A moon halo spotted over London. The phenomenon is caused by ice crystals in the atmosphere, specifically when thin cirrus or cirrostratus clouds are present at an altitude of about 20,000 feet (6,100 metres). During cold weather these clouds can contain millions of hexagonal ice crystals, and as light from the moon - which is just reflected sunlight - passes through the atmosphere, it refracts in a particular way within the crystals. The bright ‘star’ to the left of the moon in the image is Jupiter, which is making its closest approach to Earth until 2019 at the moment.
