Halos appear almost scribed on the sky with compass and ruler. Their precision follows from the exact geometry of the sky’s ice crystals. Some are plate-like, some are long columns, others are stranger. Whatever their shape, habit is the precise crystallographer’s term, their faces always slope at the same angles. Moon and sun rays refract and reflect between these never changing planes of ice to chisel halos onto the sky.?
Top: A diffuse 22 degree halo circles the moon. A paraselenic circle crosses it. Moondogs, paraselenae, bark to its left and right.??
The winter moon, unlike the sun, rides high over Sweden. Ice halos surround it in this scene captured by Göran Strand.