New Delhi: Skywatchers and space enthusiasts
on Thursday 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
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 common people 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 C B 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 6:12 am and set at 6:12 pm.
"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.