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Rare lunar halo – an odd-radius halo – over India

Rare lunar halo: A bright light in the center (the moon) with multiple light rings around it against dark sky.
View at EarthSky Community Photos. | Soumyadeep Mukherjee in Kolkata, West Bengal, India, caught this rare lunar halo – an odd-radius halo – on August 21, 2021. See the multiple rings? Plus the image has something extra. That’s Jupiter to the left, and Saturn to the right, of the moon.

A rare lunar halo

The image above shows a rare lunar halo. Most halos around the sun or moon are common 22-degree halos. They’re caused by ice crystals in the upper air. The ice crystals are typically plate- or column-like hexagonal crystals. But the multiple halo you see in Soumyadeep Mukherjee’s image above is different. It comes from pyramidal ice crystals.

According to sky optics guru Les Cowley, pyramidal crystals tumble more in the air. And so they usually form only rather fuzzy circular halos. Les’ page on these halos, which are sometimes called odd-radius halos, mentions halos of 9-, 18-, 20-, 23-, 24- and 35-degree radius: multiple rings around the moon.

Thank you, Soumyadeep, and thank you, Les!

Here’s another photo of lunar pyramidal crystal halos at Les’ website Atmospheric Optics

Here are two more odd-radius halos, plus a simulation, at Les Cowley’s great website Atmospheric Optics.

Another odd-radius halo, below

Overexposed moon with multiple light rings around it against dark sky, with housetops below.
View larger. | EarthSky friend Greg Redfern of the blog What’s Up? The Space Place captured this odd-radius halo in Virginia on June 4, 2017. He wrote to EarthSky: “I have never seen one like this in 5-plus decades of skywatching.” Les Cowley commented on this photo, “In Greg’s image, we have 9-, 18-, 20- and 23-degree radius rings compared to the common 22-degree halo … Note the bright spots above and below the moon on the inner 9-degree halo in Greg’s photo, plus the bright areas to the moon’s left and right on the 18-degree circle [see a diagram here]. These are the pyramidal crystal equivalents of paraselenae, or ‘moondogs.’” Thank you, Greg and Les! Image via Greg Redfern.

Bottom line: Photos and info about multiple halos around the moon – caused by pyramidal ice crystals – called odd-radius halos.

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