestial Navigation,
    and the AstroMedia/Wurzburg cardboard sextant with a bubble horizon from
    Celestaire.  With a little tweaking (added a horizontal slit over the
    eyepiece to stabilize the eye position and bring the horizon line into
    focus) and practice I am getting within 15-20' of the reference Hc taken
    from Omar's sight.  My location is determined by my Garmin GPS 76 and
    plugged in along with GMT.
    
    For sight-reduction-form practice I find I can take Omar's HC for the body
    and use it as the Ho, and then proceed as usual.  If done correctly, the
    plotted LOP is dead-nuts on in reference to my plotted "DR" position, so
    errors are immediately evident.  So far I am encouraged enough with my
    progress to consider a real sextant and 2102-D before my summer cruise(s) on
    the Great Lakes.
    
    PROBLEM AREAS
    
    Now the other shoe drops.  I am observing the center of the sun and moon as
    it is easier with the cardboard sextant and bubble horizon and marker line,
    especially before I added the slit to bring the marker horizon line into
    focus.  Given the slop of a bubble horizon and ? 5 to 10 minutes accuracy I
    am not able to resolve the issues attendant to using a bubble horizon
    empirically; and don't know enough about how parallax, semidiameter, and
    refraction are employed to arrive at the Sun and Moon altitude corrections
    and HP tabular values, so can't work it out logically.
    
    To add to the confusion, Howell and the instructions in the Nautical Almanac
    Moon Corrections page seem to be--for the neophyte--at odds.  (Excerpts from
    both follow.).  Given, Howell is mainly addressing an artificial horizon and
    the almanac a bubble horizon, but it strikes me there is something missing.
    I am guessing the almanac gives no instructions for bubble horizon
    observations of the Sun as they are assuming you are using a real horizon
    with the Sun, while you may be using a lighted bubble for the Moon.
    
    I understand why dip would be ignored in either case.  Howell suggests
    ignoring temp/pressure corrections while the almanac says to include them.
    Perhaps the artificial horizons double angle cancels that out while a direct
    shot with a bubble would not?
    
    The almanac does not state which limb of the moon is observed with the
    bubble horizon.  Are bubble observations traditionally made on the center of
    the body?  I am guessing the center of the moon, as they have the reader
    average the UL and LL corrections and subtract 15', while and UL sight would
    subtract 30'.
    
    Howell says that since you are observing the center of the sun, no
    correction is needed except for refraction taken from the star table and
    moon parallax.  No mention of 15'. It strikes me in the case of the Sun the
    total of UL and LL corrections (UL treated as a positive) is greater that 2x
    the semidiameter, and that the UL correction is usually greater than SD, and
    the LL correction is less than the SD, so the actual center of the Sun is
    lower than its observed center by the difference of LL corrections - LL
    corrections.  That value differs from the refraction correction for stars.
    
    Suffice it to say I am deeply confused about what corrections to make to the
    Sun and Moon observations made from the center of the body with a bubble
    horizon.  I need to rely on the kindness of strangers to solve this
    conundrum.
    
    1.  Can anyone provide a cookbook version on what corrections I need to make
    or ignore? Temp and pressure, altitude and/or HP correction for the Moon,
    and star refraction or UL/LL difference for the Sun? And nothing or -15' for
    the moon?
    
    2.  If time and temperament allow, a bit of theory as to why in terms a
    beginner might grasp?
    
    Thank you
    
    Bill
    
    EXCERPTS
    
    From Susan Howell's Practical Celestial Navigation
    
    In correcting the altitude of an artificial horizon sights first apply the
    index correction.  There is no dip correction or temperature correction
    necessary. The sextant altitude with I.C. applied is then divided by two
    after which the main correction is applied.  The main correction is figured
    as for a normal sight except in the case when the Sun or Moon are
    superimposed upon themselves.  Here the center of the body has already been
    observed so there is no correction needed for semidiameter, phase or
    augmentation.  For the Moon, there is still a parallax correction needed.
    The values for upper and lower limb corrections are averaged.  Refraction
    correction is still necessary for the superimposed Sun and Moon sights, this
    refraction value taken from the star altitude correction table.
    
    Therefore, for most artificial horizon sights Ho =  (Hs ? I.C.)/2 ? main.
    
    Ho is the corrected sextant altitude.
    
    From the Nautical Almanac
    Moon Correction Table
    
    The correction is in two parts; the first correction is taken from the upper
    part of the table with argument apparent altitude, and the second from the
    lower part, with argument HP, in the same column as that from which the
    first correction was taken. Separate corrections are given in the lower part
    for lower (L) and upper(U) limbs.  All 30' degrees is to be subtracted from
    the altitude of the upper limb.
    
    For corrections for pressure and temperature see page A4.
    
    For bubble sextant observations ignore dip, take the mean of upper and lower
    limb corrections and subtract 15'from the altitude.
    
    App. Alt. = Apparent altitude = Sextant altitude corrected for index error
    and dip.
    
    
    

       
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