The transit of Venus -- a planetary spectacle that won't occur again until 2117 -- will appear as a small moving beauty mark on the sun, too tiny to block the sun's light. Extremely hot Venus is one of Earth's two neighbors and is so close in size to our planet that scientists at times call them near-twins.
"In terms of rarity, to be here at a time when it's happening, you almost have to look at it," said Geoff Chester of the U.S. Naval Observatory, who saw the last transit in 2004. "It ain't going to happen again in my lifetime."
The transit is happening during a 6-hour, 40-minute span starting just after 6 p.m. EDT in the United States. What you can see and for how long depends on what the sun's doing in your region during that exact window, and the weather. Those in most areas of North and Central America will see the start of the transit until the sun sets, while those in Western Asia, the eastern half of Africa and most of Europe will catch the transit's end once the sun comes up.
Hawaii, Alaska, eastern Australian and eastern Asia including Japan, North and South Korea and eastern China will get the whole show since the entire transit will happen during daylight in those regions.
Don't stare directly at the sun without eclipse glasses, a properly filtered telescope or a strong welding visor. Permanent eye damage could result.
Astronomers across the globe are using the rarity of the moment to spark scientific curiosity among the public, and to document the transit with the latest technology available.
Sul Ah Chim, a researcher at the Korea Astronomy and Space Science Institute in the central South Korean city of Daejon, said he hoped the event will help people see life from a larger perspective and "not get caught up in their small, everyday problems."
"Most people consider 105 years to be a very long time," Sul said. "But when you think about it from the context of the universe, 105 years is a very short period of time, and the Earth is only a small, pale blue spot."
In Hawaii, university astronomers planned viewings at Waikiki Beach, Pearl Harbor and Ko Olina. At Waikiki, officials planned to show webcasts of the transit as seen from telescopes from volcanoes Mauna Kea on the Big Island and Haleakala on Maui.
NASA planned a watch party at its Goddard Visitor Center in Maryland with solar telescopes, images from its Solar Dynamics Observatory Mission and expert commentary and presentations. The observatory will produce "Hubble-quality" images, according to NASA's website.
Groups of scientists from the University of North Texas planned to watch from points in Alaska and Hawaii to recreate the 1769 expedition of British Capt. James Cook to Tahiti, part of an effort to use the transit to measure the solar system. The scientists will use atomic clocks, GPS and high-end telescopes to take measurements, and will use high-end video gear to capture time-lapse video.
Experts from Hong Kong's Space Museum and local astronomical groups were organizing a viewing Wednesday outside the museum's building on the Kowloon waterfront overlooking the southern Chinese city's famed Victoria Harbor. The transit begins there around 6 a.m. local time.
The transit also coincides with a national holiday in South Korea. Choi Hyungbin, head of the Daejon Observatory, said he was expecting more visitors than might otherwise come out to watch the transit. Local media urged curious residents to visit observatories for the event, reiterating the danger of looking directly at the sun.
This will be the seventh transit visible since German astronomer Johannes Kepler first predicted the phenomenon in the 17th century. Because of the shape and speed of Venus' orbit around the sun and its relationship to Earth's annual trip, transits occur in pairs separated by more than a century.
It's nowhere near as dramatic and awe-inspiring as a total solar eclipse, which sweeps a shadow across the Earth, but there will be six more of those this decade.
The Associated Press contributed to this report.