It doesn't work like that. The full Moon might change an otherwise Bortle-8 sky to Bortle 9 -- or not.
At a site with little or no artificial light pollution, even a thin Moon alters the sky greatly. At a site with lots of directly visible streetlights overhead, the full Moon is just another streetlight.
Hi Tony, as always I read your posts with great interest. I must say that even though I am in what has been evaluated as a Bortle 8 zone (Milano, town center… shows grayish-white on a map) the full moon yesterday made a very big difference. As said, I can usually see the major constellations without any trouble, and if I strain a little on a good night I can see Beta Mon naked eye. Yesterday night I could see clearly Capella, Sirius… barely Polaris. My guesses from reading your post:
1. Bortle ratings are always very "rule of thumb", and in fact, from my 5th floor balcony above the street lights, my situation is better than one would surmise on moonless nights – so when the Moon is out, there's more difference. I'm actually pretty convinced of this.
2. MeteoBlue gave the night as entirely cloudless, but there was in fact some humidity or haze in the air. I did not seem to notice while observing the Moon with my telescope, but looking up naked eye the sky was unusually "milky" (not in an attractive way!) and the Moon cast quite a halo around its disk.
I suspect both are true.
(Apologies to OP for piggy-backing on this very interesting discussion!)
Edited by radiofm74, 24 February 2021 - 08:47 AM.