The blue sky is definitely polarised but not because of the ash cloud. The blue colour is caused by scattering. You can look at the post about this on this blog last September. The scattering process polarises light. That explains a thing that I noticed - the polarised light was in every direction excpet that close to the Sun. Light close to the Sun is less scattered.
Also, I was surprised by how quickly Mercury disappeared. I was confidently telling a group on Friday night that it was just behind a cloud, but it clearly wasn't. It must have moved a long way across the sky in a week and have changed to a position that is below the horizon. Mercury was the speedy messenger of the gods in mythology. That's why the fast planet got its name. Now I have seen it for myself.
