A ring around the moon, or moon halo, is made by moonlight passing through millions of tiny six-sided ice crystals in thin, high clouds, usually the veil-like cloud called cirrostratus. The crystals bend the light by about 22 degrees, so the ring always has the same size: roughly the span of your outstretched hand, thumb to little finger, held at arm's length. The old saying "ring around the moon, rain soon" has a real basis, because those high ice clouds often arrive ahead of a storm system, but plenty of halos come and go with no rain at all.
How ice crystals make a 22-degree ring
High clouds, often 20,000 feet or more above the ground, are made of ice rather than water droplets. Many of those ice crystals are tiny hexagonal columns, like pencils too short to sharpen. When moonlight passes into one side face of a crystal and out through another, the two faces act like a prism with a 60-degree angle, and the light is bent.
The bend isn't the same for every ray, but there's a minimum: no light gets through that kind of crystal deflected by less than about 22 degrees, and a great deal of it piles up close to that minimum. With countless crystals tumbling at random angles, the light concentrated at 22 degrees reaches your eye from every direction around the Moon at once, and you see a ring. EarthSky notes that the ring's radius is always approximately 22 degrees, whatever the Moon's phase or height.
Three details give a real halo away:
- The sky inside the ring looks darker than the sky outside, because the crystals send almost no light inward of 22 degrees.
- The inner edge is sharper than the outer edge, which fades out softly.
- Lunar halos look mostly white. Solar halos often show a reddish inner edge, but moonlight is too faint for our eyes to pick out much color.
You'll see them most with a bright Moon, near full or gibbous, shining through a thin, even veil of cloud. For tonight's phase, check our moon phase today page, and for the next bright nights, the full moon calendar.
Does a ring around the moon mean rain?
Sometimes, and the reason is the cloud, not the Moon. Cirrus and cirrostratus are often the first clouds to arrive ahead of an approaching weather system, particularly a warm front, which can bring rain or snow within a day or two. So a halo can be an early sign that the weather is about to change.
Folklore turned that into rules, and it often added details that no meteorologist would sign. The 1920 collection Kentucky Superstitions, compiled by Daniel Lindsey Thomas and Lucy Blayney Thomas, recorded dozens of moon-ring sayings from the state, and they disagree with each other cheerfully:
- "Circle around the moon, rain soon; Circle around the sun, rain none."
- "When the moon has a ring around it, there will be as many rainy days that week as there are stars in the ring."
- "If there are seven stars in a circle around the moon, there will be rain for seven hours."
- "A ring around the moon with a star in it brings clear weather."
- "If there is a ring around the moon, rain may be expected. The nearer it is, the sooner will the rain come."
The general warning holds up reasonably well. The star counts don't: how many stars you see inside the ring depends mostly on how thick the cloud is and how bright the Moon is. And the last saying can't work as stated, since a true halo is always the same size. For an actual forecast, the high cloud is a cue to check your weather service, not a substitute for one.
Halo or corona? Two rings that look alike
Not every ring around the moon is a halo. A lunar corona is a set of much smaller rings that hug the Moon closely, often with faint bands of color. It forms differently: by diffraction, as light passes among tiny water droplets (or unusually small ice crystals), often in thin, lower clouds than the ones that make halos. Its colors usually run from bluish white on the inside to reddish on the outside, the reverse of a halo's order. A planetarium guide from Tasmania's Queen Victoria Museum notes that a larger corona means smaller particles, and that pure colors mean the droplets are all about the same size.
The quick test is size. A corona fits well within a hand's width of the Moon, sometimes only a few Moon-widths across. A halo is a big ring with the Moon in the middle and a hand's span of sky between the Moon and the ring.
Moon dogs, moonbows and rarer sights
The same ice clouds can produce other effects on a bright night:
- Moon dogs are bright patches of light about 22 degrees to the left and right of the Moon, on the same level as it. They come from flat, plate-shaped ice crystals drifting horizontally, like falling leaves, and they show up best when the Moon is fairly low. Their daytime cousins are sun dogs.
- The 46-degree halo is a second, much larger and fainter ring, about twice the size of the common one. It's rare, and when you see it the 22-degree halo is usually there too.
- A moonbow is a rainbow made by moonlight, which needs a bright Moon low behind you and rain or spray in front. It usually looks white to the eye, because moonlight is too dim for color vision, though a long camera exposure will show the colors.
How to photograph a moon halo
A 22-degree halo is about 44 degrees across, wider than many people expect, so a normal zoomed-in Moon photo will miss it entirely.
- Use the widest lens you have. On a phone, switch to the ultra-wide camera.
- Hide the Moon. Stand so a tree, chimney or streetlamp post blocks the Moon itself. That cuts the glare and lets the ring show.
- Steady the camera on a wall or fence, or use a small tripod, and try night mode.
- Include the ground. A rooftop or treeline gives a sense of how big the ring is.
What a moon halo means in folklore and spiritual practice
In most of the folklore that survives, a moon ring is a weather sign first and an omen second. The Kentucky collection also lists "a sign of cold weather," and some sayings use the number of rings to count days to rain.
Modern spiritual readers tend to give halos gentler meanings: a sign of protection, a prompt to pause, a nudge to pay attention to what's coming. Those are newer interpretations rather than old traditions, and there's no harm in them as long as they sit next to the forecast rather than replacing it. A halo is one of the most beautiful ordinary things the night sky does, and noticing it is reason enough to stop. If you enjoy reading the sky's moods, our guide to why the moon is orange and why the moon looks bigger cover two more of them.
Questions readers ask
Is a ring around the moon rare?
No. Halos are fairly common wherever thin, icy high clouds pass in front of a bright Moon, in any season. Many people simply don't look up at the right moment, or mistake the ring for a smudge of cloud.
How long does a moon halo last?
As long as the right cloud layer stays in front of the Moon, which can be a few minutes or several hours. If the cloud thickens, the halo fades and the Moon disappears behind it.
What does it mean if there are two rings around the moon?
It could be a 22-degree halo with the rarer 46-degree halo outside it, or a halo with a small corona inside it close to the Moon. Folklore says two rings mean snow within a day, which is a weather saying, not a rule.
Does a moon halo happen in winter?
It can happen at any time of year. The ice crystals form high in the atmosphere, where it's below freezing even in summer, so halos aren't limited to cold weather on the ground.
