This rare phenomenon is caused by light passing through high-altitude cirrus clouds. The sight only occurs if the cloud's hexagonal ice crystals are shaped like thick plates with their faces parallel to the ground. When light enters through a vertical side face of such an ice crystal and leaves from the bottom face, it refracts in the same way that light passes through a prism. If the crystals are aligned just right, the whole cloud lights up in a spectrum of colors. Normally only visible during daylight hours, this arc was invisible to the naked eye. I was photographing an interesting ridgeline at the spot where the moon had just dipped behind the horizon. Not until viewing the camera's LCD screen after the 30-second exposure did I realize I had captured the arc.