Millions of people around the world watched the planet Venus move slowly across the face of the sun Tuesday, an event which occurs roughly only twice every century. The transit could not be seen in parts of North and South America, but it was clearly visible in the eastern United States and in much of Asia, Africa and Europe.

The Paris Observatory held a rare open house Tuesday, and local residents and tourists took advantage of it. By midmorning on the sunny, blazingly hot day, a long line snaked down the observatory's sidewalk, as people flocked to view Venus crossing the sun. Several telescopes and astronomers were on hand for the occasion.

School groups packed the observatory's green lawns, listening to astronomers explain celestial events, including the Venus transit, which began at about 7 a.m. local time. As elsewhere on Earth, the event was a major news topic in France, and by midday the observatory had received so many visitors that it was already out of the special protective solar glasses it was handing out free of charge.

Six graders at Stanislas primary school in Paris were at odds about what they'd just seen through the telescope. They agreed there was a flat yellow disk - the sun, with the single, tiny dark spot of Venus moving slowly across it. But some kids said the spot was blue. Others believed it was black.

Scientists once used the Venus transit to calculate the distance between the earth and the sun. Today, says Fabienne Casoli, an astronomer at the Paris observatory, the sighting holds little scientific interest.

But Mrs. Casoli says exceptional events like the Venus transit or a solar eclipse offer scientists a wonderful opportunity to explain to the public the mysteries of the solar system. She says children generally ask questions about major phenomena like the Big Bang, a theory about the origin of the universe. Adults generally stick to more practical topics, like when the observatory was built.

The Paris observatory was hardly the only place deluged with Venus watchers. Millions of people gathered in front of telescopes in Europe, Australia, Africa, and in parts of North America, where the phenomenon could be seen early in the morning. Others watched online broadcasts on their computers.

In the Middle East, an astronomical society in the United Arab Emirates provided an air-conditioned tent, telescopes, chocolates and water for visitors. In South Africa, A retired man set up two telescopes for viewing at a nature preserve. Even in Iraq, a group of reporters and photographers took time off from covering the tumultuous daily news, and watched the transit through cardboard glasses. And in India, astrologers warned the event might lead to a surge in sex crimes. It's unclear if they were proved right.

Standing in line outside for the Venus viewing at the Paris Observatory, 20-year-old Christine Tannous said she had penciled the event into her calendar weeks ago.

A Lebanese university student studying in Paris, Ms. Tannous said she loves astronomy and is considering changing her focus from philosophy to science next year.

Nor was the Venus transit limited to non-scientists. American physicist Kimbell Milton took time off from an astrophysics conference in Paris to watch the event.

"I think people are really interested in astrophysical things," said Kimbell Milton. "It gets a lot of press. because we get all these fantastic images of the planets, and it seems exciting."

But not everybody gets excited about astronomical wonders. A few blocks from the observatory, British tourist Bob Rolands said he had completely forgotten about the Venus transit.

"I realized we'd be on holiday, and although I'm of a scientific bent, I'm not into astronomy and things of that nature," he said. "And it's so difficult to see anyway, you can't look at it with the naked eye, you need equipment. So rather than get involved during my holiday to view it, I'd just forgotten about it."

Others, like businesswoman Marie Oneissi, had no idea what the Venus transit was about.

Was it something spectacular? Mrs. Oneissy asked. Did it have a luminous effect? When informed she still had one viewing hour left before Venus completely crossed the sun - shortly after 1 p.m. local time - Mrs. Oneissy said she would head for the Paris Observatory before lunch. The next transit of Venus take places in June 2012. Then, the prime viewing will be in East Asia and the Pacific.