Twice in a century Venus fly-by | Science| In-depth reporting on science and technology | DW | 05.06.2012
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Twice in a century Venus fly-by

A rare astronomical event took place from Tuesday to Wednesday when the planet Venus passed between the sun and the earth. It will not happen again until 2117.

Eager astronomers and scientists witnessed on Tuesday and Wednesday "the planet of Venus passing directly between the earth and the sun," said University of New South Wales astrophysicist, Jonti Horner. "It's quite an unusual event, because most of the time when Venus passes between the sun and earth it is not lined up and we don't get to see one of these transits," he said.

A speck on the horizon

The transit always occurs in pairs eight years apart. This was the second in this pair (the first occurred in 2004), and it enthralled sky gazers across seven continents, including Antarctica. "You will see a dark blob, a circle, one fortieth the size of the sun which will block out part of the sun's disk, and that circle will be Venus. It will slowly make its way across the lower half of the sun in the southern hemisphere, and the upper half of the sun in the northern hemisphere," said Horner before the six hour and 40 minute event that concluded early Wednesday morning. "It's quite a small speck in reality, so that gives you an idea of the scale of the solar system."

The sun partly behind a could at sunset, with Venus in the bottom left-hand segment

In 2004, Venus appeared as just a spot on the sun

It is hoped data collected from the sky show will reveal why Earth and Venus, two planets almost equal in size and orbit, ended up being poles apart. With an atmosphere 100 times denser than Earth, Venus' surface heats up to 900 degrees (482 degrees Fahrenheit). Clouds of sulphuric acid continually whip across the planet, so that showers of acid rain are a regular occurrence.

"In modern astronomy, transits of planets like the one [that took] place on Tuesday are a very important way of finding planets orbiting other stars," said Fred Watson, astronomer in charge at the Anglo-Australian Observatory in Western New South Wales.

Scientists will also use this transit to learn more about the atmosphere of what is the second planet in the solar system in the hope of understanding changes occurring in the Earth's own atmosphere.

The NASA space agency had the Hubble Space Telescope pointed at the moon during the transit of Venus.

"They use the moon as a giant mirror, a very poor mirror, but the light we see from the moon is sunlight being reflected from the moon back at the telescope back at earth. They collect that light and from that scientists will use this to better understand how we can work out what is in the atmosphere of planets around other stars and how we can detect planets around other stars."

Best seat in the house

People looking at the transit of Venus in 2004

Those unable to watch the event live will be able to watch online

Also speaking before the event, Watson said astronomers in Australia would have the best view of the entire event, while onlookers in Europe and the Americas would be able to see either the beginning or tail end of this astronomical rarity. "The Eastern hemisphere and the US will see the beginning of the transit before sunset, Australia will see the whole of the transit, Europe will only see the beginning on Wednesday morning, as the sun rises. There are some countries that will not see the transit at all," said Watson of the event which was streamed live online, and was available to view via a smartphone app for those unable to see the event unfold live.

Historic significance

This year's transit is only the eighth since Hans Lipperhey invented the telescope in 1608. Jeremiah Horrox, an English astrologer first documented the Venus transit in December 1639. "The historical aspect of the Venus transit is extremely important," said Watson, as it was responsible for the shift in how we view the planets in our solar system.

Venus against the sun

This pair of transits is only the fourth since the telescope was invented

Following Horrox's finding, scientists set about plotting the length of time it took Venus to cross the disk of the sun, which, with a few mathematical equations enabled them to determine how far the earth was from Venus. "Once you know how far away Venus is from the earth, then you know how far away the sun is from the earth," said Watson. "You can relate the period of revolution of a planet to its distance from the sun, once you know one planet's distance from the sun, you know them all."

In 1769 Englishman Captain James Cook set sail for Tahiti to make measurements of the Venus transit. He was one of hundreds of scientists who traveled all over the world at a time when travel was difficult and equipment heavy. Their combined measurements allowed astronomers to accurately calculate the distance from the sun - changing the way scientists viewed what was beyond the earth's ether.

It was the first ever example of a large-scale international scientific cooperation such as is standard practice today - and this at a time when scientists didn't even have standard measurements.

'Giant clockwork mechanism'

Watch video 00:52

Venus' last transit of the century

While previous Venus transits gave scientists the opportunity to amass data to figure out the size of the solar system and the distance between the sun and the planets, earth's significance in the grand scheme of the solar system leaves it as just a drop in the ocean.

"The transit of Venus has the great benefit of opening people's eyes to the fact that, yes, we live on a planet, but really we are just microbes on a cinder left over from the formation of the sun. But our planet is relatively small, and it is just part of a giant clockwork mechanism which is the solar system. We are part of something very big indeed," said Watson of this rare and special event.

Author: Jessie Wingard
Editor: Michael Lawton / za

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