Why does the Moon have a big ring around it sometimes, 30th July southern Hemisphere.?

Answer 1

When the Moon is seen through thin clouds that carry ice crystals (e.g., Cirro-stratus), the light is reflected by ice crystals at an angle of 22 degrees.

Nice picture at
http://antwrp.gsfc.nasa.gov/apod/ap000515.html

Answer 2

Well, the date and the hemisphere has absolutely nothing to do with it at all. But here's a page that explains it quite well.

http://home.hiwaay.net/~krcool/Astro/moon/moonring/

Answer 3

Generaly, that's an indication of ice crystals high in the atmosphere. Sometimes it means the approach of a storm, but generally it's due to crisp, cold conditions high up.