While the whole article is interesting, the highlighted part is especially so. Aisa bhi hota hai! 
http://www.nature.com/nsu/040531/040531-7.html
On 8 June, the planet Venus will appear to cross the surface of the Sun. No living person has ever seen this happen; indeed, it will be only the fifth time that the event has been recorded.
The âtransit of Venusâ was a valuable tool for early star-gazers trying work out the distance between the Earth and the Sun. But next week, millions of professional and amateur astronomers across the world will be watching the transit simply because it has not happened since 1882.
Transits happen when Earth, Venus and the Sun all lie in a straight line. They are rare because the planetsâ orbits are slightly tilted with respect to each other, which also means that they occur at the irregular intervals of 8, 121.5, 8 and then 105.5 years. The next transit will happen on 6 June 2012, but the one after that will not arrive until December 2117. Transits of Mercury are much more common: there are 13 or 14 each century, the next being in November 2006.
The event is visible from Europe, as well as most of Asia and Africa. On the East Coast of the United States, it will already be in progress by sunrise, whereas Australians will see it during sunset. The transit will begin at 5.19 am GMT, and will last for about six hours. For those unable to watch it directly, various science museums and astronomical societies will broadcast the spectacle on the Internet.
If you are watching the transit in the flesh, remember that looking directly at the Sun can blind you: the best way to view the transit is to use a lens to project an image of the Sun on to a piece of white card kept in shadow.
âAn old-fashioned refractor telescope would do nicely, or even a pair of binoculars with one lens blocked off,â says Gordon Bromage, an astronomer at the University of Central Lancashire in Preston, UK, which is hosting public viewings of the transit.
Hold the lens up to a chink between two curtains and make sure it is pointing at the Sun. Move the white card until the image is focused sharply; the shadow of Venus should look like a black penny crossing a shining dinner plate.
Star-studded story
The British astronomer Jeremiah Horrocks made the first recorded observation of a Venus transit on 24 November 1639, from his home near Preston. Although his German counterpart Johannes Kepler correctly predicted the preceding transit on 6 December 1631, he never got to see it, partly because the event was not visible from Europe, but mainly because he died the year before.
Horrocks predicted the 1639 event after correcting an error in Keplerâs planetary tables, and instructed his friend William Crabtree to watch the transit. Both were self-taught amateur astronomers. âThis was a historic moment for astronomy because it proved that Venus was a solid planet like our own,â says Allan Chapman, a historian of science at the University of Oxford, UK. John Flamsteed, Britainâs first Astronomer Royal, described the moment as the beginning of English astronomy.
Astronomers subsequently tried to use the event to work out how far the Earth is from the Sun, making ever more elaborate expeditions to catch Venus in its traverse. The famous British explorer James Cook stopped off in Tahiti to watch a transit in 1769, during his first circumnavigation of the globe.
French astronomer Guillaume Le Gentil sailed to India to observe the 1761 transit, but missed the event after his ship was attacked by the British. He decided to wait in the Far East for the 1769 transit, but when the momentous day arrived cloudy skies obscured his view. Defeated, Le Gentil returned home, arriving in Paris after two shipwrecks to find that he had been proclaimed dead and his property divided up between friends and relatives. âIt was probably the worst observation campaign in history,â says Bromage. :eek:
The black drop
Even those who managed to observe a transit struggled to determine the Earthâs distance from the Sun. The basic geometry of their measurements was sound, but in practice the calculations were sketchy because of difficulties in judging precisely when Venus began and ended its transit. The small black disk seems to âdragâ on the Sunâs edge instead of passing cleanly across the boundary, so that it looks like an oily droplet instead of a perfect circle.
Eighteenth-century astronomers believed that this âblack drop effectâ was caused by the thick venusian atmosphere, but Mercury, which has no atmosphere, shows a similar drag. In fact, scientists have recently concluded that it is an optical effect caused by the Sunâs outer edge being darker than its centre.
Today, astronomers hope that transits across other stars will aid the hunt for planets outside our Solar System. They should cause dips in starsâ brightness that can be detected by sensitive orbiting telescopes. In a bid to prove this principle, astronomers at the European Space Agency will scrutinize next weekâs Venus transit for the predicted 0.0076% change in the Sunâs intensity.
© Nature News Service / Macmillan Magazines Ltd 2004