Astronomers around the world on Monday were getting set for Tuesday's "Venus Transit," when earthlings will be able to see the planet cross the face of the Sun, a rare phenomenon that last occurred in 1882.
As many as five billion people may be able to watch a celestial alignment that no human alive today has ever seen.
Unlike a solar eclipse, no-one will note any perceptible darkening: the Sun's light will dim by just a tenth of one percent, and Venus will appear as a small black disc inching its way, from about 0513 to 1126 GMT, across the star's surface.
People should view it indirectly by projecting the image through binoculars or a telescope onto paper, or by watching it on television or the Internet.
Anyone looking at it directly must wear proper filters. To look at the Sun without protection can cause blindness.
"During a little more than six hours, planet Venus will cross the face of the Sun, offering a wonderful show for everybody to admire," the European Southern Observatory said.
"Nobody should miss the opportunity to witness this great event. And - good luck! - it appears that the observing condition prospects are rather favourable in large areas of the world."
Only six transits have ever been recorded: in 1631, 1639, 1761, 1769, 1874 and 1882. The next will be in 2012, but the one after that will be in 2117.
"It's an extremely rare event," said Gordon Bromage, a professor of Astrophysics at the University of Central Lancashire, a focal point for British observations about the transit (http://www.transit-of-venus.org.uk/vt-2004/).
The reason is because Venus, the second planet from the Sun, has an orbital plane that is slightly tilted to Earth, the third planet.
Until the advent of the radar and satellites, the big interest in the alignment was to measure the exact distance between Earth and the Sun.
Today, the main interest is sentimental and educational, although scientists who are interested in extra-solar planets - planets that orbit distant stars - will find it a great occasion for calibrating their instruments.
They will be especially interested in the spectrum of light deflected by Venus's cloudy surface - a telltale as to the atmosphere's chemical composition, in fact a deadly mix of suffocating carbon dioxide and poisonous sulphuric acid.
The world's leading space organisations and astronomy groups have been preparing for the event for years.
Two satellites, TRACE and SOHO, have been dedicated to the transit. Some agencies have set up specialist web pages to record observations or show the transit live.
Newspapers in many countries have allotted big coverage to the run, and some radio and television stations are running special programmes on the day.
As of early Monday, 1,562 "observing groups" had signed up to make their observations to the European Space Agency (http://www.esa.int/esaSC/SEMD3H2VQUD_sensations_0.html), "making this by far the largest internationally co-ordinated effort of its kind," according to ESA.
"Over 600 of these are school classes (in about 45 countries) and 265 are astronomy clubs, the rest are mostly individuals groups.
It is still time to join this unique programme - it would be good to have more observers, in particular in the Russian Federation (especially Siberia) and Africa."
Europe and Africa will see the entire happening, as will most of Asia, while East Asia and Australia will miss the end.
North and South Americas will miss the beginning, and well-heeled American astronomers have flown to sunny parts of Europe and the Middle East for a view.
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