Space enthusiasts throng Jantar Mantar to witness equinox

Skywatchers and space enthusiasts today thronged the Jantar Mantar here to witness equinox, a celestial phenomenon when the day and night are approximately of equal duration.

The event takes place twice a year on March 21 and September 23.

During equinox, 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, thereby making the night and day of approximately equal duration.

Science Popularisation Association of Communicators and Educators (SPACE), an NGO, in collaboration with Nehru Planetarium has organised a public outreach programme at Jantar Mantar on the occasion of Autumnal Equinox.

School students, foreigners and the common man visited the observatory and took active part in the event.

"The word equinox derives from the Latin words 'aequus' (equal) and 'nox' (night). In reality, the day is longer than the night at an equinox, 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," SPACE director CB Devgun said.

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), he said.

"This year the autumnal equinox was at 08:39 am. The exact time of the equinoxes are not fixed. They generally occur about six hours later each year, amounting to one full day in four years which in turn makes a leap year," he said.

In Delhi, day and night will be equal on September 27 when the Sun will rise at 6:12am and set at 6:12pm.

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