A
halo of bright, somewhat colourful sky, forming a circle about with an
angular radius of about 22½º degrees around the
sun or
moon.
It is caused by refraction through randomly-oriented ice crystals. A cirrostratus very often gives rise to this; sometimes scattered cirrus clouds do the same. Either way, it is often a sign of low pressure approaching, creating ice-crystal clouds as it does so. Expect rain and wind and cold within the next day or two. Just don't be too surprised if they don't turn up.