Science enthusiasts will measure the shortest shadow at Wednesday noon on the equinox day, when the Sun shines directly overhead as viewed from the Earth, making the night and day equally long.
Equinox occurs twice a year (around March 20 and September 22) when the day and night have approximately equal length. The name equinox is derived from Latin words aequus (equal) and nox (night).
In Delhi, an NGO, SPACE (Science Popularisation Association of Communicators and Educators) will encourage school students and science enthusiasts to observe the equinox day by conducting various experiments using sundial at Bara Pullah here.
Under the NGO's 'Project Paridhi: Measure the Earth Again', students will take actual measurements of the shadows made by the Sun to compute the size of the Earth and its shape as was done centuries ago by Greek astronomer Eratosthenes, the first person to calculate the circumference of the Earth.
"The Project Paridhi will help in calculating the circumference of our planet using shadows of the Sun, backyard tools and simple trigonometry and geometry concepts, SPACE Director C B Devgun said.
On the equinox day, the Sun moves across the celestial equator, which lies directly above the Earth's equator. During the crossing, the Sun rises exactly in the east and sets exactly in the west.
The exact time of the equinoxes are not fixed but fall about six hours later every year, amounting to one full day in four years which in turn makes a leap year, Devgun said.