An eclectic mix of academics, astronomers and space enthusiasts gathered at Chennai’s planetariums and beaches before dawn on Wednesday, to witness the Venus transit, a rare celestial phenomenon.

At an event hosted by the Tamil Nadu Astronomical Society, Exnora International and Breakthrough Science Society, observers watched Earth’s sister planet make its way across the Sun’s disc. Venus was visible as a dark dot against the Sun.

Every century, the Venus transit occurs in pairs. The double of Wednesday’s event was seen in 2004. “Before 2004, the transit happened in 1872. This gap makes each instance invaluable. It lets us make angular measurements of the Earth’s distance from the sun and how that’s changing,” said D. Vijayakumar, president of the Tamil Astronomical Society.

Curiosity was as big a draw as the science behind the event. Morning walkers, vendors and nature photographers saw the gathering and walked in to participate. “This is not going to be seen for another 105 years. I’m just thrilled to be here,” said Haripriya, a student of Anna University.

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