A lunar halo usually forms when moonlight passes through tiny, six-sided ice crystals in high, thin clouds. The crystals refract the light, creating a broad ring that commonly appears about 22 degrees from the Moon.
Ice crystals bend moonlight
High clouds can contain flat or column-shaped hexagonal ice crystals. As moonlight enters and exits a crystal, it bends through the ice. Because many crystals have similar angles, light reaching your eye is concentrated into a ring around the Moon. [1]
Why the ring has a familiar size
The common halo is about 22 degrees in radius. That angle comes from how light refracts through the geometry of ice crystals; it is not a physical circle surrounding the Moon. [2]
Why the halo can look brighter in one place
Cloud thickness, crystal orientation, and the Moon’s brightness affect how visible the ring is. The crystals are usually oriented randomly, so the halo appears circular even though each individual crystal has a particular shape.
Does a lunar halo mean rain is certain?
No. The high clouds that create a halo can accompany changing weather, but the optical display alone does not guarantee rain at a particular place or time.
Sources and evidence
- NASA Astronomy Picture of the Day — A Halo Around the Moon ↗
Explains refraction through elongated hexagonal ice crystals and the 22-degree radius of a lunar halo.
- NASA Earth Observatory — A Halo Above the Horizon ↗
Describes ice crystals refracting light to create halo-like features.
Sources consulted: 15 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; the explanation was checked against linked institutional, medical, or scientific references. This article was prepared with AI assistance and has not received independent expert review.