Sunday, 5 February 2017

2D slices of 3D tangent arc

On November 25, 2016, Sauli Koski photographed Y-pillars over outdoor lights at Levi ski resort. In some of his photos colored arcs are seen along the path of the pillar, the most striking example shown above. I understand these to be intersections of spatial tangent arc in a clumpy diamond dust. The clumps act as surfaces in which the 2D cuts of the 3D tangent arc show up. For reference see e.g. figures 5.3 and 5.4 in Walt Tape's Streetlight halos part I (pdf page 82). The arcs in the photo above have different shapes depending on their location and they represent upside turned lower tangent arcs at different light source elevations. Below is another image. Koski's first and second post in Taivaanvahti have more photos.

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