A once in a lifetime event: the transit of Venus

by Rebecca Priestley / 30 May, 2012
How to watch the transit of Venus.


When the transit of Venus occurs on June 6, “it’ll be the last time we see the little black disc of the planet Venus passing in orbit between our planet and the Sun”, says John Field from Wellington’s Carter Observatory. “It’s a chance to see the solar system in action.”

Transits of Venus occur in pairs every 125 years. The last transit, in 2004, was not visible from New Zealand. But on June 6, if the skies are clear, New Zealand will be one of the best places in the world to see “the dark side of Venus silhouetted against the bright disc of our star, the Sun”, says Field. At 10.15am, Venus will begin its journey between the Earth and the Sun. After 18 minutes the entire planet will be in front of the Sun, visible as a tiny and intensely black dot. Venus will reach mid-transit at 1.29pm and will make its final egress at 4.43pm, when the Sun is low on the western horizon.

One of the simplest ways to observe the transit, says Field, is with “eclipse glasses”, available from observatories and astronomical societies around the country. These glasses fi lter out 99% of the Sun’s rays – and you can use them again in November, during the partial solar eclipse. At Carter Observatory and Auckland’s Stardome Observatory, visitors will be able to view the transit through telescopes fitted with special filters, but Field has some advice on some other fun ways you can watch it at work, school or home.

“You can project the image of the Sun using a pair of binoculars,” says Field. “Take the cap off one of the lenses, hold the binoculars up to the Sun and project the image of the Sun onto a piece of paper or a white wall – you’ll see the black dot of Venus slowly move, or transit, across it.” Or, if you’re feeling particularly handy – and want to avoid the risk of someone getting their eyes between the binoculars and the white wall – you can make a Sun funnel. “You need a paper cup and some wax paper,” says Field. “Cut a hole at the bottom of the cup and fit it to the lens of the binoculars. If you fix a circle of wax paper over the open end of the cup you’ve got a screen, and you can project an image of the Sun, with Venus moving across it, onto the paper.”

When we see Venus in the evening sky, we see the bright glow of the planet reflecting the Sun’s light. During the transit, we’ll see the planet as a black circle against the Sun. But what’s it like on the surface of the planet? “Venus is hell,” writes Marcus Chown in his recent book Solar System. “Beneath impenetrable sulphuric acid clouds lies a surface hot enough to melt lead.” Venus’s extreme surface temperature – about 470°C – and atmosphere of 96% CO2 are the result of a “catastrophic runaway greenhouse effect” and is “a sober warning to us all”, says Chown.

No matter where you are on Wednesday, June 6, this is a once in a lifetime event. The next transit of Venus is not until 2117, so it’s unlikely that anyone alive today will be around to see it. And remember, don’t look at the Sun with the naked eye, with sunglasses, through smoked glass and especially through binoculars or a telescope. Looking at the Sun can permanently damage your eyes – looking at it through a telescope can blind you.

For full Listener coverage of the transit of Venus, and the transit of Venus forum, click here.

Send questions to science@listener.co.nz
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