Bengaluru to experience Zero Shadow Day today

Bengaluru: Bengaluru will experience Zero Shadow Day on Thursday at exactly 12.17 pm. Objects will cast a shadow exactly downward, causing objects on the ground to have no visible shadow.

According to astronomers at the Indian Institute of Astrophysics, Bengaluru (IIA), this phenomenon occurs twice a year, once around April 24 or 25 and a second time around August 18.

At local noon, the sun will be at its zenith (highest point in the sky), but this highest point is not exactly overhead on most days. The sun transits north or south of it due to the 23.5-degree tilt of the Earth's axis of rotation with respect to the plane of Earth's revolution around the sun. On Zero Shadow Days, however, the sun is directly overhead, causing vertical objects to cast no visible shadows.

When the sun transits directly above the Tropic of Cancer (+23.5° latitude), it is called the summer solstice (around June 21), and when the sun transits directly above the Tropic of Capricorn (-23.5° latitude), it is called the winter solstice (around December 22).

The sun transits across the equator twice a year on days called equinoxes. The arc made by the sun in the sky changes throughout the year. The arc moves southwards starting on the summer solstice and passes over the equator around September 23 (autumnal equinox), subsequently transiting directly over the Tropic of Capricorn during the winter solstice. After this, the arc makes its journey northward. The arc's transit between one tropic and the other takes six months.

Therefore, for people who live in the tropics (+23.5° to -23.5° latitude), the sun passes directly overhead twice a year — once when the arc transits northward and once when the arc transits southward.

Zero Shadow Days occur on different days for different cities when the sun transits directly overhead based on their latitude.

On Zero Shadow Day objects will cast a shadow exactly downward causing objects on the ground to have no visible shadow.

Activities planned at IIA

Talk: 10 am

Hands-on activities: 11.30 am

Computation session: 12.30 pm

IIA is coordinating with a team in Bhopal to compute the diameter of the Earth through shadow length measurement.