When the Sun is exactly overhead on a specific location, it is referred to as a Zero Shadow Day. On this day sun rays hit the surface at a perpendicular angle to the surface. So at a specific point, one cannot observe the shadow of anything around.
A zero shadow day is an astronomical event. On this day the Sun does not cast shadow of an object at noon, when it will be exactly at 'Your Zenith'. Zero shadow day happens twice in a year at tropical regions.
Between the Tropic of Cancer (+23.5) and Tropic of Capricorn (-23.5) are the areas where you may observe it. The dates will be different for different regions on the Earth.
Zenith is an imaginary point directly overhead from a particular point on the ground. It lies on an imaginary celestial sphere. It is also expressed as exact opposite directional point opposite to the apparent gravitational force at that location. In simple words it is a "highest" point overhead on the celestial sphere.
When the Sun comes at this point you experience Zero Shadow Day at your location.
Here is a setup guide to observe Zero Shadow Day from your location. It can be observed from your terrace or balcony. In the wake of current COVID 19 situation, it is not adviced to venture out. Use your discretion while safely observing the event. Do not look at the Sun with un-aided eyes, observing the Sun is not a part of this activity.
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Share pictures with us on Twitter by using hashtag #zeroshadowday and @boxofscience
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Check this list to know dates for ZSD event at your place. We have tried to cover many regions. If your place is not listed, see any nearby district or city that is listed here. Else write us for details, we will facilitate you with for any further information.