Venus about to make Sun passage
By Jonathan Amos
BBC News Online science staff
Event lasts about 6 hours
UK timing: 0620-1224 BST
Viewing must use protection
Web and TV is safest option
The planet Venus is set to make a very rare passage across the face of the Sun
Tuesday's six-hour transit has not been witnessed for 122 years and observers will position themselves in Europe, Asia and Africa to get the best view.
Scientists will use the event to test technologies they will soon deploy to detect similar sized worlds orbiting stars tens of light-years away.
Venus will appear as a tiny black disc against our star but no one should look for it without the proper equipment.
Looking directly at the Sun with the naked eye, and worse still through an open telescope or binoculars, can result in blindness.
It is recommended people attend an organised viewing where the transit will be projected on to a screen; or they can visit one of the many institutional internet sites planning to stream pictures.
Rare opportunity
The latter may be the very best option, especially if the local weather is cloudy - something UK skywatchers know only to well.
But this is an event that should be caught in some fashion.
"Something wonderful, something marvellous is happening on 8 June and will be witnessed and experienced by millions of people all over the world," said Professor Gordon Bromage, head of the University of Central Lancashire's Centre for Astrophysics.
"It represents a fantastic opportunity to fire the next generation of astrophysicists with enthusiasm for scientific discovery.
"It is an extremely rare event."
There have only been six transits in the telescopic age - in 1631, 1639, 1761, 1769, 1874 and 1882.
Scale of everything
The last Venus transit to be viewable in its entirety from the British Isles was in 1283, when no one knew it was happening.
The special alignment of the Earth and Venus required to bring about this event occurs in irregular pairs, with the next scheduled for 2012.
The phenomenon has particular historical significance. The 17th and 18th Century transits were used by the astronomers of the day to work out fundamental facts about the Solar System.
Employing a method of triangulation (parallax), they were able to calculate the distance between the Earth and the Sun - the so-called astronomical unit (AU) - which is about 149.6 million km (or 93 million miles).
This allowed scientists to get their first real handle on the scale of the Solar System and the Universe beyond.
The first person to predict a transit of Venus - the 6 December, 1631, event - was Johannes Kepler but he died before it occurred.
Courageous scientists
Jeremiah Horrocks, the young English astronomer, was probably the first to record the phenomenon when he and a friend, William Crabtree, made separate observations of the passage on 24 November, 1639.
The 18th Century transit expeditions were sagas of utter courage
Dr Allan Chapman, Oxford University
By the time the transits of 1761 and 1769 came around, they had become major scientific events. Expeditions were despatched all over the globe to get the data necessary to calculate the AU.
One such expedition was undertaken by Captain James Cook whose epic voyage in the Endeavour took in the "new lands" of New Zealand and Australia.
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"The 18th Century transit expeditions were sagas of utter courage," the astronomy historian Dr Allan Chapman, of the University of Oxford, told BBC News Online.
"These people went to sea on four or five-year-long missions at extreme danger to their lives just to witness the same thing you can take a jet to see today. And this was especially true of the 1761 transit because England and France were at war."
In the modern age, thanks to radio signals emitted by spacecraft as they pass behind Venus, we now have very precise numbers on planetary positions and masses, as well as the distance between the Earth and the Sun.
But to the early astronomers, just getting good approximate values represented a huge challenge.
Tiny fraction
This is not to say the 2004 Venus transit will be regarded as just a pretty show of no interest to scientists.
The movement of the planet in front of the Sun will bring about a tiny drop in the brightness of our star.
And several research groups will be looking for this dip in light with detectors that will soon fly on space telescopes looking for Earth-sized planets around distant stars.
Current telescope technology cannot resolve small, rocky planets many light-years away, but it may be possible to detect their presence if they, too, bring about characteristic changes in light when passing in front of their parent stars.
The Europeans have the Eddington mission seeking funds. This would use the transit method to study around 500,000 stars and is predicted to find as many as 20,000 planets - of which about 100 would be comparable to Earth.
And the US space agency, too, will scrutinise around 100,000 stars in a similar mission to discover new worlds.
"One has to be sensitive to a change in light of less than a hundredth of one percent," said Dr Andrew Coates, from the Mullard Space Science Laboratory of University College London.
"For this transit, we will be using a CCD (charged coupled device) which was originally devised for the Eddington mission to look for a dip of 0.0076% in the light coming from the Sun."
