How does zero shadow day occur?

Zero shadow day is a special celestial event that occurs twice a year at places between +23.5 and -23.5 degrees latitude. During this time, no shadows appear of any object or living being when the sun is at its highest point in the sky. According to the Astronomical Society of India (ASI), the Sun will not cast a shadow on an object – when it is exactly at the zenith position.

“For people living between +23.5 and -23.5 degrees latitude, the Sun’s declination will be equal to their latitude twice – once during Uttarayan and once during Dakshinayan. On these two days, the Sun will be exactly overhead at noon and will not cast a shadow of an object on the ground,” the ASI wrote on its website.

This phenomenon occurs when the Sun’s declination becomes equal to the latitude of the location. While this happens only for a second, its effect can be seen for up to two minutes.