Indian Institute of Astrophysics celebrates Zero Shadow Day

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Indian Institute of Astrophysics in Bengaluru under the blazing Sun celebrated Zero Shadow Day on 24 April 2024. More than 60 people gathered at IIA on this occasion. They engaged in hands-on experiments to measure shadow lengths to verify the phenomenon, and also witnessed some demonstrations set up by the institute, following a talk on the subject. The program was organised by the Science Communication, Public Outreach, and Education Section of IIA. IIA institute is an autonomous research institution under the Department of Science and Technology of Government of India.

The Sun does not rise exactly in the east or set exactly in the west every day, and neither does it pass directly overhead every day. This is because of the axial tilt of the Earth of 23.5 degrees, which is the cause of seasons as well. The Sun is directly overhead the Tropic of Cancer on 21 June (summer solstice in the northern hemisphere) and then journeys southwards as seen from the Earth, called Dakshinayana. The Sun is then directly overhead the Equator on 23 September (autumnal equinox for northern hemisphere), and is overhead the Tropic of Capricorn on 22 December. It is then seen to journey northwards, called Uttarayana, and is overhead the Equator again on 21 March, and so on. Hence, the Sun is directly overhead locations between the two tropics twice a year, and this day varies with latitude.

When the Sun is directly overhead, the shadow of a vertical object falls under it, and hence no shadow will be seen at local noon. Hence, this is called the Zero Shadow Day (ZSD). ZSD for Bengaluru (and places at the same latitude like Mangaluru and Chennai) occurs on 24 April and 18 August. Local noon in Bengaluru on 24 April is at 12:17, when the Sun will be directly overhead.

IIA invited students and the public to witness the ZSD phenomenon, and more than 60 people including students from Surana College attended the event. After a talk on the subject, they stationed themselves at various exhibits. Four gnomons (vertical sticks on the ground, called Shanku in Indian astronomy) were kept ready for them to mark the shadow every few minutes, and they were able to verify that the shadow was indeed absent at local noon. In addition, there were demonstrations arranged by the institute where, for example, the shadows of vertical hollow pipes became exact circles, and sunlight passed through multiple holes aligned vertically to form patterns on the ground, at local noon.

“In addition, we partnered with Aryabhat Foundation in Bhopal, which is roughly at the same longitude value as Bengaluru. Their students measured the length of the shadow at our local noon and were able to tell our participants its value”, said Niruj Mohan Ramanujam, the Head of science communication and outreach at IIA. Using the distance to Bhopal and the method of Eratosthenes devised in 240 BC, the participants at IIA were able to calculate the radius of the Earth to within 7% accuracy with just this simple equipment. Such an exercise has been carried out by pairs of cities across the world during ZSD as well. “We had also organised a similar public event at Mysuru on 22 April, which is the ZSD for Mysuru, under our COSMOS project”, added Niruj Mohan Ramanujam. Zero Shadow Day was also celebrated on 16 April at Kodaikanal Solar Observatory, which is operated by IIA.

Annapurni Subramaniam, Director of IIA, said “This is an excellent occasion for students to understand the motion of the Sun and the Earth, and also to make astronomical measurements themselves using simple equipment”

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