Why do some moons have halos, and how fast is our galaxy moving?

We explore the curious questions that science can answer

The moon with a halo over the city of Tromso in arctic Norway (Getty/iStock)

Sometimes you can see a halo of light around the moon. What causes this?

There are two possible explanations. It could be a corona – a round patch of brightness, two or three times the diameter of the moon, often with a hint of colour around its edge. This is caused by diffraction of moonlight by tiny drops of water in a thin veil of misty cloud, usually at low levels in our atmosphere. A corona often precedes a warm front, when the thin veil of cloud thickens and produces drizzle, and then possibly heavy rain.

Alternatively, there is an effect that is, in fact, known as a “halo”. This is a much larger, bright circle around the moon (radius 22 degrees) which is only bright at its outer edge. The halo is caused by reflection of moonlight from ice crystals in a thin veil of high-level, cirrus-type cloud. A halo is often seen in spells of fine weather.