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    Re: Iterative Lunar method
    From: Wendel Brunner
    Date: 2019 Feb 25, 18:57 -0800

    Hello Frank,

    I appreciate your comments. I realize I have much to learn about various ways of clearing the lunar distance and the observations required, and especially the descriptions and methods outlined in older navigation texts when that method of finding Longitude was actually used in practice. Particularly the understanding that one doesn't have to make terribly precise measurements of altitudes to clear the Lunar Distance using many methods.

    When I started on Lunars in 1992, what I knew about that process came from reading "Carry On, Mr Bowditch" in middle school, some terse references in the modern navigation books I had available, the Hornblower and Jack Aubrey references (fiction, indeed), and especially the Slocum account in "Sailing Alone Around the World".

    I planned to measure the LD and take a series of altitudes of the sun and moon before and after, and then interpolate. Lacking the Mythical Three Midshipmen, that seemed to be the only way to do it. But when I couldn't get a Lunar altitude during my observation opportunity sailing to Hawaii - and frequently I have found that because of clouds or fog you just can't get an observation- I had to come up with some other method that works. An iterative method is often used for a variety of calculations, from square roots to polynomial solutions, so it was natural to try it for this problem. Iterative methods lend themselves to programmable calculator/computer solutions; I believe the StarPath calculator uses the same approach to calculating Lunars.

    And while Slocum's account may be just a footnote or less in the body of navigation information on Lunars, his book has had a major impact on inspiring modern sailors. I had heard about Slocum's Lunar feat from my older sailing mentors. After I read that book and in contemplating my own ocean voyage, like many others I wanted to recapitulate some part of Slocum's experience for myself. Sailing single headed through the Straits of Magellan was probably not in the cards for me, but with my celestial skills, I could attempt to emulate Slocum's lunar feat.

    So that gets to your basic question, "why do you want to do Lunars"? Certainly not to find Longitude. In fact, there is really no practical reason for celestial navigation at all anymore, although I suspect there are many on this forum who would bristle at that suggestion. The apocalyptic lightning strike that fries all the electronics including the spare pocket GPS tucked away in some shielded place seems unlikely.

    From reading you describe your rich investigation of the manuals and methods, as well as the myths and misconceptions, of how Lunar navigation was developed and taught in the time when it was important for navigation, I would guess that for you one reason is to be able to do Lunars in a way similar to how it was done in the 18th and 19th Centuries, but perhaps better. Or maybe I am just thinking that is what I would like to learn now.

    For me the reason to do a Lunar is also linked to appreciating how we came to our understanding of the world. While the explorers opened up the oceans, the astronomers expanded the heavens, breaking out of the confining celestial spheres of the earth-centered Ptolemaic theory and into the vast expanse of of the Copernican universe. Kepler, Galileo and Newton clarified the celestial realm, and sailors equipped with crude instruments to use that knowledge headed out into unknown terrestrial oceans-and sometimes came back.

    There are certainly better ways to get position than a Lunar Distance, or for that matter, quicker ways to cross an ocean than to be blown about in a sailboat. Staring at the dials of a GPS tells precisely where you are, but a carefully worked out Lunar does more- it connects us with our past, with the navigators of the oceans and the mind who puzzled out the heavens and the earth. The GPS tells position, but a Lunar fix reminds you also where you have come from. As Joshua Slocum wrote in 1899,"The work of the Lunarian, although seldom practiced in these days of chronometers, is beautifully edifying and there is nothing in the realm of navigation that lifts one's heart up more in adoration"

    Wendel Brunner

       
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