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    Almanacs, theory and use.
    From: Mike L
    Date: 2007 Nov 20, 03:39 -0800

    We live in Scotland and my mother lives in England, so I thought it
    would be  pretty simple to prove to my kids using a sextant that the
    world was  spherical - so I bought an EBBCO on eBay. Whilst I've
    proved to myself I can  use a sextant to find out where I am, I'm
    still to convince the children either that you can, or that you would
    want to.
    
    I started by using some software into which I put the time,
    approximate  longitude, latitude and sextant reading, and (after
    working out I had to  subtract half the sun's diameter) I finally got
    something average on our  location on average about 3 miles from our
    location. Unfortunately, as far  as a kid is concerned, if you have a
    PC, you may as well look up  google/streetmap rather than waste time
    with a sextant, so I need to find a  PC-less way to find out where I
    am.
    
    So, using a bit of trig (with some software from the web) I created my
    own  single-page weekly tables (the sun don't shine everyday!), giving
    altitude  and direction of the sun for a given location for each
    minute of the day.  This allows me to create a table for any given
    place which most children who  can add two digit numbers, and use a
    ruler/protractor could use by  themselves (with instructions) to plot
    a line giving their location (to  within 10miles I hope!), which if
    repeated twice in a day should give an  "exact" location.
    
    Now, I know how my "Almanac" works, but even having figures for every
    minute  of the day, for a known location and interpolating results for
    seconds, I  will still be pushing it to get tabular errors less than
    1'. From what I  have been able to discern about real almanacs they
    contains a fraction of  this information with only hourly figures for
    every location in the world.  Although, I've downloaded a few
    worksheets to "calculate" the figures, I  can't understand how these
    are used (I neither have a worked example, nor do  I have an almanac,
    nor do I have a theoretical explanation for the tables -  but I don't
    see that as a fundamental problem!) Surely getting from these  figures
    in the Almanac to one at any time for a particular location but
    involve some complex trigonometry and rather hectic sinusoidal
    interpolations - neither of which are apparent on the worksheets!
    
    What I really want to know is how my "almanac" relates to a real
    almanac,  and how, could and should I make my "almanac" more like a
    real almanac and  still have it useable by children? I've tried
    searching the internet, for  any explanation of how to use an almanac
    (with the theoretical background) -  any help would be greatly
    appreciate (remembering I am not familiar with  SHA, GHA, and whilst I
    learnt spherical geometry at University, I'm a little  rusty)
    
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