A Halo around the Moon in Athens
A halo around the Moon. This fairly common sight occurs when high thin clouds containing millions of millions tiny ice crystals cover much of the sky. Image credit: Anthony Ayiomamitis (TWAN)
The above picture was taken in Athens, Greece.
The distant planet Jupiter appears by chance on the halo‘s left.
Each ice crystal acts like a miniature lens. Because most of the crystals have a similar elongated hexagonal shape, light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the Moon Halo. A similar Sun Halo may be visible during the day.
Exactly how ice-crystals form in clouds remains under investigation.