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The Transit of Venus, 2012

Credit: Geoffrey Wyatt, Sydney Observatory/Powerhouse Museum

Geoffrey Wyatt’s prize-winning image of the 2004 transit was taken with a special filter that only allows through the red light of hydrogen atoms. Credit: Geoffrey Wyatt, Sydney Observatory/Powerhouse Museum

By Nick Lomb

Transits of Venus allowed astronomers to calculate the scale of the solar system, and led to the discovery of Australia. On 6 June this year Australians and New Zealanders will have a ring-side view of one of the most famous events in astronomy – and the last one for another 105 years.

Next month the planet Venus will pass in front of the Sun for the last time until the year 2117. This event is the rare and famous transit of Venus, which has only been seen six times in recorded history – the first sighting was in 1639 and the last in 2004.

Transits of Venus are of exceptional significance to Australians because the voyage of James Cook that led to the settlement of the country by the British was to observe the transit of June 1769 from Tahiti. To astronomers of the

18th and 19th centuries, transits of Venus were of great importance as they provided an opportunity to measure the distance to the Sun and hence the scale of the solar system. In order to make the necessary observations for this measurement, astronomers undertook daring and adventurous journeys to far-flung places such as Tahiti, Siberia, India and South Africa.

Only Mercury and Venus – the two planets that circle the Sun within the path of the Earth – can ever be seen in transit across the face of the Sun. Transits of Mercury are more common as they occur on average 13 times each century. Those of Venus generally occur in a pair of transits 8 years apart, and then not for a century or more.

17th Century Transits
German astronomer and mathematician Johannes Kepler was the first to predict the possibility of transits by predicting the transits of both Mercury and Venus in 1631. At least one astronomer saw the Mercury transit that year, but no one managed to see the Venus transit for it occurred mainly during the night in Europe.

Although Kepler had not realised that another transit was due 8 years later, a young English astronomer named Jeremiah Horrocks used the astronomical tables that he had produced to work out that a transit was likely in 1639. Having alerted a friend in Manchester, Horrocks successfully observed the event from a small village called Much Hoole near Liverpool. The half hour of Venus moving across the Sun’s disc that he managed to see before sunset was enough to allow Horrocks to refine the path of Venus around the Sun and, of course, to make him famous.

18th Century Transits
There was much greater knowledge of and interest in the following transit in 1761. This was largely due to the suggestion by another famous English astronomer, Edmond Halley of Halley’s Comet renown, that transits of Venus could be used to establish the distance from the Earth to the Sun.

Halley suggested that astronomers at widely spaced locations on the Earth should time when Venus first crosses onto the disc of the Sun and when it leaves the disc. By comparing the durations of the transits from different places, astronomers could work out the tiny shift in the path of Venus across the Sun due to parallax, which is the apparent shift in the position of an object when seen from different viewpoints. A simple example of parallax is putting a finger in front of your face and noting its apparent jump as you shut one eye and then the other.

Once astronomers knew the shift in the position of Venus due to parallax, they could use simple geometry to work out the distance from the Earth to the Sun. That distance gave the scale of the solar system using Kepler’s third law, which is a relationship between the distance of a planet and the observable period that the planet takes to circle the Sun.

The British expedition of the astronomer Charles Mason and the surveyor Jeremiah Dixon was one of the 100 or so astronomical teams that set out to observe the June 1761 transit. One of two expeditions sent out at the instigation of the Royal Society, Mason and Dixon not only made successful observations of the transit but eventually had a significant impact on American history.

Mason and Dixon’s expedition did not start out well. They were meant to make their observations from the British settlement of Bencoolen in Sumatra, which is the modern Indonesian city of Bengkulu. Three days after setting sail on the Royal Navy frigate HMS Seahorse, the ship was involved in a gunfight with a French warship. By the time the Seahorse escaped it had lost 11 of its men with 38 injured. Back in port, the two intrepid astronomers refused to begin the trip again until the Royal Society threatened to destroy their reputations and take them to court.

When the Seahorse again set sail it was too late to take the astronomers to Bencoolen. Instead the pair opted for the Dutch Colony of Cape Town. There their observations were so successful that shortly after the transit they were selected to measure the disputed border between the two American colonies of Pennsylvania and Maryland. This was a complex task that they accomplished, again with great success, during the years 1763–67. A century later the Mason–Dixon Line that they had measured became the crucial boundary between the two sides in the American Civil War, and their names became an immutable part of the history of the United States.

Eight years later a voyage to observe the following transit of Venus was to become crucial in Australian history. This was the voyage of Lieutenant James Cook, captain of the Endeavour, to observe the June 1769 transit from Tahiti. The newly discovered island of Tahiti had been selected because Rev. Nevil Maskelyne, the Astronomer Royal, had indicated that observations from that area of the South Pacific would be particularly advantageous for comparison with those from the northern hemisphere.

The day of the transit was clear and Cook and his astronomer Charles Green made good quality observations of the event. However, they were disturbed by the unexpected sight of a ring of light around the planet that was probably due to the Earth’s atmosphere and their relatively small telescopes. As a result, Cook and Green were both despondent for they thought that they had failed in their allotted task. In fact, their observations were better than those of most other astronomers observing the same transit.

