On July 22, parts of India will witness a total solar eclipse in the early hours of the day. Debiprosad Duari, the director of research & academic at MP Birla Planetarium, decodes the total eclipse of the sun for Metro and explains why Calcutta will be partially denied

Q: How rare is this eclipse?

A: This is rare and spectacular, since one will have to wait another 105 years for the next total eclipse of the sun — to be seen again from the Indian subcontinent on June 3, 2114.

Also, the uniqueness of this particular solar eclipse is that it will be of the longest duration in this century. At its maximum, the total eclipse will continue for 6 minutes 39 seconds over a region on the Pacific Ocean. In India the duration of the eclipse will be an average of three-and-a-half minutes, with the maximum being at Cooch Behar, lasting 3 minutes and 53 seconds.

Q: From where will the total eclipse be visible in this part of the world?

A: People keen to view this eclipse should find a location from where the eastern horizon is clearly visible. The zone of totality will start from the north of Malda and cover the entire northern part of the state. The central line of totality will pass near Siliguri, Jalpaiguri, Cooch Behar, Darjeeling and its neighbouring areas.

Q: Why will Calcutta witness only a partial eclipse?

A: A solar eclipse happens when the sun, moon and the earth come in a straight line. The shadow of the moon has two distinct regions, called the umbra, a conical-shaped area where the shadow is darkest and no light from the sun can reach, and the other is the penumbra, where light rays from the sun partially illuminate the shadowed region.

Only when the tip of the umbral cone touches a particular region on the surface of the earth can people in that region experience a total solar eclipse, whereas the penumbral shadow falling on earth produces a partial eclipse. That is what skygazers in Calcutta will witness.

Q: When will the eclipse be most visible in Calcutta?

A: The obscuration in the Calcutta sky will be about 90 per cent starting at 5.28am and ending at 7.30am. The maximum eclipse in Calcutta will be at 6.26am, when the sun will be obscured by 90.2 per cent. At the maximum phase of the eclipse in Calcutta, the sun will be at an altitude of about 17 degrees from the eastern horizon.

Q: How should one view the eclipse?

A: One must be cautious and not look at the sun directly when the eclipse is in its partial phase. The best way is either to see the phenomenon through a No. 14 Welder’s Glass or aluminised Mylar filter, which reduces the intensity of the sun’s glare. The other option can be to project the image of the sun on a clean surface.

Enthusiasts can make pin holes and project the sun on to a plain surface and observe the projected image of the sun.

Only when the sun is completely obscured by the moon can one look directly at the totally eclipsed sun, so skygazers in Calcutta should avoid staring at the sun directly.

Q: Isn’t the monsoon a bother?

A: One can only hope that clouds don’t play spoilsport and blur one of the most interesting and beautiful celestial phenomena of the century.