Zero Shadow Day Event 2015

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Oct 27

2015

by Milan Patel

The event named Zero Shadow Day was organized at Vadodara on 3rd June, 2015. The definition of noon in some text books is given as when the sun is overhead. Which actually is not the case of every day observation? The long shadows of objects at sunrise start becoming smaller and smaller as the day progress until the shadow is smallest.  At this point the shadow 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 (21st/22nd December) and June solstice (20th/21st June) and then between June solstice and December solstice.In India this motion of the sun is clearly identified. The northward travel of the Sun is called Uttatayan and the southward is called Dakshinayan. For 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.On day when latitude of a place equals the declination of the Sun, the sunrays are perpendicular on this latitude and exactly at the 'local' noon the sun is right overhead and shadow of a vertical object falls right below.Having said this we must also consider two points 1) the sun has angular diameter of 30min of arc and 2) closer to the tropic of Cancer or Capricorn the Sun change in declination is rather slow  so observationally it is possible to experience this phenomenon on two days and for more number of day near the tropical lines.This is a very enjoyable phenomenon to watch that carries a high value of edutainment. All one need to do is to keep a vertical object in the sun and watch its shadow falling right below it.