Trying to photograph a spiderweb against the sun to get a completely forward-scattered image of it led me to the discovery of the “spiderweb-halos” (Foto 1). I got a rather dim image of the web against the darker background, but I was struck by a much brighter circle-like structure. This was the moment of the discovery of “spiderweb-halos”.
All you need to get them is a spiderweb between yourself and the sun and a device to block the sun. I used my self-made filter usually used when photographing halos, made of a small piece of black slide film inserted on one end of a ball pen’s tube. It has the right transparency to show also the contour of the sun disc on the photographs (Foto 2). One of the best pictures I get is this (Foto 3).
The spiderweb-halos belong to the general phenomenon called “Circles of light in treetops” (M. Minnaert, Light and Colors in the Outdoors (1993), §29: Foto). As seen on the pictures, the bright forward scattering part of each thread is that one, which is perpendicular to the sun. These segments form an (interrupted) circle between the spiderweb centre and the sun.
Taking several pictures I discovered the dynamics of the spiderweb-halos (Foto 4). The diameter of the circle increases with increasing distance of the sun from the centre of the web (Foto 5), until the halo becomes a dim and open circle segment when the sun is far outside the web (Foto 6). With the sun in the centre of the web, the whole web will be lit up as a “full halo” (not perfectly realized in Foto 2).
An outer circle segment may also be seen, depending on the construction of the web (Foto 4). Another feature of the spiderweb-halos known to halo observers is the “pillar” between the sun and the center of the web. The pillar may vary in appearance from a truly pillar-like structure (Foto 7) to a triangular segment (Foto 8). The pillar may also be seen outside of the sun (if the sun is located near the centre of the web; Foto 9) and extend to the other side of the center of the web (“counter-pillar”, Foto 7).
The full-scale of “spiderweb-halos” is shown in this very schematic diagram – it visualizes also why I termed them “halo”: all appearances are known also to the atmospheric halo observer.
Author: Christoph Gerber, Heidelberg, Germany