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    Re: Duttons Navigation and Piloting 12th edition
    From: Gary LaPook
    Date: 2016 Jan 25, 08:13 +0000
    I have four editions of Dutton's, 5th ed., 1934, 7th ed.,1942, 12th ed.,1969, and 14th ed.,1985. Neither the 1934 nor the 1942 editions  mention irradiation or sea-air temp difference. The 14th ed. says that thousands of observations were taken and the irradiation correction  of 1.2' for the  upper limb of the sun was determined and 0.11' correction per degree of sea-air temp difference. The 1985 edition says neither are used in normal navigation. 
    The 1938 Bowditch doesn't mention either. 
    To max out on correction look at the 1962 Bowditch. Tilt correction for the ball recording sextant; sea-air temp difference; wave height correction; sea tilt correction; deflection of the vertical; irradiation  correction for the sun of 1.2' 
    There is a table of all the corrections on page 436:

    Sea-air temp. diff.
    Wave height
    Sea tilt
    Deflection of the vertical
    Air Temp.
    Atmospheric pressure



    From: Frank Reed <NoReply_FrankReed@fer3.com>
    To: garylapook@pacbell.net
    Sent: Sunday, January 24, 2016 11:47 AM
    Subject: [NavList] Re: Duttons Navigation and Piloting 12th edition

    Bob Crawley, you wrote:
    "Correction for irradiation."
    I don't know dates on editions of Dutton's, but the presence of this topic alone suggests it's from circa 1970. Just so you know, the fascination with the idea of "irradiation" and the corrections for it which were all the rage in the 1960s eventually faded away. It's a perceptual effect and proper use of shades renders it irrelevant. The inclusion of an "irradiation correction" in the Sun altitude corrections in that period is now generally regarded as a mistake.
    "Correction for air-sea temperature difference."
    Doesn't work. It would be nice if it did, and in theory it should. Unfortunately, the variable driving anomalous dip is the air temperature gradient in the shallow layer between the sea surface and the observer's height of eye, and it turns out that this is only loosely correlated with the air-sea temperature difference. This idea, of correcting for unusual refraction based on simple air-sea temperature comparisons, has come and gone several times in the history of celestial navigation. Chauvenet thought it would work as early as the 1860s. It's good physics, in principle, and the math is solid in simple cases. Nature is not a simple case.
    "Astigmatising shades - where can I get these?"
    They were a standard feature on some sextants in the 1960s and 70s. The argument in their favor was founded on the idea that it was "hard" to swing the arc in some circumstances. Or was it? I believe that this was a reflection of the "bad habit" of swinging the arc around the horizon axis which developed in mid-century, apparently originating in USN practice. If you swing the arc properly, keeping the Sun or star centered in the field of view, then swinging the arc always works, and an astigmatizer becomes superfluous.
    All that said, I don't want to diminish the value of the book. An older copy of Dutton is a great thing! Both as a historical document and as a textbook on celestial from an era when people still depended on it. That mission-critical dependence --quite naturally-- promoted better pedagogy in many textbooks.
    Frank Reed
    Conanicut Island USA

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