Zero-Shadow Day (ZSD) - What, why, how and when
What:
This is a day when at a certain time, close to noon, the sun comes exactly overhead, and therefore we do not have shadow. The exact time differs from place to place. This event occurs only in the area of the Earth within the Tropic of Cancer and Capricorn. It can be seen from anywhere in Puducherry.
Why
Common sense tells us that at noon the sun should be overhead  every day and zero shadow should happen at noon everyday. . But that is not true. Because the sun does not rise in the exact east and set in the exact west every day. Our Earth is tilted by 23.45°, and that it is due to this tilt that we have seasons like summer and winter. This also means that the sun appears to rise to a bit north-east every day, and then after a particular day it starts moving towards south east. So the length of the shadows at noon, change every day. And on two days of the year, it is zero.
Observing the lengths of the shadows of various objects is a great learning experience and a fun activity best done by observing a zero-shadow day. We can calculate our latitude; we can calculate the maximum altitude (height) of the Sun from the horizon by ZSD activities. Â In addition, we can also realise that the Sun appears to rise in the east and sets in the west but in reality the Earth rotates from west to east. This event is useful for overcoming superstitions related to celestial bodies.
This astronomical phenomenon was used in ancient times to accurately measure the length of the year, predict the change of seasons, measure the time of a day using the shadow of a vertically erected pole, and some even measured the radius of the Earth using the same shadow.
How and When
No scientific equipment is required to witness this rare event that occurs only twice a year. It never occurs everywhere on the same day. When our latitude and the declination of the sun on the celestial sphere are equal, a zero-shadow day will occur. To celebrate this awesome and scientifically important day. Every year PSF organizes an event across many schools and in the public for children and adults to experience this. In our Puducherry region, a zero-shadow day will occur on April 21st in the northward direction and on August 21st in the southward direction. See this picture below. This April close to one lakh individuals witnessed it. Â
Dr. Sundararaman
