Zero Shadow is a phenomenon when the sun is exactly overhead and the shadows of symmetrical and vertical objects vanishes. This happens for locations between the tropics and is caused by the northern and southern motion of the sun during the course of a year.
The lack of shadow is beautiful too. And it happens twice a year, for places between +23.5 and -23.5 degrees latitude. The Sun is almost never exactly overhead at noon, but usually transits a bit lower in altitude, a bit to the north or a bit to the south. We have all studied in school that the Earth's rotation axis is inclined at 23.5 degrees to the plane of its revolution around the Sun, which is why we have seasons. This also means that the Sun, in its highest point of the day, will move from 23.5 degrees south of the celestial equator to 23.5 degrees north of the equator (Uttarayan), and back again (Dakshinayan), in a year. Of course, the northern most and southern most points are the two solstices, and the crossing of the Sun across the equator are the two equinoxes.
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. This Zero Shadow Day will clearly be different for different places on earth.
You can find the two Zero Shadow Days at your location using an interactive map.
An android app is also available to tell you the ZSD and local noon for your location or any other place, lets you interactively explore the length of shadows and the position of the Sun for any date, time and place, and also have animations that will help you understand and teach others about the ZSD. To download the app, click here.
Source : Astronomical Society of India
Last Modified : 5/1/2023