Zero shadow day

A zero shadow day is a day on which the Sun does not cast a shadow of an object at solar noon, when the sun will be exactly at the zenith position. Zero shadow day happens twice a year for locations in the tropics (between the Tropic of Cancer at latitude 23.4° N and the Tropic of Capricorn at 23.4° S). The dates will vary for different locations on Earth. This phenomenon occurs when the Sun's declination becomes equal to the latitude of the location.[1] On a zero shadow day, when the sun crosses the local meridian, the sun's rays will fall exactly vertical relative to an object on the ground and one cannot observe any shadow of that object.[2]

Approximate subsolar point dates vs latitude superimposed on a world map, the example in blue denoting Lahaina Noon in Honolulu

Students performing experiment on a zero shadow day