New Delhi (PTI): Skywatchers and space enthusiasts are gearing to celebrate equinox day on Monday by taking measurements of the shortest shadow at noon and calculate the latitude of the place.

Equinox day is the time when the Sun crosses the plane of the Earth's equator, making night and day of approximately equal duration. It takes place twice a year on March 21 and September 22.

An NGO, Science Popularisation Association of Communicators and Educators (SPACE), and its members will encourage school students and enthusiasts across 16 states to observe the equinox day by conducting various experiments.

"We will celebrate equinox day by measuring the shortest shadow (local noon) and trying to measure the latitude of different locations where the experiments are conducted," SPACE President Chander Bhushan Devgun told PTI.

The word equinox derives from the Latin words aequus (equal) and nox (night). It means that equal day and night. But 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.

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.

This year the autumnal equinox is on September 22 at 15:46 hrs UT (21:16 IST), he said.

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.

In Delhi, day and night will be equal on September 27.

This day is commonly referred to as the "equilux" to distinguish them from the equinox. The equinox is a point in time, but the equilux is a day.