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    Re: A lesson in a photo
    From: Frank Reed
    Date: 2016 Aug 28, 10:43 -0700

    David Fleming, you wrote:
    "What I find unusual is the raggedness of the horizon."

    Yes! That's just what I was going for. We all know that the sea horizon becomes unreliable at an angular scale of a fraction of a minute of arc, and this raggedness is part of the problem. It's caused by small variations in refraction. So how big are those ragged bumps in the horizon? Except perhaps in extreme conditions, like in the arctic, they would not be larger than a minute of an arc, probably not bigger than half a minute of arc. But that raggedness is frequently visible with 10x magnification, so on the order of a tenth of a minute of arc or larger would not be unusual. This then sets a very rough angular scale on the image. The "ragged band" is perhaps a quarter of a minute of arc thick. Knowing this angular scale, how far away is the spacecraft under its parachutes? What would be reasonable ranges on that distance?

    Frank Reed

       
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