The city and its surroundings in the North and South 24 Parganas district witnessed a celestial phenomenon of a perfect ring with the sun at the centre on Wednesday morning.
Hundreds of curious sky gazers saw a dazzling halo of the sun starting at around 10:45 AM and continuing for nearly over an hour and 15 minutes till around noon.
This is known as ‘22 degree halo of the sun’, because the angular distance of the light ring is exactly 22 degrees from the Sun at the centre, Director, Research and Academic, MP Birla Institute of Fundamental Research, MP Birla Planetarium Dr Debiprosad Duari told PTI on Wednesday.
It occured because of interaction of sunlight with the ice crystals suspended in the air in a particular type of cloud, he said.
“The ice crystals are responsible for halos which are typically suspended in cirrus or cirrostratus clouds in the upper troposphere, which are present typically at a height of 5-8 km from the earth’s surface,” he said.
According to Duari, the shape and the orientation of the crystals are also responsible for the type of halo.
“It is hypothesised that the water ice crystals present in the cirrus clouds are typically hexagonal in shape.
When sunlight falls on these crystals, passing through hexagonal crystals it gets refracted and bent by 22 degrees or more resulting in the formation of a bright ring of light around the sun at an angle of 22 degrees,” he said.
The passage of the sunlight through millions of these crystals causes the white light to get dispersed in all the colours of the rainbow.
“This colour of a rainbow was witnessed in Wedensday’s halo. It was with a reddish tinge on the inside and a bluish tinge at the outer part of the lighted ring. Since all the sunlight gets refracted by at least 22 degrees the inner part of the halo looks a little darker than other parts of the sky,” Duari said.
The phenomenon was not “very rare” and occurs whenever there is moisture laden Cirrus clouds in the sky near or around the sun and is a local phenomenon. “This happens quite regularly at different places depending upon the favourable atmospheric conditions,” he said.
The duration of the ‘22 degree halo of the sun’ depends on the cloud formations and the air currents in the atmosphere, he added.