At 12.07 pm on April 24, children brought together by Chennai-based NGO Pudiyador jumped in the air to be able to see their shadows. The sun was at its zenith, their shadow exactly under them. “Lie down on your back and imagine the sky is a giant dome,” a poster made by an assistant professor at the Chennai Mathematical Institute explained. “The highest point of this sky dome is called the zenith. When the sun reaches the zenith, your shadow will be exactly under you! This happens only twice a year, on Zero Shadow Days.”
What is Zero Shadow Day?
For every point on Earth between the Tropic of Cancer and the Tropic of Capricorn, there are two ZSDs a year. For Chennai, for instance, the next one is on August 18. The ZSD is restricted to locations between the tropics; places north of Ranchi in India are out of it. “One falls during the Uttarayan when the Sun moves northwards, and the other is during Dakshinayan when the Sun moves southwards,” says Niruj Ramanujam, member of the public outreach and education committee of the Astronomical Society of India (ASI). It was this committee that put out a public call, asking for people to participate in ZSD.
Why does ZSD happen?
Uttarayan (movement of the Sun from south to north from winter solstice to summer solstice) and Dakshinayan (back from north to south) happen because Earth’s rotation axis is tilted at an angle of roughly 23.5° to the axis of revolution around the Sun. Ramanujam explains that the Sun’s location moves from 23.5°N to 23.5°S of Earth’s equator and back. All places whose latitude equals the angle between the Sun’s location and the equator on that day will experience Zero Shadow Day, with the shadow beneath an object at local noon.
If it has always been happening twice a year, why the new focus?
Since last year, scientists have been looking at ZSD as a fun way — jumping in the air to see one’s own shadow, lodging a stick vertically into level ground to see how its shadows play around — to help school students as well as adults learn about the motions of the earth around the Sun, the seasons, and the length of days among several concepts. These concepts can be made simple to understand, the ASI feels. “Measuring shadows involves a lot of things that students learn in schools. Geography, basic astronomy, optics, basic geometry and trigonometry, concepts that through this experiment can be learnt in a fun and hands-on way,” says Ramanujam.
Why push a stick into the earth?
This is a concept practised in many civilisations and associated with Shanku, an Indian astronomical instrument used by mathematician-astronomer Aryabhata. Ramanujam goes further back, to 200 BC and Eratosthenes, the Greek astronomer, who was living in Egypt. “He observed the shadows of two vertical poles in two cities on the same day. He knew the length of the shadow at noon in the city he was living, and based on these two numbers he calculated the diameter of the earth,” Ramanujan said. “Essentially, schoolchildren can understand what is perceived to be fairly complicated concepts by measuring the shadow of a stick on the ground.”
What is the ASI’s public outreach programme about?
Last year, the ASI decided to make ZSD a national campaign for parts of India south of the Tropic of Cancer. Since then, it has organised several workshops to train teachers and offered free resource material in English as well as various Indian languages. The ASI has also launched a free Android app that provides information on ZSD, and its website offers upcoming dates for other parts of the country and an interactive map.
This year, the outreach programme has led to the Institute of Mathematical Sciences in Chennai getting involved. “We worked with the NGO Pudiyador that works closely with the fisherfolk community and runs an after-school programme with children. This activity was part of their on-going summer programme for children,” says IMSc outreach associate Varuni P. “Since the 3D arcs and paths make it a difficult concept to describe orally, we drew 3D diagrams on poster to help us explain the phenomena more clearly.” The IMSc also appealed to Chennai residents through its Twitter page to go out at 12.07 pm on April 24 and check for shadows.
High Jump at High Noon
At 12:07 on April 24, Zero Shadow Day in Chennai, the sun was at its zenith and the shadows directly under the children, who had to jump in order to be able to see their shadows
Twice a Year: At any place south of Tropic of Cancer in India, Zero Shadow Day occurs once during Dakshinayan (June 21-December22, when the Sun’s arc moves southward) and once during Uttarayan (December 22-June 21, Sun’s arc moves northward).
ZERO SHADOW: The moment arrives when the sun reaches its zenith, or the highest point in the arc it describes over Earth.
Source: Institute of Mathematical Sciences & Chennai Mathematical Institute