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    Re: iPhone as artificial horizon
    From: Frank Reed
    Date: 2008 Sep 10, 06:49 -0400

    Paul, you wrote:
    "It turns out that Apple's iPhone and iPod Touch make a nifty
    artificial horizon.  Both devices have built-in angle/acceleration
    sensors, so If one downloads Labyrinth Lite (currently the most
    popular game download for the iPod), it has a set-up feature that
    displays two right-angle leveling bubbles.  Use a couple of Post-its
    to shim it into level and the screen makes a small but effective
    sighting surface!  I haven't actually tried to measure the leveling
    sensitivity, but my wild guess (based on how hard it isto chase the
    bubble) it is something like a 10 - 30 min bubble equivalent."
    A simple app that could be written would measure the angular tilt of the
    device and record it when the screen is tapped. One could then aim the
    device at the Sun by minimizing its shadow, and then immediately read out
    the altitude. As you say, these altitudes are only accurate to 30 minutes of
    arc or maybe somewhat better, but you might still be able to extract some
    useful navigational information out of it (and this would work even if the
    device has no access to network information/cell phone capability). It might
    make a very good digital compass...
    There are off-the-shelf digital levels available that measure angles to
    about +/-10 minutes of arc reliably (at 6' nominal precision). You could
    stick a simple sighting tube on top of one and you have a digital "sextant"
    in no time. But a device like this is not that useful for manual
    observations. It abandons the most important trait of the sextant:
    double-reflection. Without double-reflection, we're right back to the days
    of the backstaff or the kamal, and shaky hands are the limiting factor in
    taking sights. So I guess in the end, I would call a device like this a
    "digital kamal" rather than a digital sextant.
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