Context: At 12:17 PM IST on April 25, Bengaluru and at all places along the 130  north Latitude experienced a ‘Zero Shadow Day’.

What is ‘Zero Shadow Day’(ZSD)?

• A ZSD 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(highest point in the sky)

• ZSD happens twice a year for locations in the tropics (between the Tropic of Cancer at +23.5 degrees of latitude and the Tropic of Capricorn at -23.5 degrees of latitude). So, places north of Ranchi in India does not have Zero shadow day.

• One ZSD falls during Uttarayan (movement of the Sun from south to north from winter solstice to summer solstice) and one other during Dakshinayan (back from north to south).   

• The dates will vary for different locations on Earth.

• Every point on Earth between the Tropic of Cancer and the Tropic of Capricorn would experience two Zero Shadow Days a year.

• One occurs during the Uttarayan i.e. when the Sun appears to move northwards, and the other occurs during Dakshinayan when the Sun appears to move southwards.

• Uttarayan and Dakshinayan take place because Earth’s rotation axis is tilted at an angle of roughly 23.5° to the axis of revolution around the Sun and Ramanujam explained that the Sun’s location moves from 23.5°N to 23.5°S of Earth’s equator and back. 

• All the places whose latitude equals the angle between the Sun’s location and the equator on a specific day experience zero shadow day.

• Since Zero Shadow Day is restricted to locations between the tropics, the places north of Ranchi in India do not experience this phenomenon.

• Bengaluru will experience another Zero Shadow Day on August 18, 2023.

Why does a ZSD happen?

• Uttarayan and Dakshinayan happen because Earth’s rotation axis is tilted at an angle of roughly 23.5° to the axis of revolution around the Sun.

• When the Sun is at the zenith its rays make the shadow exactly under it, making it look like no shadow.