What does it mean?
The sun will be exactly overhead at any place and will cast almost no shadow of objects on the ground, two times a year, between places 23.5 degree latitude on both hemispheres. On this day, in a place, a vertical pole will cast no shadow.
When will it occur?
At 12.25 p.m on April 14 in Kochi and nearby places and on adjacent days on other parts of Kerala. Watching Zero Shadow Day (ZSD) during the lockdown at the front yard or the terrace of the house will be an interesting activity, and it can trigger discussions on science.
How can you observe the phenomenon?
One can just stand in the sun and observe his shadow becoming zero or can use a vertical pole fixed to the ground, or keep a hollow tube upright on a clean sheet of glass, which is balanced perfectly horizontal and watch the exact thin circular shadow cast during the zero shadow time. For better viewing, put the glass plate on four inverted paper cups placed at its corners. It is important that the end of the tube that would rest on the glass should be perfectly perpendicular to its axis.
Text by Joe Jacob, associate professor of physics, Newman College, Thodupuzha.
