New Delhi, Jun 6 (IANS): It was a visual treat for Delhiites Wednesday as they witnessed a rare celestial event when Venus - the second planet from the Sun and the Earth's neighbour - crossed the Sun's face in a once-in-a-lifetime celestial phenomenon.

Venus appeared as a small black dot on the Sun's surface during its transit, one of the most eagerly-awaited celestial events.

Public viewings of the rare occurrence, which will not happen again until 2117, were organised at the Nehru Planetarium with telescopes and solar filters put up at Teen Murti lawns in Chanakyapuri in the national capital.

"I was awestruck. It was a special moment that I will never forget in my life," said Ramandeep Singh, a Class Nine student of Gyan Mandir Public School in west Delhi.

"I have watched several eclipses but this was beautiful. Knowing that it will not happen again till 2117 makes it all the more special," said 18-year-old Geetika Suneja who aspires to be an astronaut.

Science Popularisation Association of Communicators & Educators (SPACE), an organisation working to spread awareness about astronomy, took 50 students to Shanghai, China, to observe the transit, where it was visible for the longest time.

According to scientists, transits of Venus occur only on very rare occasions when Venus and the Earth are in a line with the Sun. At other times, Venus passes below or above the Sun because the two orbits are at a slight angle to each other.

Transits occur in pairs separated by eight years, with the gap between pairs of transits alternating between 105.5 and 121.5 years - the last transit was in 2004.