June 4, 2004
By Chandrea Gerber
CELESTIAL skies herald a rare opportunity in June, when half of the planet gets to see an event so extraordinary it has not been seen by any living person.
This event is the Transit of Venus, which will occur on Tuesday 8 June, and was last seen in 1882. On this day, we will be able to watch the black disc of the planet Venus moving across the bright orb of the sun, as it lines up directly between the sun and the earth.
"It's like an eclipse by a very small thing," explains Dr Claire Flanagan, director of the Johannesburg Planetarium. Only to be viewed with the correct safety devices, what we will see is Venus, appearing as a black splotch one-thirtieth the size of the sun, moving over the sun over the course of about six hours.
While a little black spot on the sun may seem like a simple act, the transit of Venus is not. Historically, the movement of Venus between earth and the sun was instrumental in defining our place in the cosmos. Global expeditions timed the transit of Venus to determine how large the solar system was, and where everything stood in relation. This was first realised by Edmund Halley, and while inaccurate, his findings gave astronomers their first good basis for the size of the solar system.
Flanagan explains that although there are vast pages of complex mathematical equations to explain this, it essentially means that the position of Venus in relation to both the sun and the earth helped astronomers calculate the distance of the earth from the sun, known as the astronomical unit (AU), and in turn the solar system.
Nowadays, more accurate devices such as radar have been developed to determine distance, but astronomers use spacecraft and other techniques to seek earth-class planets transiting distant stars. Transits help us find out more about where we fit in the grander scheme of the universe.
The first recorded observation of the rare transit of Venus was by Jeremiah Horrocks in 1639, three decades after the invention of the telescope in 1609. Since then, transits have been witnessed in 1761, 1769, 1874 and 1882.
The transit path.
Photo courtesy of SOHO EIT consortium
Two transits occur in the space of eight years, and then again only about 120 years later. The next transit of Venus will be in 2012, and after that only in 2117 and 2125. The 2004 Transit of Venus will only be seen in Europe, Africa, Asia and the eastern parts of America, so this really is an event not to miss.
But, Flanagan warns, you won't see anything if not wearing the correct viewing device, and viewing it without such filtered devices can cause damage as great as blindness.
Suitable eyewear includes No.14 shade welding glasses, eclipse shades or a pinhole projector. No matter what technique you use for viewing the sun, do not stare continuously at it.
The 2004 transit of Venus will occur on Tuesday, 8 June, from 7.17am to 1.29 pm. Many events are planned around Joburg to experience this special event safely. For more information, go to the Johannesburg Planetarium website: http://www.wits.ac.za/planetarium/, or call them on 011 717 1392.
For more information on suitable viewing techniques, go to http://www.transitofvenus.org/safety.htm
For more information on the transit of Venus go to http://www.transit-of-venus.org.uk, or http://www.transitofvenus.org, very interesting and informative websites about this rare phenomenon.
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