Last night(after I was long in bed) my good friend and swim training partner Jake sent me this photo on Facebook with the question: “Why is there a ring behind the moon?” It was a great photo of the phenomena so I thought it would be a great opportunity to explain it to everyone who might have wondered.
In the photo you can see a subtle light ring around the moon nearly to the edge of the photo. This effect is known as a Corona.
A lunar corona is produced by the diffraction of the moon’s light through small water droplets and or ice crystals, usually high up in the atmosphere. What does this actually mean? The light waves coming from the moon encounter the water droplets or ice in a thin cloud and bend around them. In certain situations this bending around the obstacles, known as diffraction will produce a ring pattern as seen in the photo.
This effect can also occur with the sun’s light during day time, when the sun is shining through a thin layer of cloud. The effect is often far greater at day time, with colourful patterns often appearing. The colour patterns can also occur with lunar corona, but are usually less noticeable.
Other visual effects caused by ice crystals and water droplets are halos, sun dogs and light pillars. More information on atmospheric coronas can be found in this Wikipedia article.
The best time to see these sorts of atmospheric phenomena is when there is a thin layer of high ice clouds. Hope this answers your question!