Halo summer! ‘Stunning’ ring round the sun a solar sell of warmer days to come

A halo around the sun appears just before midday. The red line on its inside is because red light bends slightly less than blue light. Photo: Eddie Jim Swinburne University research fellow Alan Duffy said 22-degree halos appear different on different planets. Photo: Arsineh Houspian
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Solar halos are relatively common. This one was seen in Sydney last month. Photo: Richk Rycroft

Melbourne Planetarium astronomer Tanya Hill. Photo: Jason South

As if to assure warmth-deprived Melburnians that it’s still up there and on duty, the sun put on a spectacular sky show to mark the first day of summer on Thursday.

A stunning halo encircled the sun at lunchtime, thanks to the presence of tiny ice crystals high in the atmosphere.

Melbourne Planetarium astronomer Tanya Hill said the halo was caused by sunlight passing through the ice crystals, which refracted or bent the light.

“They are high up, part of the thin wispy cirrus clouds,” Dr Hill said.

Known as a 22-degree halo, the phenomenon is visible on as many as 100 days a year – much more frequently than rainbows.

Swinburne University research fellow Alan Duffy said keen observers would note that there was a red inner rim to the halo.

This was created because red light bends slightly less than blue light, which can be seen on the outer rim.

“It’s a beautifully clear image, it’s really stunning,” he said.

Dr Duffy said the ice crystals causing the light to refract were evenly distributed in the atmosphere, some six kilometres from Earth.

Around the width of a human hair, the ice crystals are shaped like hexagonal logs.

The 22-degree halo gets its name from the minimum angle of a bend you get going through the ice crystal.

“It’s like you have nearly a billion of these tiny ice crystals all bending the sunlight at just about the same angle,” he said.

One of the most common types of halo, the optical phenomenon also occurs with moonlight.

In folklore, lunar halos have been suggested as a sign storms are on their way.

However Dr Duffy said there was no evidence this was the case.   

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