Unless, of course, it rains.
Despite the forecast, which is for clouds and possible rain, astronomy groups are setting up viewing sites around the city, including Union Square, the High Line, Riverside Park South and 125th Street in Harlem, where the temporarily star-struck can go to see the transit safely. It occurs when the orbits of Venus, Earth and the Sun put them into alignment along the same plane. Watching it with the naked eye is dangerous, and all but impossible, given the Sun’s blinding glare.
At the viewing sites, amateur astronomers and academics will have telescopes with special solar filters, as well as projection devices and solar glasses, available to the public. “In a sense, it is an eclipse,” said Summer Ash, director of outreach for Columbia University’s astronomy department, who will be stationed in the southeast corner of Union Square. “It’s the same phenomenon. It’s just that Venus is so much farther away than the Moon.”

In New York City, the transit of Venus, weather permitting, will be visible until sundown, at 8:24, letting New Yorkers see just the first third of the event.
In the 1700s, scientists realized that the transit of Venus could help determine the distance between the Earth and the Sun. Mathematical models and diagrams stemming from the 1769 transit put the distance at 95 million miles, just 2 million more than what is now accepted to be the average distance.
“If we know the distance between the Earth and the Sun, we can use that to get distances to other stars,” said Jason Kendall, a board member of the Amateur Astronomers Association of New York and a volunteer “solar system ambassador” for NASA. “Ultimately, the transit of Venus gave us a fundamental measuring stick for all distances in the universe.”
Mr. Kendall will oversee a viewing platform on a pier in Riverside Park South, near 68th Street, on the Upper West Side. He will bring a projection device, fashioned from an inexpensive telescope and a funnel covered with a fabric screen. He is expecting about a dozen other amateur astronomers to bring their own telescopes outfitted with solar filters.
Two other prime locations are also along the Hudson River, with clear views of the sunset: a section of the High Line park, at 14th Street, and the concrete pier at the Intrepid Sea, Air and Space Museum, at 12th Avenue and 46th Street. At the Intrepid, a nonprofit group, NYSkies Astronomy Inc., will have several specially equipped telescopes available.
John Pazmino, the group’s co-founder, remembers watching the 2004 transit from Carl Schurz Park on the Upper East Side, when the event was in progress as the sun rose. Conditions were such that the transit was visible with the naked eye. “There was a very dense haze so that you could look at the Sun directly and most people, including myself, did see Venus,” he said. “The sun was orangy, like a pumpkin, and it looked like someone took a pin and poked it.”
In Harlem, Columbia’s astronomy department will invite the public to have a look from a sidewalk on 125th Street and Adam Clayton Powell Jr. Boulevard. There will be projection devices, which allow for multiple views at one time, along with solar glasses and special solar telescopes. “We’re bringing astronomy to people where they are not necessarily looking for it,” Ms. Ash said. “Instead of prime locations where a lot of the amateur astronomy community will be, we prefer crowded streetscapes.”
If the clouds conspire to turn the transit into a nonevent here, there is always video. NASA’s Web site will be live-streaming the transit from an observatory atop Mauna Kea in Hawaii starting at 5:30 p.m., Eastern time.
The American Museum of Natural History will link to NASA’s webcast, projecting a portion of the transit on a large screen in its Cullman Hall of the Universe, to be followed by a related film in the Hayden Planetarium. Mr. Kendall was already feeling the tug of the museum’s big screen. “If it’s very cloudy, I might pack up my stuff and go there,” he said. At the Intrepid, Mr. Pazmino vowed to wait and watch, no matter the weather. “Our philosophy,” he said, “is to let Mother Nature kill the event, not human nature.”
And while, according to NASA, there have been 53 transits since 2000 B.C., this is believed to be the first one with its own Twitter hashtag: #venustransit.