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    Re: Sun Moon Lunars to 155 degrees
    From: Frank Reed
    Date: 2010 Apr 13, 15:39 -0700

    Antoine you wrote:
    "A 3'50" dip correction !! That should give us a Height of Eye somewhere around 15 to 16 feet ??"

    Yep.
    You can almost always assume 4' of arc for dip throughout this period. It was so much of a "rule" to assume 4' of dip, 16' for the Sun's semi-diameter, and no refraction, that even as late as the 1880s, Lecky felt the need to protest the practice in "Wrinkles". The common calculation --and it was very, very common-- was to combine 4' of dip, 16' for SD (reasonable), and nothing for refraction. The total of 12' was then subtracted from 90 degrees so that a noon latitude would be worked by taking the observed altitude and subtracting it from 89° 48'. As I say, this was very common. The mistake was when it was applied regardless of specific circumstances. So Lecky wrote: "Some men drift into a very common though extremely reprehensible habit of finding the sun's meridian zenith distance by subtracting their noon altitude from the constant 89° 48'; and this they do under all circumstances—whether they are standing on the bridge of a high-sided steamer, or the deck nearly awash of a coasting schooner—whether the sun is almost overhead, or only a few degrees above the horizon. Some do it through pure ignorance—others, because never having taken the trouble to investigate the matter, think "it is near enough." But it is not near enough; and the man who does such a lazy trick, to save himself at most half-a-dozen figures, is not fit for command."

    Lecky is very quotable. :) He's also somewhat over-stating the case here.

    The difference between this specific value, quoted above, of 3'50" and 4' is, of course, taking things entirely in the opposite direction, and 10" difference in dip is probably meaningless, but it's typical of the over-done fascination with seconds of arc that you will find, not just with Cook's calculationally-obsessed astronomers, but even among more typical practical navigators. It's not unusual to see "rough-and-ready" tricks like the 89..48 calculation worked out to the nearest second of arc in old navigational notes.

    By the way, Did I miss something here? Do you folks not have access to the original book by Wales??

    -FER


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