The halo is a luminous circle seen sometimes to surround the sun or moon. Often two, or even three, circles are seen and occasionally bands of light, crosses, and arcs of circles tangent to or intersecting each other have been observed. Halos frequently show prismatic colors. In other instances they appear simply as white light. Halos are caused by rays of light falling upon crystals of ice, drops of rain, or the minute particles of moisture that form clouds. To understand the phenomenon fully one must understand the changes light undergoes when it comes in contact with new media.
The halo, or nimbus, as it is called, has been used in art by both ancients and moderns to surround the heads of sacred persons. Ancient poets speak of their deities as being crowned with circles of light when they appear to mortal vision. In Christian art the halo, usually in circular form but sometimes as rays forming a cross, is represented about the head of Christ, and of the Virgin Mary, and often about the heads of saints and martyrs.
