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    Re: Lunars: 10/27/2013
    From: Frank Reed
    Date: 2013 Oct 29, 11:47 -0700

    "Could a r/r track vanishing point be used as a horizon? I think there is a little bit of an optical illusion to the vanishing point."

    Only if you know in advance that the tracks are level. If the tracks are climbing a 1° grade, then the vanishing point will be 1° above the true horizon. But you can certainly use this trick in cities with modern buildings. Take any pair of floors (or rows of windows or similar) in a properly constructed building and extend the converging perpsective lines of those floors until they cross. That point lies on the true horizon. If you can then find some feature to serve as a pseudo-horizon in that direction (maybe a roof-line of a distant building), you can then measure the angular offset from the true horizon to the pseudo-horizon and use that for sights with the offset as an effective dip.

    You mentioned the "optical illusion" of the vanishing point. I wouldn't call it an optical illusion really. The problem is that we never can see to the vanishing point since the Earth is curved. So even if you're looking down perfectly straight railroad tracks on a salt flat that you know is flat as defined by the last water that created it, it's still a curved surface because the Earth is nearly a sphere (or more accurately because the gravitational equipotential is nearly a sphere). You can use any small-ish nearby length of the track and extend the parallel lines to "infinity" visually. That will point to the true horizon. But the tracks themselves, if you look out far enough, will begin to curve away from those lines as they follow the curvature of the Earth. Eventually, at the visible horizon, the tracks will disappear over the curve, naturally before they have actually "converged". If the horizon is 3 nautical miles away (about 18000 feet) and the track gauge is about 5 feet, then the gap between straight, perfectly level rails at the horizon would be very nearly one minute of arc, easily resolved with binoculars or a small telescope.


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