On 8 June, the planet Venus will pass directly between the Earth and the sun in a rare event that last took place in 1882. Venus will appear as a small black speck on the face of the sun for several hours and will be visible from most of Europe and Asia.
London, 4 June 2004 (RFE/RL) - The "Transit of Venus" -- as a rare alignment of the sun, Venus, and the Earth is known -- will take place on 8 June for the first time in 122 years.
On that day, the orbital path of Venus will take it between the Earth and sun, leaving Venus visible to earthlings as a small black speck against the sun's bright face.
The transit will start around 0518 GMT and end about six hours later.
While it won't be as spectacular as an eclipse, novices will be able to watch, provided they take precautions against looking directly into the sun.
People with good eyesight will be able to spot Venus -- about 1/30th of the sun's diameter -- using specially made, dark eclipse-glasses. Properly shielded telescopes or simple binoculars for projection will offer a better view, as Dr. Andrew Coates, an astronomer working at the Mullard Space Science Laboratory, University College of London, explained.
"You can either use a telescope if you have one, or binoculars to watch and not look directly, but project the image," said Coates. "The other simple way of looking at it is actually to use a mirror, and have a circular cut-out in front of a mirror -- a piece of cardboard or something like that. Aim that mirror on to a wall, which is in the shade at the time. So, the same precaution compared to a total solar eclipses, because it is possible to be blinded."
Coates stresses the safest method of seeing the transit without endangering eyesight will be on the Internet, with some sites covering the event live.
The transit of Venus happens so rarely because of the relative angles of the Earth's and Venus's orbits. Coates says if the orbits of the two planets were on exactly the same plane, the transit would happen every year because Venus is closer to the sun.
Dr. Jacqueline Mitton is the spokeswoman at the British Royal Astronomical Society.
"I think the main reason why people are interested is because it is such a rare astronomical phenomena. In the whole of history, it's only been observed by human eyes five times," Mitton said. "The first time was in 1639, and the first person to observe it was a young British astronomer called Jeremiah Horrocks."
Because of the mathematics of the orbits, the next transit will take place in eight years -- but the next one after that will take place only in 2117. Anyone wanting to see the transit again from parts of Europe will have to wait until 2247.
The event was first predicted in the 17th century by astronomer Johannes Kepler. It was Kepler who discovered the laws that guide the movements of the planets. Kepler never had a chance to view the transit himself.
The Venus transit is important in history for another reason: Scientists soon figured out it could be used to calculate the distance from the Earth to the sun if the exact time of the transit could be recorded from two distant places on Earth.
To that end, the British Royal Society in 1769 decided to dispatch explorer Captain James Cook to the far reaches of the Pacific Ocean.
Mitton explained: "They chose Captain Cook because he was already interested in astronomical things, and actually made an observation of an eclipse of the sun in 1766. So it was suggested he would be an ideal person to lead the expedition, take some telescopes and some other scientist down to Tahiti."
Mitton said Cook succeeded in observing the transit but failed to note the exact timing -- and the effort was for naught.
The distance between the Earth and the sun has since been calculated by other, more precise means. Yet the Venus transit is still worth observing -- if only because no one alive today has ever seen it.