Moon halos, moonbows and other lunar sky effects

A page about the Moon's sky effects has lived at this address since 2004. It has been rewritten from scratch to cover what we know and how we photograph these things today.

Most of the Moon's best tricks have nothing to do with the Moon itself. They happen a few kilometres overhead, in ice crystals and water droplets, and the Moon simply supplies the light.

The 22-degree halo

The classic "ring around the Moon" is a large circle about 22 degrees in radius, roughly the span of an outstretched hand from thumb tip to little finger. It forms when moonlight passes through tiny hexagonal ice crystals in thin, high cirrostratus cloud. Each crystal acts as a small prism, and the geometry of a six-sided column bends light by a minimum of about 22 degrees, so light piles up at that angle and forms the ring. The sky just inside the ring often looks darker than the sky outside it.

There is an old rule that a ring around the Moon means rain is on the way. It has some truth to it. Cirrostratus often runs ahead of an approaching warm front, so a halo can arrive a day or so before cloud thickens and rain sets in. It is a hint, not a forecast, and a quick look at MetService will tell you more.

Corona or halo?

People often mix up halos with the lunar corona. A corona is much smaller, hugging the Moon within a few degrees, and it is made by water droplets rather than ice. Light diffracts around the droplets and produces a soft bright disc, sometimes with faint coloured rings, bluish inside and reddish outside. Coronas show up in thin altocumulus or a patchy layer of mist. If the cloud droplets are all a similar size the colours become surprisingly clean.

A rough test: if you can cover the effect with your thumb at arm's length, it is a corona. If you need to spread your whole hand, it is a halo.

Moonbows

A moonbow is a rainbow lit by the Moon. It needs three things together:

• a bright Moon, within a few days of full
• a low Moon, less than about 42 degrees up, so the bow clears the horizon
• rain, spray or mist in the part of the sky opposite the Moon

Because moonlight is faint, our eyes see a moonbow through rod vision and it usually looks pale grey or white. A camera on a tripod, given a few seconds of exposure, records the full colours. Waterfalls are reliable moonbow makers, since they supply spray on demand. Stand with the Moon at your back and the spray in front, and on a clear full-moon night in a gorge you may be rewarded.

The moon illusion

A rising full Moon looks enormous over the hills or the sea, then seems to shrink as it climbs. Measure it and you will find it is the same size, or even fractionally smaller near the horizon, because it is then slightly further from you. The effect lives in our brains, which judge size against nearby landscape and the perceived "flattened dome" of the sky. Atmospheric refraction does squash the Moon a little near the horizon, but it never enlarges it.

Photographing lunar effects

A halo is wide, so a wide-angle lens is your friend. Some practical starting points:

• Use a tripod and manual focus set on a bright star or the Moon itself.
• Start around ISO 400 to 800, f/4, and a couple of seconds, then adjust. The Moon will overexpose; that is fine for a halo shot.
• Put something in the foreground, such as a tree line or roof, to give scale.
• For moonbows, try 5 to 20 seconds at ISO 800 to 1600 and expect to experiment.
• Shoot raw if you can, so you can recover colour in the cloud later.

Phones in night mode do a decent job on halos and coronas, though they tend to brighten everything and lose subtle colour.

For diagrams and a vast catalogue of rarer arcs, the Atmospheric Optics site is the standard reference. Daytime cousins of these effects are covered on our sun halos and sundogs page, and the Moon's darker moments on our lunar eclipse notes.