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    Finding the true horizon
    From: Frank Reed
    Date: 2013 Mar 26, 09:25 -0700

    The true horizon is invisible but easily defined. You find the vertical (gravitationally, with a level or a plumb line or similar) at some point on the Earth's surface and then you locate all points that are 90° from that vertical line. That's the true horizon. It's important to note that this horizon and the vertical it's derived from are locally variable. It's only good luck for us earthling navigators that the Earth's gravity field is rather smooth, closely approximating a spheroid. If we lived on a gravitationally lumpy world, celestial navigation would still be possible, but the connection between coordinates drawn on the sphere and the readings of celestial navigation would be much more complicated.

    There's a trick for finding the true horizon that works extremely well in some cities. Buildings are constructed "plumb". The floors are supposed to be level. If you live in a city with modern buildings, and if those buildings aren't in danger of collapse or settling over many decades, then you can count on floors being level. If you look into the distance and sight down the side of a building, perspective will lead you right to the true horizon. This is especially easy in photographs. I've attached a photo trimmed from a random photo looking down an avenue in New York City. The lines drawn from floors on the building on the left converge in the distance. That crossing point is located right on the true horizon for this observer's location. Note that a building at great distance wouldn't work since there would be some rotation due to the curvature of the Earth.

    You can use this trick in photographic experiments in astronomical position-finding. Get the horizon from the lines of neighboring buildings. Note that this technique finds the true horizon so, of course, there's no correction for dip, refracted or otherwise.


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