"Some people have been waiting all their lives for this. Nobody alive has seen it," said astronomer Peter Boyce, who plans to watch the "transit of Venus" from Massachusetts' Nantucket Island.
People across much of the Earth will be able to see it, but it won't be visible in the western United States. In the Midwest and East, viewers can watch its final stages for a couple hours starting at sunrise, if skies aren't clouded over.
Transits of Venus occur twice - eight years apart - about every century, when the sun, Venus and Earth precisely line up. Past transits - the last pair were in 1874 and 1882 - helped astronomers calculate Earth's distance from the sun. This time, Tuesday's transit and one in 2012 carry little scientific significance, but they've still stirred up interest.
Thousands of planetariums, libraries and astronomy clubs are hosting programs to educate the public about the transit and give them a chance to view it safely.
In New York City, the Hayden Planetarium at the American Museum of Natural History plans to set up a bank of telescopes in Central Park to give people a view once the sun rises above Manhattan's skyline. The sun's image will be projected onto white screens so that dozens of people can watch the passage of Venus, which will appear as a small black dot. Other scopes will give them a direct view through dark solar filters.
"If there are sunspots you'll actually see this dot overtake them as it slides slowly across the sun's disc," said Neil deGrasse Tyson, the planetarium's director.
In cities such as Denver where the transit won't be visible, enthusiasts will watch live webcasts of the event from Greece and other countries.
About 30 children and their parents have signed up for a "Pajamas in Space" sleepover at the Denver Public Library. Although Tuesday night's sleepover begins hours after the transit ends, the kids will unfurl sleeping bags under a large screen onto which a recording of the full transit will be projected, said librarian Kristin Arnold.
"This is a really significant event and we want them to experience it," she said.
In Muncie, Ind., a space-themed outdoor concert Monday night will feature the uplifting "Transit of Venus March" that John Philip Sousa composed after the 1882 transit.
The next morning, members of the Muncie Astronomy Club will be one of 12 groups across the nation helping NASA make observations of the event.
Other enthusiasts have taken steps to give American soldiers serving in Iraq, Afghanistan and Kuwait - all prime viewing sites -- a chance to watch the transit.
Chuck Bueter of Mishawaka, Ind., helped send about 4,100 solar shades to 15 sites where soldiers are serving along with safe-viewing tips in English and local languages.
"Frankly, it would be a shame if anyone in this ideal viewing zone were unable to experience this dance of planets that's occurring above their head," he said.
By Rick Callahan