Sky-gazers around the world held up their telescopes and viewing glasses Wednesday to watch Venus slide across the sun -- a rare celestial phenomenon that will not happen again for more than 100 years.

The spectacle began shortly after 2200 GMT Tuesday in parts of North America, Central America and the northern part of South America, and was seen, with magnification, as a small black dot on the solar surface.

All of the transit was visible in East Asia and the Western Pacific, although poor weather conditions spoiled the view for some.

"This is a once in a lifetime thing and if you miss it you have to wait until 2117," said Jong Tze Kian from the National Planetarium in the Malaysian capital Kuala Lumpur, where clear skies afforded prime viewing.

"So people are very excited to come and see the transit."

Australia -- for which the movement of Venus carries a special historical interest -- was one of the best places to watch with the nearly seven-hour transit visible from eastern and central parts of the country.

Although broken cloud hampered the view for some, Sydney Observatory held a sell-out event with 1,500 people buying tickets to witness the rare passage.

"It's not like an eclipse where you've got something blotting out the sun," Fred Watson, astronomer-in-chief at the Australian Astronomical Observatory, told AFP.

"Venus is 100th of the diameter of the sun so it's essentially just a black spot superimposed on the disc of the sun, but it moves across from one side to the other."

Europe, the Middle East and South Asia got to see the end of the phenomenon, while North America saw its opening stage.

"Everyone's having a great time," NASA scientist Richard Vondrak told AFP from the Goddard Space Flight Center in the US state of Maryland, where 600 people gathered to observe the fiery planet of love.

The passage between the Earth and the sun of the solar system's second planet should only be viewed through approved solar filters to avoid the risk of blindness, experts warned.

The event has special significance to Australia as a previous transit in 1769 played a key part in the "discovery" of the southern continent by the British navy's James Cook.

Captain Cook set sail for Tahiti on HMS Endeavour to record the transit that occurred that year, and after a successful observation he was sent to seek the "great south land" thought to exist in the Pacific Ocean.

During the voyage, he charted the east coast of Australia, staking a British claim in 1770.

Planetary transits have enduring scientific value.

"Timing the transit from two widely separated places on the Earth's surface allows you to work out the distance to Venus and hence the size of the solar system," explained Watson in Australia.

Scientists say it also allows them to learn more about how to decipher the atmospheres of planets outside our solar system as they cross in front of their own stars.

Only six transits have ever been observed -- in 1639, 1761, 1769, 1874, 1882 and 2004 -- because they need magnification to be seen properly, though the event has happened more than 50 times since 2000 BC.

US space agency NASA promised "the best possible views of the event" through high-resolution images taken from its Solar Dynamics Observatory (SDO), in orbit around the Earth.

The European Space Agency's Venus Express is the only spacecraft orbiting the hot planet at present and will be using light from the sun to study Venus's atmosphere.

ESA and Japan's space agency also have satellites in low-Earth orbit to observe as Venus passes in front of the sun.

And the NASA Hubble Space Telescope, which cannot view the sun directly, will use the Moon as a mirror to capture reflected sunlight and learn more about Venus's atmosphere.

For further details:

NASA webcast at http://venustransit.nasa.gov/transitofvenus.

For a list of transit times in international locations:

http://eclipse.gsfc.nasa.gov/transit/venus/city12-1.html