Marking the beginning of spring, the Sun will cross the plane of the Earth's equator tomorrow morning, making day and night of approximately the same duration.
The event known as equinox is eagerly awaited by space enthusiasts as it takes place only twice a year, on March 21 and September 23.
On the equinox, the sun moves across the celestial equator, which lies directly above the Earth's equator.
When the Sun crosses the plane, it rises exactly in the east and sets exactly in the west.
"This year the phenomenon (vernal equinox) will take place at 4:51 am (23:21 UT)," Science Popularisation Association of Communicators and Educators (SPACE) president Chander Bhushan Devgun said.
SPACE, an NGO, has planned a programme at Jantar Mantar in New Delhi on the occasion.
Telescopes and solar filters will be set up for solar observations.
Skywatchers and space enthusiasts will celebrate equinox day on Monday by taking measurements of the shortest shadow at noon and also calculate the latitude of the place.
"We will celebrate equinox day by measuring the circumference of the earth," Devgun said.
The word equinox is derived from the Latin words aequus (equal) and nox (night). It means equal day and night.
However, in reality, the day is longer than the night at an equinox.
This is because the Sun is not a single point of light, but appears to be a disc.
So when the centre of the Sun is still below the horizon, the upper limb is already visible and emits light, he said.
Furthermore, the atmosphere refracts light downwards, so even when the upper limb of the Sun is still below the horizon, its rays already reach around the horizon to the ground.
These effects together make the day about 14 minutes longer than the night (at the equator, and more towards the poles), said Devgun.