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    Re: Learn the stars, by phone
    From: Frank Reed
    Date: 2009 May 14, 13:10 -0700

    George H, you wrote:
    "If that were possible, I could see an obvious application arising on my own 
    boat, and no doubt so would many others. The marine world would beat a path 
    to its door."
    Oh, I don't think it (the SkyScout or similar technology) would work on a 
    boat... at least not very well. There's a way to make one work better under 
    conditions with complex accelerations, but I don't think anyone would 
    optimize these consumer devices for such applications. There's just no need 
    for it.
    And you wrote:
    "Brad hasn't addressed that claim of 1? of accuracy, for direction, but Frank 
    has taken it further, by writing- "it can determine where you're pointing in 
    the sky from anywhere on Earth at any date and time ... with an accuracy of 
    about 0.5 degrees." , but he still doesn't state where this figure comes 
    from, what instrument it refers to, and under what conditions it applies."
    It refers to the SkyScout, which I have tried out myself. That half-degree 
    claim comes from the official specs. It's a believable claim, based on 
    performance, but the exact level of accuracy (whether it's 0.5 or 0.75 or 1.0 
    degrees) is not critical in any way to the device's use. It simply works as 
    advertised and works well. But bear in mind, this is an educational toy. It's 
    a very fine educational toy, but it's not designed or marketed as a 
    "measuring instrument". It has no such function.
    "He tells us "It is sensitive to local ferrous materials and in fact informs 
    you when the deviation is greater than some internal value.". I'm sceptical 
    about that magic ability. How could that be done?"
    As for me, I don't know. The design is proprietary. But here's a speculation: 
    measure derivatives of the field. The terrestrial field is quite uniform over 
    considerable distances. Also, in order to deal with built-in fields in the 
    device, like those from the batteries, I would speculate that it might record 
    the field derivatives on start-up and compare changes to that initial field. 
    [oh and before someone says, 'you don't know! You're just speculating!', 
    yes... that's why I have used the word "speculation" :-)]
    "It could, perhaps, by detecting any significant difference in the total field 
    strength, or the dip angle, from the value predicted for that location on the 
    Earth. But could it establish, by such means, a deviation that changes the 
    magnetic direction by 1?? I don't believe it!"
    I agree -- that wouldn't work.
    And George, you wrote:
    "Frank, as is his style, belittles that problem by addressing it in 
    exaggerated terms- "Clearly, as you suggested, if there's significant 
    magnetic or acceleration interference (you wouldn't want to use it inside an 
    iron carousel), then it would have problems.. ". Well, of course, I wasn't 
    considering an "iron carousel", I was talking about real-life difficulties 
    that interfere with real-life compasses, difficulties that need serious 
    Well, *I* was talking about the subject of this thread: devices that will soon 
    be much more widely which will facilitate learning basic star identification. 
    And as I say, I highly doubt that these devices, in particular the Skyscout, 
    would work on an "iron carousel" (in other words, anywhere with ferrous 
    materials which would modify the magnetic field, or anywhere with significant 
    accelerations, rotational or otherwise). I was not talking about a compass 
    you might want to acquire for your boat. 
    PS: If you feel that I am "belittling" a problem then you are taking yourself waaaaaay too seriously.
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