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    Re: Star finder tip: subtract one degree
    From: Brad Morris
    Date: 2017 Feb 1, 11:24 -0500
    Hello Herbert

    I will reply to your comments in a civil manner.  I think that the polite exchange of ideas is important.

    I have indeed "looked at" a 2102D. I wrote a nifty Excel spreadsheet which provided the polar stereographic base and the azimuth equidistant overlays. I provided the overlays in 1°steps, +/-90° in addition to an automatic sun and all the 57 navigational stars.  Frank, in the past, has used the overlays and has complimented that part of the tool.  So I may know just a bit about the mathematics and how it works behind the scenes.  Thank you for asking.

    You asked if I needed help in converting RA to SHA.  Certainly I don't need that help.  Thank you again for asking!  You are correct, of course, that marine navigators have not used RA in many decades, which further moots the main thrust of Frank's complaint, but let us continue.

    An example was provided, using Hamal to demonstrate that the arithmetic result, when compared to thr NA, was in error.  That may be true, yet we did not finish the problem.  

    As stated in the instructions, the point is to identify an unknown star.  After we arrive at the arithmetical result, the 2102D is to aid you in determining which one of the 57 navigational stars you cannot identify.   Note that we cannot identify billions of stars, only which one of the 57 stars.  

    I used the data provided and note the following 
    1) of the five stars around Hamal, none is closer in LHA by more than 10°
    2) the closest in LHA is Menkar (12°or so) , yet the declination also differs by more than 10°
    Consequently, we are left with only one of the 57 navigational stars to chose from, to wit, Hamal.

    The magnitude of the complained error (1°) is an order of magnitude less than the nearest other star, in any direction, to wit 10°.

    I would claim then, we have identified the unknown star, even with that error.  The polar stereographic plot, when combined with the azimuth a equidistant overlays still determined the correct answer (what navigational star is that?) even with 50+year old data. 

    Lastly, I would like to examine the complaint that the ephemeris plotted on the polar sterographic base is wrong, when compared to today's ephemeris. 

    An update to the ephemeris is warranted, yet the update will change very little of the design functionality, as demonstrated in the Hamal example.  I contend that updating the ephemeris isn't a design flaw.  

    We wouldn't try to navigate precisely today using a 1961 NA, we use the 2017 NA.  Is that a design flaw of the NA?

    I do so hope, Herbert, that I have provided you with a clear presentation for your review and that my presentation is intelligible. 

    Brad


       
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