Bengaluru sees Zero Shadow Day: What is it, why does it happen

What is Zero Shadow Day?

As The Indian Express has explained earlier, for every point on Earth between the Tropic of Cancer and the Tropic of Capricorn, there are two Zero Shadow Days a year.

For Bengaluru, the next one is on August 18. The Zero Shadow Day is restricted to locations between the tropics, and so places north of Ranchi in India are out of it.

“One falls during the Uttarayan when the Sun moves northwards, and the other is during Dakshinayan when the Sun moves southwards,” Niruj Ramanujam, member of the public outreach and education committee of the Astronomical Society of India (ASI), had told The Indian Express in 2018.

Why does a Zero Shadow Day happen?

Uttarayan (movement of the Sun from south to north from winter solstice to summer solstice) and Dakshinayan (back from north to south) happen because Earth’s rotation axis is tilted at an angle of roughly 23.5° to the axis of revolution around the Sun.

Ramanujam explained that the Sun’s location moves from 23.5°N to 23.5°S of Earth’s equator and back. All places whose latitude equals the angle between the Sun’s location and the equator on that day experience zero shadow day, with the shadow beneath an object at local noon.

“We have all studied in school that the Earth’s rotation axis is inclined at 23.5 degrees to the plane of its revolution around the Sun, which is why we have seasons. This also means that the Sun, in its highest point of the day, will move from 23.5 degrees south of the celestial equator to 23.5 degrees north of the equator (Uttarayan), and back again (Dakshinayan), in a year. Of course, the northern most and southern most points are the two solstices, and the crossing of the Sun across the equator are the two equinoxes,” the ASI says on its website.