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© APThis photo of a sun halo viewed from behind a street lamp was actually taken in Kolkata on July 5, 2010.
On Friday morning, residents of the city and its suburbs witnessed a natural phenomenon around 11.15 am. A beautiful, bright halo was seen around the sun, forming a perfect circular ring. The rare sighting triggered intense excitement as word on the celestial wonder spread. The ring lasted about an hour, after which it gradually faded away.

According to MP Birla Planetarium director (research & academic) Debiprosad Duari, the angular distance between the sun and the ring was almost 22'. "This celestial feature, that is sometimes known as 22' halo of the sun, occurs due to refraction of light against ice-crystals present in the thin wisp-like strands of clouds in the sky," he said.

The clouds, known as cirrus, are so extensive at times that they are virtually indistinguishable from one another, forming a sheet of cirrus called cirrostratus at altitudes of 5-10 km. The ice crystals formed in these clouds often take the shape of hexagonal crystals and sunlight passing through these gets refracted by an amount of 22', thereby causing the ring-like shape.

"If one noticed closely, one would have seen the ring was slightly redder in the inside and bluish on the outer edges. The area within the ring was also less brighter compared to other parts of the sky. This happens because the sunrays passing through these ice crystals on the cirrus clouds behave like prisms and deflect light by at least 22' and, hence, no light gets deflected within a region of 22'," Duari added.

Scientists have shown that the geometry of the ring or halo is such that its size is roughly 44 times the size of the sun. This natural phenomenon can happen anytime during the day if the sky is covered by cirrus clouds and the atmospheric conditions are right.