"The hook that got people was that there was no one in our lifetime who had ever seen it. My son Daniel got gripped by that," said Debbie Musselwhite of London, who came with 10-year-old son to join several hundred people at the Royal Observatory in Greenwich to catch a glimpse of the event.
"It's a brilliant opportunity to know the mechanics of our solar system," said another visitor, Shereeza Feilden, who came with her father.
"The black dot crossing the surface of the sun is absolutely marvelous," added a man visiting the observatory.
"Very, very lucky to be able to say that we've seen the Transit of Venus," said another amateur astronomer.
"It is of more social interest and educational interest these days than actual scientific crucial observations," said Robin Scagel, vice president of the Society for Popular Astronomy.
Some people were waiting in line at 6 a.m. for a chance to use one of the filter-equipped telescopes provided by the observatory, said Emily Winterburn, curator of astronomy.
The Royal Observatory, beside the Thames in southeast London, has a historic connection to the transit, which occurs twice — eight years apart — about every 121½ years. In 1716, Edmond Halley of comet fame observed the transit at Greenwich to calculate the distance between the Earth and the sun.
The last time the transit occurred, in 1882, John Philip Sousa wrote a march, "The Transit Of Venus." Concert bands around the world have been pulling it out of their archives for performances this month.
"It looks like some one put a black dot on a painted orange disc," said a first grader named Emon who was with a school group in Dhaka, Bangladesh.
In Greece, two American experts stationed themselves at opposite ends of the country — the southern island of Crete and the northern city of Thessaloniki — in hopes of unlocking the mystery behind the "black drop effect," which makes Venus appear teardrop shaped instead of a circle when it aligns with the edges of the sun.
"It's unbelievable!" gushed Jay Pasachoff, an astronomer at Williams College in Massachusetts, as he watched the event from the Observatory of the Aristotle University of Thessaloniki. "It's perfect ... It's not a media event like the Oscars, but it's like a fine, French wine for the people who know about it and enjoy it."
Glenn Schneider of the University of Arizona observed the transit from Crete.
Pasachoff's team, which includes six students from the college and Greek colleagues, collected data on Venus' atmosphere. They also took advantage of the transit to refine techniques for studying so-called exoplanets orbiting distant stars. The used 12 telescopes and NASA's Transition Region and Coronal Explorer spacecraft, known by its initials as TRACE, to observe the transit.
Planetariums the world over — from India's eastern city of Bhubaneswar to New York City — set up telescopes with the solar filters needed to watch the event, while two observatories in Spain's Canary Islands planned to use the transit to recalculate the distance between the Earth and the sun.
A blue sky over Sydney gave about 40 people looking through telescopes at the city's observatory a clear view of the transit as it began in mid-afternoon local time. Telescopes were set up on lawns and inside, while an image of the transit was projected onto a white screen for safe viewing.
"Venus is just marching into the sun," said Andrew Constantine, an education officer at the observatory. "It's very exciting."
The sight had special significance for Australians — this country's east coast was "discovered" by British explorer James Cook on his way home from viewing the 1769 transit in Tahiti.
But in many places clouds obscured the show, with observatories in Japan reporting rain and the transit coinciding with the cloudy monsoon season in Thailand.
It also was cloudy in Hong Kong, but that didn't stop more than 100 people, including students, senior citizens and children, queuing up at the Hong Kong Space Museum, where several telescopes were waiting.
At an air-conditioned tent in Abu Dhabi, the Emirates Astronomical Society provided telescopes, literature and lectures along with chocolates and water for those coming in from the sunny, 111 Fahrenheit outdoors. In Bahrain, state-run television aired documentaries on Venus and provided live coverage from a university.
"This is so educational. ...It teaches you about the universe and God Almighty's wonders," said Nemr Ramzi, a 10-year-old Palestinian, who was in the tent. "One day, I want to be a pilot and reach up there."
Greg Skulmoski, a Canadian professor at Zayed University, came with his wife Tracy and their baby. "This is life, this is the universe, this is humanity," Skulmoski said.
In Thailand, hundreds of people flocked to observatories across the country. But an overcast sky disappointed many.
Many people in predominantly Buddhist Thailand believe in astrological implications of planetary movements, especially rare ones.
"The Venus eclipsing the sun means that Venus will gain huge power, but the power will have a negative affect on finance and love," Phingyo Phongchareon, a well known astrologer, was quoted as saying in an interview with the Matichon newspaper.
Beijing's Ancient Observatory set up a slide show to explain the phenomenon to visitors and had telescopes to watch the transit.
People in Africa, Europe and the Middle East would see the entire transit, while the northeast corner of the United States and Canada would see only the tail end of the event, said Darren Osborne, education spokesman with Australia's Commonwealth Science & Industrial Research Organization, which set up a live Webcast of the transit.
In India, about 5,000 people were expected to turn up at the Pathani Samanta Planetarium in Bhubaneswar, where organizers had brought in several telescopes and binoculars fitted with solar filters.
The two observatories in the Canary Islands were recalculating the distance between the earth and the sun, but "don't expect anything new," said Luis Cuesta of the Canary Islands Astrophysics Institute.
A key viewing location in Britain was Carr House in Much Hoole, near Preston in northwest England. A telescope was set up in the bedroom where astronomer Jeremiah Horrocks observed the transit for the first time on Nov. 24, 1639.