A halo, or Sun-ring, is caused by sunlight refracting through ice crystals in thin cirrus or cirrostratus clouds in the upper troposphere, typically 5 to 10 km above the ground. The image above is an example of this phenomenon, snapped last month above the Canberra Deep Space Communication Complex (CDSCC) 70-metre antenna, Deep Space Station 43. (Image credit: @CanberraDSN)