Polarisation of haloes and double refraction
Published in Weather 32, 467-468 in 1977
G. P. Können
Because of birefringence, 22° halos consist of two polarised components, mutually shifted by 0.11°, which is about one quarter of the angle subtended by the moon. By rotating a polarizer before the naked eye we could be easily observed that the halo's inner boundary is about 0.1° closer to the sun for polarized light for which the E vector is radial with respect to the sun. To our knowledge, observations of this kind have never been published.