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    Re: New to the list
    From: John Huth
    Date: 2009 Jun 2, 07:10 -0400
    Thanks for your kind welcomes.  

    A few quick comments - Stephen Thomas went on to be the host of "This Old House" on PBS.  He was quite young when he wrote "The Last Navigator."    I think he's still alive.     If you want more depth on Polynesian navigation, I'd recommend "We the Navigators" by David Lewis, who spent a substantial fraction of his life studying this.   The book is quite long and written in a more academic style, but it goes into great depth not only about the Polynesian "etak" system of sidereal compasses, but also about wind compasses and the use of swell patterns, birds, even bioluminesence as navigational clues.

    In terms of our own "tricks" we developed during the seminar last year:  there is a straightforward way to memorize the equation of time.   Also, I developed a way of being able to create a solar declination table that's accurate to about 0.3 degrees or so (averaging over several years).    The challenge I gave to my students was to use a wrist watch and a home -made device of their making to determine latitude and longitude to some precision. 

    We'd synchronize our watches to NIST time - there's a nice applet you can download that takes into account the internet communication delay.   After tracking your watch, you can figure out how much time it gains or loses per day (mine gains 0.1 sec/day).   

    With a fairly crude device, you can measure the altitude of the sun to about 20 minutes.   Mine involved an aluminum tube, and I made a quadrant by successive bisecting of angles, rather than rely on some template that had degrees on it.   Some of the kids in my class used sextants fashioned from straws, and using teabags from the school cafeteria as a plumb-bob.

    It turns out that if you measure the altitude of the rising and setting sun, and noting the time, you can get a fairly precise value of longitude by taking the mid-point (e.g. draw lines tracing the rising and setting values as a function of your watch time).   If you then use the memorized equation of time, you can get a decent estimate of longitude.  

    Using the trick to memorize solar declination, we also got latitude from the midday sun.

    We did a bit better in longitude than latitude, but the typical uncertainties were about15-20 miles, which is not the kind of precision you can get with real tables and a good sextant, but it was surprisingly good give the fact that we were using home-made devices and tricks to get memorized tables.

    A guy at our astronomy department teaches a course on conventional sextant/table/chronometer navigation.   His first exercise is to have the students make a quadrant from a straw, string, rock, and protractor.   They use a time-average analema (ie. averaging over years) and got roughly the same precision we got.

    On Mon, Jun 1, 2009 at 10:50 PM, Anabasis <jcaoy---.com> wrote:

    Hi John, welcome to the list.  This should be right down your alley
    with the mathmatical discussions.  There are also a number of real and
    created celestial problems that you and/or your students can play

    I read "The Last Navigator" by Stephen D. Thomas which is about his
    efforts to learn from one of the last Polynesian navigators still
    using the old methods.  Given the age of the book, I wouldn't be
    suprised if he is gone now, or at least not taking to sea any longer.
    It is a good read in any case.


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