The circular halo of the sun that was visible in the morning sky on Monday

Those who had looked up at the sky between 10.30am and 11.30am on Monday were treated to the sight of a rare red-and-blue ring around the sun.

Known as the 22 degree circular halo of the sun, this atmospheric phenomenon is caused when the sun’s rays get deflected through hexagonal ice crystals present in cirrus clouds, said Debiprosad Duari, the director (research and academic) of MP Birla Planetarium.

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

Subrata Manna, a railway employee and an amateur astronomer, was in the middle of his shift at Bally when he noticed the disc.

“It was more beautiful than anything I could have imagined. I showed it to all my colleagues and even called up my 10-year-old son at home and told him to take a look,” said Manna.

The 22 degree halo was last observed in September 2008 in Siliguri. “The occurrence is rare and cannot be predicted,” said Duari.