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    Re: celestial accuracy
    From: George Huxtable
    Date: 1999 Oct 30, 5:48 AM

    Thanks to Paul Hirose for a useful contribution to this discussion. In
    particular, his quotation from Bowditch 1984 Vol 1 is very relevant, and I
    repeat it here.
    
    >"An investigation by the Carnegie Institution of Washington showed
    >that of 5,000 measurements of dip at sea, no value differed from the
    >tabulated value by more than 2.5', except for one difference of 10.6'.
    >Extreme values of more than 30' have been reported and even values of
    >several DEGREES have been encountered in polar regions."
    
    In my recent mailing on sextant sight accuracy, the section dealing with
    horizon refraction commented on Jim Manzari's analysis as follows-
    
    "For the Manzari approach to be workable, horizon refraction effects have
    to be of two distinct kinds; those that are major, rare, and obvious, and
    those that are totally negligible: nothing in between. Does he, or anyone
    else have evidence to support this unlikely contention?".
    
    Well, here I have to concede that the passage to which Paul Hirose has
    drawn attention does indeed provide significant evidence in that direction,
    in favour of what I had called Manzari's "unlikely contention". It would be
    interesting to read the Carnegie report that was referred to, if anyone can
    provide a reference to it.
    
    Dutton (1968)  refers to a correction that could be made to the dip
    resulting from sea-air temperature difference as follows-
    
    "The Japanese Hydrographic Office, by considerable empiric testing, found
    the value to be 0.11 minutes per degree Fahrenheit.; other values, ranging
    up to 0.21 per degree Fahrenheit, have been suggested.  ...When the
    Japanese value of 0.11 was applied to several hundred observations made
    along the Atlantic coast between the Virgin Islands and New England
    throughout a year, it proved to be generally satisfactory and improved some
    98 per cent of the observations."
    
    It would be interesting to see further details of those observations.
    However, one might infer that if making this air-sea temperature correction
    really did improve 98% of the observations, then-
    1) It seems likely that refraction near the horizon must have been the
    major error remaining, after all other corrections had been made.
    2) The correction must have been a reasonable approximation to the effect
    being corrected.
    
    Perhaps other readers can inform us what is the likely range of air-sea
    temprature differences at sea, the air temperature being measured near to
    the eye-height of the observer, the sea temperature near the surface, from
    the cooling-water intake or with a bucket. My rather uninformed guess would
    be that a range of 0 to 25 degrees Fahrenheit (0 to 14 Celsius) in the
    difference (air - sea) would be rather conservative, and this would
    translate, according to the Japanese assumption, into a range of 2.75
    minutes of arc correction.
    
    Presumably, standard values for dip include a correction for horizon
    refraction which is based on an average value of this temperature
    difference. In that case, if no correction is made for air-sea difference
    (which is probably true for all of us) a resulting scatter up to 1.4
    minutes either side of this average would not be surprising.
    
    So, not an overwhelming effect, this horizon refraction, but by no means
    negligible either. When other observing conditions are good (calm sea,
    clear horizon) then it may well be the dominant error remaining.
    
    It may be useful for navigators to be aware of the sign of the
    temperature-difference correction. Assuming the air is warmer than the sea,
    then the horizon is raised and the dip is decreased, compared with its
    standard value. Trouble is, until we know what air-sea temperature diffence
    the standard dip correction has been calculated for, we can't properly
    allow for changes.
    
    Please note that I am NOT proposing that we should all arm ourselves with
    buckets and thermometers and make such corrections. It's just worth
    thinking seriously about what errors remain when we have made as accurate
    an altitude observation as we can, and corrected it as well as we can.
    
    MEASURING DAY-TO-DAY CHANGES IN DIP.
    
    Both Paul and I have suggested that measurements could be made with a
    theodolite from shore. But for any reader who is fortunate enough to have a
    view of the open sea from his window, useful observations might be made
    with much simpler gear than that, needing no theodolite. All it would need
    is for a vertical scale to be made by sticking a vertical measuring-tape to
    the inside of his window, and a post to be placed, say 10 metres away,
    towards the sea. Knock a nail horizontally into the side of the post, at a
    height where, when seen from a convenient height within the window, it
    aligns with the horizon, more or less. If in tidal waters, note a mark on
    the shore that the waves just lap, somewhere near mid-tide.
    
    Now, whenever the tide is at or within a few inches of that mark (which
    gives four possible shots per day) and when a clear horizon can be seen,
    then line your eye up with the nail and the horizon, and note the
    corresponding reading on the scale. Note the air temperature outside, too.
    The sea temperature could be monitored from time to time, but won't change
    quickly.
    
    The most useful observations would be made from a residence close to the
    sea, within say 100 or 200 metres, as the aim is to gauge refraction over
    sea, not refraction over land. It would be best if the elevation was no
    higher than a ship's bridge, preferably less. And a view directly out to
    open sea, rather than out through a creek or over a harbour, would give the
    best approximation to the seagoing conditions we wish to simulate.
    
    With the dimensions I have suggested, a change in dip by one minute would
    shift the reading on the scale by 2.9 mm, so day-to-day variations, if they
    are of the magnitude suggested in the section above, should be readily
    discernible, and might make an interesting plot against time.  Trouble is,
    the unaided eye can only resolve a minute of arc or so. An improvement
    would be a small telescope or binocular which can be fixed to a frame and
    jacked vertically up and down against a scale. How about it, anyone?
    Unfortunately, not me, as I live 80 miles from the sea, which in Britain is
    nearly as far as it's possible to get.
    
    George Huxtable.
    
    
    ------------------------------
    george---.u-net.com
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel, or fax, to 01865 820222 or (int.) +44 1865 820222.
    ------------------------------
    

       
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