Weather-permitting, skygazers can track its transit on Tuesday between 05:13 and 11:26 GMT – the first time the eclipse has been seen in more than 122 years.

But don't expect anything as spectacular as a solar eclipse, the Sun will hardly be dimmed.
  
Instead, Venus will appear like a little black dot, crawling across the face of our star for some six hours.
  
Europe and Africa will see the entire event, as will most of Asia, while East Asia and Australia will miss the end and both North and South America will miss the beginning. 
  
Odd pattern

The eclipse is "one of the rarest of predictable viewing phenomena," says Joe Rao, astronomy columnist with the specialist website space.com.
  
"Venus transits recur like clockwork, though in an odd pattern." The last was on December 6 1882 - and after Tuesday - the next will be in June 2012.
  
The Venus transit was first predicted accurately by the 17th-century mathematician Johannes Kepler. Since then, six have been recorded: in 1631, 1639, 1761, 1769, 1874 and 1882. 
  
Opportunity

But although an old phenomenon, a small, radical branch of astronomers will use the transit to finetune techniques for observing planets that orbit other stars.
  
Accurate measurements may help our understanding of other extrasolar planets, of which more than 100 have been spotted, when they pass in front of their parent star.
  
The drop in observed light can help deduce the planet's diameter and the spectrum of deflected light gives clues as to the planet's atmosphere.