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Dr. Debiprosad Duari : Director, Research and Academic at M.P Birla Institute of Fundamental Research. Kolkata, 5th. June 2021.Whenever there is light, there should be a shadow. But people, all around the world, living between the Tropic of Cancer (+23.5 degrees latitude) & Tropic of Capricorn (-25 degrees latitude), loses their shadows, albeit momentarily, twice a year. Those two days are called Zero Shadow Days. For Kolkata the first zero shadow day was on 5th June, 2021 and the zero shadow moment was at around 11:35 hrs.

Young students and enthusiasts in spite of the critical situation all around because of the pandemic showed interests and enthusiasm to observe this phenomenon. This event also reminds us of motions and position of celestial bodies with respect to Earth which have direct links to events on Earth like change of seasons, tides, eclipses and zero shadow moments . It reminds us also, maybe momentarily, our presence on Earth with a cosmic perspective.
Zero Shadow Day is a special celestial event that occurs twice a year. During this time, no shadows appear of any object or living being when the sun is at its highest point in the sky. The lack of shadow is beautiful too. And it happens twice a year, for places between +23.5 and -23.5 degrees latitude. The Sun is almost never exactly overhead at noon, but usually transits a bit lower in altitude, a bit to the north or a bit to the south. The Earth’s rotation axis is inclined at 23.5 degrees to the plane of its revolution around the Sun, which is why we have seasons. This also means that the Sun, in its highest point on a particular day, will move from 23.5 degrees south of the celestial equator to 23.5 degrees north of the equator, and back again in a year. On a zero shadow day, when the sun crosses the almost overhead position, the sun’s rays will fall exactly vertical relative to an object on the ground and one cannot observe any shadow of that object. On these two days, the Sun will be exactly overhead at noon and will not cast a shadow of an object on the ground. This Zero Shadow Day will clearly be different for different places on earth.

This year, from Salt Lake, in the north eastern fringe of Kolkata, the zero shadow day and the moment was observed mostly from the rooftops and terraces. It was cloudy at times, but the beauty of the event could be captured through the camera.
The exact zero shadow moment at Kolkata was 11:35 hrs. On its return path (called Dakshinayan) Sun again will be exactly overhead in Kolkata casting no shadows, on 7th July, at around 11:41 hrs.

Astronomical Society of India Public Outreach & Education Committee (ASI POEC) has created huge number of resource material on the subject and visibility from different locations in India. A beautiful mobile App can be downloaded, created by one of the prominent member, Alok Mandavgane , which gives detailed map of the event across India.

Pictures : Taken by Debiprosad Duari, from the rooftop in Salt Lake at different moments, ending in the Zero Shadow Moment, 11:35 hrs. 9:45 to show the decrease in shadow length, ultimately becoming zero at the zero shadow moment. The Sky was not very clear, but gave the opportunity to take images. Even a plastic basket cast a perfect symmetric shadow at the moment.