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    Horizon refraction: photos of changes
    From: Frank Reed
    Date: 2013 Apr 19, 09:07 -0700

    There's a spot on Conanicut Island here, right by the Newport Bridge tolls, where one can look north up Narragansett Bay past a pier two miles out on the north end of Gould Island, then towards the Mount Hope Bridge about 10 miles away, then to the cooling towers of the power plant across from Fall River about 16 miles away, and finally to the apparent horizon which is a ridge of hills about 20 miles out (distances are statute miles). I took a few photos with 20x magnification two days ago and then a few more yesterday. The scale is approximately 20 pixels per minute of arc. The camera location, atop a tripod, was shifted by only a few inches horizontally and about one inch vertically.

    Some differences to notice:
    --There is water visible beyond the pier before the bridge in the first photo. That water was NOT visible the very next day.
    --The bases of the bridge piers are elevated and visible in the first photo. They were hidden the following day.
    --The deck of the bridge, the main roadway of the Mount Hope Bridge, is considerably higher in the first photo.
    --The cooling towers are considerably higher in the first photo.
    --Most noticeably, the distant "hill" horizon is elevated quite a bit and the gap between that line and the underside of the bridge is significantly smaller in the first photo.

    If you can, open the images in an image viewing program that lets you align them and then swap back and forth between them.

    The overall pattern is clear enough. In the first photo, there was more refraction and distant objects were lifted up, allowing me to see into the bay beyond the pier and showing more of the bridge piers. The difference in position is on the order of a minute of arc in ten miles. This is roughly equivalent to the constant, k, for the terrestrial refraction in the equations for dip, etc., changing from 0.16 to 0.26. The increased refraction is not simply proportional to distance as it would be in a model atmosphere with a constant temperature lapse rate. The cooling towers are lifted by almost the same amount as the Mount Hope Bridge even though they're quite a bit further away. Also notice that the tops of the towers are not shifted as much as the bases. Both of these were days with unremarkable, pleasant weather. There was no hint at all that they might have significantly different refraction constants. I was not expecting to get such clear results in just two days of photography.


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