Venus passes across the face of the sun from about 10:15am this morning and will be finished by 4:43pm. The next transit will not be until 2117.
Watch the transit live below:
Captain James Cook's observed the Transit of Venus in Tahiti as part of a scientific mission for the Royal Society and it prompted his visit to New Zealand in 1769.
For astronomers, Venus passing in front of the sun is not just a rare planetary spectacle - it won't be seen for another 105 years. It's also one of those events they hope will spark curiosity about the universe.
Sul Ah Chim, a researcher at the Korea Astronomy and Space Science Institute in the central South Korean city of Daejon, said he hoped people see life from a larger perspective, and "not get caught up in their small, everyday problems".
"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," he said.
As astronomers use the latest technology to document the transit of Venus, stargazers gathering across the world should only look at the celestial event with a properly filtered telescope or cardboard eclipse glasses.
If viewed directly, permanent eye damage could result.
"It's absolutely not an old wives' tale," said Steven Nusinowitz, associate professor of ophthalmology at the UCLA Jules Stein Eye Institute. "Looking directly at the sun is foolish and not advisable."
Extremely hot Venus is one of Earth's two neighbours and is so close in size to our planet that scientists at times call them near-twins. During the transit, it will appear as a beauty mark moving across the face of the sun.
"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 US Naval Observatory. "It ain't going to happen again in my lifetime."
The transit is happening during a 6-hour, 40-minute span starting at about 10:15am in New Zealand. 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 Australia 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.
In Hawaii, university astronomers planned viewings at Waikiki Beach, Pearl Harbor and Ko Olina. At Waikiki, officials planned to show webcasts 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 Centre in Maryland with solar telescopes, "Hubble-quality" images from its Solar Dynamics Observatory Mission and expert commentary and presentations.
Amateur astronomers 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.
They will use atomic clocks, GPS and high-end telescopes to take measurements, and will use high-end video gear to capture time-lapse video.
Most people don't tend to gaze at the sun for long periods of time because it's painful and people instinctively look away. But there's the temptation to stare at it during sky shows like solar eclipses or transits of Venus.
During the 1970 solar eclipse visible from the eastern United States, 145 burns of the retina were reported, according to the American Academy of Ophthalmology.
The eye has a lens and if you stare at the sun, it concentrates sunlight on the retina and can burn a hole through it. It's similar to when you hold a magnifying glass under the blazing sun and light a piece of paper on fire.
It can take several hours for people to notice problems with their eyes but, by that time, the damage is done and, in some cases, irreversible.
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 Harbour.
In South Korea, the transit coincides with a national holiday.
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 residents to visit observatories, 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.
3 News / AP