Other astronomers were also bothered by various unexpected effects. The most famous or infamous of these is the “black drop”. This is a dark ligament that appears to join the silhouette of Venus to the edge of the Sun as it moves onto the disc and then again as it starts to leave the disc at the end of the transit. Since that was just the time that astronomers were trying to record with great accuracy, the effect was a major nuisance to observers. It was only in the last few years that a definitive explanation could be given for the black drop: we now know that it is due to the clarity of the atmosphere and the quality of the telescope being used combined with a slight dimming of the edge of the Sun.

After the event was over, Cook opened sealed orders that instructed him to search for the Great Unknown Southern Land – Terra Australis Incognita. When he did not find this mythical and non-existent land, he decided to head for home via New Zealand and the uncharted south coast of the land then known as New Holland. After mapping New Zealand, the Endeavour reached New Holland and Cook mapped its east coast, naming it New South Wales and taking possession on behalf of the British Crown. As a direct result, 18 years later the First Fleet, with Captain Arthur Phillip in charge, landed at Botany Bay.

19th Century Transits
Following the normal pattern of transits of Venus there was no transit until December 1874. This transit was extensively observed from Australia. Sydney and Melbourne observatories made plans to observe the transit, and each set up temporary observing sites elsewhere in case of clouds spoiling the view at the main observatory. Adelaide Observatory also planned observations while in Tasmania there were two separate American observing teams.

At Sydney Observatory, Henry Chamberlain Russell had become Government Astronomer in 1870 and immediately began preparing for the coming transit. He obtained funds from the NSW Government to order two new major instruments: one was a 29-cm lens telescope from Hugo Schroeder of Hamburg in Germany, and the other was a special telescope to photograph the Sun called a photoheliograph from the London optician John Henry Dallmeyer. For use with the latter instrument there was a device called a Janssen apparatus, which allowed 60 small photographs to be taken in quick succession on a circular glass plate. This was the forerunner of the movie camera.

Temporary observing sites were set up in Goulburn, at Woodford in the Blue Mountains and at Eden on the south coast of NSW. Russell also recruited scientific men (men only!) to staff these observing sites and trained them extensively prior to the event.

Apart from some cloud at Eden, observations on the day of the transit went well. Russell’s own observations were of considerable interest. As Venus was moving onto the Sun he saw a thin arc of light on the part of the planet still off the Sun. He described this as “very remarkable and beautiful, like a fringe of green light, through which the faintest tinge of red could be seen”. Four hours or so later, as Venus was leaving the Sun, he again saw the arc of light, though this time it was brighter near the north pole of the planet.

The thin arc of light, or aureole, that Russell saw was due to sunlight transmitted through the atmosphere of Venus. The brightening of the aureole near the pole of the planet went unexplained for more than a century until the 2004 transit. At that transit a group of astronomers made a similar observation with a space-based telescope and, referring back to Russell, suggested that the phenomenon provided important information on the structure of the planet’s atmosphere. As yet we do not know the reality or significance of the colours noted by Russell, for scientists observing the most recent transit in 2004 did not examine the colour of the aureole. It is likely that this will be rectified during observations of this year’s transit.

Scientists observed the second of the 19th century pair of transits in 1882, although by that time they had had greater success with other methods of measuring the distance of the Sun such as using the planet Mars. Russell again made plans to observe the transit from Sydney Observatory and from temporary observing stations elsewhere in the state, but in the event all the sites in NSW were clouded out and no observations were possible.

21st Century Transits
There was no transit of Venus in the 20th century, so there was great anticipation among astronomers for the June 2004 transit of Venus, which was the first to be seen by anyone alive. How would the transit look with modern equipment? Would some of the phenomena described by past astronomers be visible and could they be photographed?

From Sydney, only the beginning of the transit could be seen before sunset, but that was enough to satisfy the hundreds of visitors who lined up to see Venus crossing the Sun on a beautiful Tuesday afternoon on 8 June at Sydney Observatory. Many images of the transit were recorded from the places around the world where the transit was visible, and it was found that some of the photographs did indeed record the same phenomena that astronomers had described in past centuries.

Now the last transit for another 105 years is fast approaching on 6 June 2012. Those of us in Australia are favourably placed, for the transit will be visible from beginning to end from most of the country. From Perth the transit will already be in progress at sunrise. From Sydney the transit begins at 8:16 am and ends at 2:44 pm. With allowance for different time zones, the times are similar across Australia and New Zealand.

Extreme care has to be used when observing the Sun at any time. Ideally, visit your local public observatory such as Sydney Observatory or check if your local amateur astronomy group is holding a public observing session. If you have a small telescope, do not look through it but, with your back to the Sun, project the image onto a piece of white cardboard.

Dr Nick Lomb spent 30 years as an astronomer at Sydney Observatory, which is part of the Powerhouse Museum, and is still closely associated with it. His book Transit Of Venus: 1631 to the Present is published by NewSouth and Powerhouse Publishing, and provides the full story of the transits of Venus (see http://www.sydneyobservatory.com.au/transit-of-venus-the-book/). Sydney Observatory will run programs associated with the transit of Venus on 6 June 2012, including safe solar viewing if sky conditions permit. For details, see http://www.sydneyobservatory.com.au/transit-of-venus-at-the-obs/