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    Re: A little space navigation
    From: Frank Reed
    Date: 2018 Oct 8, 11:44 -0700

    David,

    You're absolutely correct that we can determine altitude from this image (even if it's photo-realistic artwork --always possible!). As you  have described, the trick, which I had hinted at in my original posting of the image, depends on a perspective effect. When you're closer to a planet, the proportion of the apparent disk of the planet occupied by features at the center of the disk (like North America in this case) increases. This is entirely independent of the total angular size of the planet and therefore independent of magnification. It's only useful when the observer is nearer than a few planetary diameters. This diagram may help others to see what's going on...

    Frank Reed

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