Tamil Nadu - Madurai   

Madurai witnesses ‘22 degree halo’

— Photo: G. Moorthy

Optical treat: The ‘22 degree Halo’ seen around the sun in Madurai on Friday.

MADURAI: Residents of the city were witness to a celestial phenomenon rare for the region. In the forenoon of Friday, they saw a colourful ring around the sun that attracted curious onlookers, especially children. Since it was a working day, children came out of their classrooms to look at the sky. More curious ones went home to cull out information about the sun with a ring. Two colours in the ring, red and blue, were clearly visible to the naked eye.

According to S. Krishnasamy, State president, Tamil Nadu Science Forum, it is a regional phenomenon as it arises from hexagonal ice crystals formed in cirrus clouds. The formation of a 22 degree circle around the sun, an optical phenomenon, is called a ‘22 degree halo.’ It is formed by the refraction of sunlight in hexagonal ice crystals suspended in the atmosphere.

Scientific material on ‘22 degree halo’ available in Wikipedia says that when a light beam passes through two sides of a prism forming a 60 degree angle the angle of minimum deviation is almost 22 degrees. It is 21.54 degrees for red and 22.37 degrees for blue.

“This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish. Light passing through the hexagonal ice prisms is deflected twice which produces deviation angles ranging from 22 degrees to 50 degrees. Lesser deviation results in a brighter halo along the inner edge of the circle, while greater deviation contributes to the weaker outer part of the halo. As no light is refracted at smaller angles than 22 degrees the sky is darker inside the halo.”

Similar halo could be occasionally seen around the moon in the night due to the same phenomenon, Dr. Krishnasamy said.

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