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    Re: Digital Camera Celestial Navigation
    From: Greg Rudzinski
    Date: 2008 Jul 4, 14:23 -0700

         I was able to locate the pixel count under photo info while in
    the edit mode of iPhoto.  Precision is improved by a factor of four
    over the ruler method. Neat! G
    On Jul 4, 2:11�am, frankr...@HistoricalAtlas.net wrote:
    > Greg, you wrote:
    > "Measurements are made using a ruler directly on the laptop screen."
    > Does your software show pixel positions (x,y) when you move your mouse
    > around? That should be a little more accurate than measuring with a ruler.
    > If not, just a little fishing around for graphic software should turn up
    > something that does display the exact pixel location.
    > And:
    > "The almanac diameter of the sun is a given at
    > approximately 32 minutes of arc. Knowing this will allow the ratio of
    > the laptop screen measurements to yield an altitude of the sun's lower
    > limb above the horizon in minutes of arc."
    > For a given level of zoom in your camera, the angular size of pixels (at
    > least near the center of the field of view) is a fixed quantity which you
    > could measure quite accurately. Place a meter stick 34.38 meters from your
    > camera and perpendicular to the line of sight. Take a photo. Then the
    > centimeter marks will be minutes of arc. The "pixels to minutes of arc"
    > ratio will not change (unless you select a different zoom setting). You
    > could get really fancy and measure the slight variation in angular scale
    > across the field of view --there's a little distortion like this.
    > And:
    > "Perform normal sight reduction for the GMT of the sun's photo to get an
    > azimuth and intercept."
    > Very nice! There's something else you can try with photo navigation. Have
    > you ever heard Ken Gebhart talk about getting Sun altitudes by the
    > refractional flattening of the Sun? Suppose you take a digital photo of the
    > Sun a few degrees above the horizon when the horizon is obscured somehow. If
    > you very carefully measure the vertical diameter of the Sun and compare it
    > with the horizontal, you can work backwards from the refraction tables (or
    > formulae) to determine the Sun's altitude. It works, but it requires very
    > accurate measurements. Since many modern digital cameras have fairly high
    > optical zoom magnifications, you might be able to get good results.
    > �-FER
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