BOLOGNA, Italy - In this old center of stargazing, as in much of the world, thousands watched a rare heavenly show Tuesday: the black dot of Venus inching across the blazing face of the sun.

"Ecco!" gasped one matronly woman as the big screen at Piazza VIII Agosto showed a tiny perfect circle edge into view. "There it is!"

When scientists first worked out the Earth's distance from the sun, the Venus eclipse was crucial. "This sight is by far the noblest astronomy offers," Edmond Halley, of comet fame, declared in 1691.

Now mostly a curiosity, the celestial rarity still had excited crowds lining up to peer into telescopes from Australia to the American Midwest.

"It's a brilliant opportunity to know the mechanics of our solar system," said 14-year-old Shereeza Feilden at the Royal Observatory in Greenwich, England.

Reactions were similar from people who shared a faith-straddling religious experience to those who marveled at a universal phenomenon.

"Imagine," said Bologna astronomer Corrado Bartolini. "We can't even tell when a car will reach Zamboni Gate in traffic, but we know to a fraction of a second when Venus meets the sun every 122 years."

Venus makes two passes across the sun, eight years apart, every 122 years; the next one will be in 2012. As the sun is 30 times bigger, the planet is barely visible through special dark glasses.