The parhelic circle is not the ring around the Moon; that's a 22-degree halo. Instead it is the almost-horizontal arc cutting through the Moon. The rest of the parhelic circle was too big to fit in the field of view of Strand's camera.
Parhelic circles are formed by sunlight reflecting from the vertical faces of ice crystals--millions of them floating in thin cirrus clouds spread almost evenly across the sky. In this case, bright moonlight + cirrus clouds performed the same trick.
Note: Because this circle is caused by moonlight rather than sunlight, it is most correctly called a "parselenic circle."