Many Calcuttans were glued to the sky on Tuesday morning when a ring of light appeared around the sun.

Cellphones started clicking on the streets and the shots got posted on social media.

Astrophysicist Debiprosad Duari, who shared some of the pictures he took from his Salt Lake home, said it was a “solar halo or the 22-degree halo”, which he said was visible between 10am and 12.15pm.

“It happens when the overhead sun’s rays pass through low-lying hexagonal ice crystals or crystal-bearing clouds (high cirrus clouds). Sun rays get refracted by 22 degrees because of the hexagonal shape of the crystals. As a result, a bright ring of light forms around the sun,” said Duari.

Cirrus clouds, visible as stray wisps in the sky, are formed when water vapour freezes into ice crystals at altitudes of five to 10km from the earth’s surface.

Since the passage of sunlight through millions of these crystals causes the
white light to get dispersed
in all the colours of the rainbow, one often sees, as was the case on Wednesday, a halo with a reddish tinge on the inside and a bluish one on the outer ring.

“Since sunlight gets refracted or bent by at least 22 degrees, when observed closely, the inner part of the halo looks a little darker than other parts of the sky. The 22-degree halo appears when the sky has patches of thin cirrus or cirrostratus clouds. The ring of light had a faint reddish tinge on the inner edge and a bluish hue in its outer part,” Duari said.

The celestial phenomenon is more likely in summer and monsoon months in India, he said.

Mobile phone pictures of the celestial surprise went viral on social media. “Hello; here we go. The halo in Kolkata,” said a Facebook post accompanied by shots of the ring.