Sean King captured this 22 degree halo ~75 yards from the rim of Kilauea Crater, Hawaii on October 5 with the moon at first quarter.
©Sean King, shown with permission
The 22 degree radius halo is formed by sunlight, or in this case moonlight, passing through hexagonal ice crystals in cold high cirrus cloud.
a prism side face and leave through another inclined 60 degrees to the first. To form the circular halo rather than a sundog or tangent arc (their ray paths are the same) the crystals must tumble and be randomly oriented.