Sky-gazers around the world, including Malta held up their telescopes and viewing glasses this morning to watch Venus slide across the sun -- a rare celestial phenomenon that will not happen again for more than 100 years.
The spectacle was seen, with magnification, as a small black dot on the solar surface.
"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.