A ring around the Moon, often called a lunar halo, is a natural optical phenomenon produced by light interacting with ice crystals high in Earth’s atmosphere. How it forms
- Light from the Moon passes through thin cirrus or cirrostratus clouds. The ice crystals therein act like tiny prisms and lenses, bending, splitting, and rearranging the light to create a circular halo.
- The most common halo is centered on the Moon with a diameter of about 22 degrees. A secondary 44-degree ring can sometimes be seen as well, produced by light interacting with the crystals in specific orientations.
- The effect is analogous to how a lens bends light, which is why the halo has a well-defined, nearly uniform size across sightings.
What to look for and when
- Haloes are most often observed when the sky is clear but high, thin cirrus clouds are present, typically ahead of approaching weather systems. This is why such halos are sometimes associated with changing weather, though they can appear without any weather shift [web sources summarize this link].
- The phenomenon is more readily visible under dark skies away from light pollution, with the halo visible around the Moon even when the Moon is not extremely bright.
Common questions
- Do halos indicate rain? There is folklore about halos signaling upcoming storms, because cirrus clouds often precede weather fronts. However, halos can occur without any weather change, so they are not a reliable forecast by themselves [web sources discuss both the folklore and the meteorological context].
- Can you see a halo around the Sun as well? Yes—a similar 22° halo around the Sun can occur under the same atmospheric conditions, produced by the same ice-crystal processes.
If you’d like, I can tailor a viewing tip so you can spot a lunar halo next time you have a clear night: best around a crescent or gibbous Moon, look for thin, wispy high-altitude clouds, and give your eyes a few minutes to adapt to the dark.
