Venus puts on show for first time in 721 years
Date: May 13 2004
By David Stringer
British stargazers are praying that bad weather will not scupper their
chance of witnessing one of the rarest events in astronomy - last seen 721
Amateur observers and schoolchildren are hoping to watch the six-hour
transit of Venus across the sun on June 8.
Venus will appear as a small black disc as it moves in an angular orbit
across the bottom of the sun - but will be seen in the UK only if there are
no clouds between 6am and 12.30pm.
Though the planet last made a transit in December 1882, it could not be
properly observed from the UK and the event has not been seen clearly since
Transits of Venus across the sun are among the rarest of planetary
alignments and have occurred only six times since the invention of the
telescope, in 1631, 1639, 1761, 1769, 1874 and 1882.
The events occur in irregular pairs and though another is scheduled for
2012, it will not be visible in Britain.
A spokesman for the Royal Astronomical Society said next month's event was
expected to be seen at 6.20am, shortly after sunrise.
Though Venus will be large enough to be seen with the naked eye, observers
have been warned not to stare directly into the Sun and to project the image
on to a screen through a telescope.
Transits in the 18th and 19th centuries were used to calculate accurately
the distance between the Earth and the Sun, helping define the astronomical
unit (AU), which is approximately 93 million miles.
Now scientists, aided by solar observatories stationed in space, will use
the event to test "planet-hunting" methods which it is hoped will pinpoint
more Earth-sized planets orbiting stars beyond the solar system.
The transit will be used to refine the technique of detecting the minuscule
dip in the brightness of a distant star as it is crossed by a planet.
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