The Pondicherry Science Forum on Sunday organised an outreach programme to popularise the Zero Shadow Day at the Petit Seminaire Primary School in Uppalam.
Forum secretary N. Arun said the event was held to dispel wrong understanding of the concept propagated by school texts such as that the day and night are equal on the equinox day in March and September. This is not true for every location. Besides, the definition of noon in some textbooks is given as when the sun is overhead, which actually is not the case in everyday observation, he said.
The long shadows of objects at sunrise start becoming smaller and smaller as the day progresses until the shadow is smallest when it is exactly aligned north-south.
For those between the latitudes 23.5 deg. south (the Tropic of Capricorn) and 23.5 deg. north (the Tropic of Cancer) can experience the sun right overhead at zenith — twice every year, once between December solstice (December 21 and 22) and June solstice (June 20 and 21) and then between June solstice and December solstice. (For Puducherry this falls in April and August)
Those north of Tropic of Cancer (between +23.5 and +90 degree latitude) and south of Tropic of Capricorn (between -23.5 and -90 degree latitude) will never experience the sun right overhead. Mr. Arun said there would be no shadow at noon, when the sun would be exactly at the zenith.
This happens only twice a year. When the latitude of a place equals the declination of the sun, the sun’s rays are perpendicular on this latitude and exactly at noon the sun is right overhead and shadow of a vertical object falls right below, Mr. Arun added.
