Lieutenant James Cook (right) observed the transit of Venus from Tahiti in 1769.
History of the transit of Venus
Despite being a rare astronomical event, transits of Venus have helped astronomers measure the distance to the Sun, create Internet audience records, and lead to the mapping of the east coast of Australia.
19 September 2006 | Updated 14 October 2011
The first recording viewing of a transit of Venus was made by English astronomer Reverend Jeremiah Horrocks and a friend on 4 December 1639.
He was the first to recognise that transit occurred in pairs that were then separated by more than a century.
Measuring the distance
In the late seventeenth century, English astronomer Sir Edmund Halley, devised a method of measuring the distance to the Sun, using accurate timings of the transit of Venus. These timings needed to be recorded from various locations.
Nearly one hundred years later, astronomer Mr Charles Mason and land surveyor Mr Jeremiah Dixon headed to the island of Sumatra, in what was then known as the East Indies (now Indonesia), to observe and record the transit of 6 June 1761.
However, a series of clashes with the French, resulted in them observing the transit from South Africa.
Lieutenant Cook and Mr Green
The most celebrated transit of Venus is that of 3 June 1769, which was observed from many places.
One of the observers was Lieutenant James Cook who sailed to the newly discovered island of Tahiti with the astronomer Mr Charles Green in the ship HMS Endeavour.
They arrived six weeks before the transit to allow for time to prepare. Lieutenant Cook had a small fort, called Fort Venus, which was built to protect the observing equipment from the indigenous population.
Cook and Green found that timing the important second and third contacts was difficult because of the 'black drop' effect.
Lieutenant Cook and Mr Green found that timing the important second and third contacts (when the disc of Venus enters and leaves the disc of the Sun), was difficult because of the 'black drop' effect. This was a dark thread that appeared to join the edge of Venus to the Sun, making the absolute instants of second and third contacts hard to establish.
Despite this, Lieutenant Cook and Mr Green agreed exactly on one of the times, and only differed by six seconds on the other.
The voyage to observe the 1769 transit is especially significant for Australians because after successfully completing the observations, Lieutenant Cook opened his sealed orders from the British Admiralty, telling him to search for the unknown southern continent. He did not find this mythical land, but in the process of searching for it, did claim New Zealand and New South Wales for the British Crown.
Observing from Australia
The first attempt to observe the transit from Australia was organised by New South Wales Government astronomer Mr Henry Chamberlain Russell on 9 December 1874. In addition to observing the transit from Sydney Observatory, he sent three observing parties to the Blue Mountains, Eden and Goulburn.
The transit was also viewed from Cambelltown in central Tasmania, by the United States Naval Observatory.
By the time of the next transit, on 6 December 1882, scientists had devised new methods of calculating the distance to the Sun, and therefore interest in transits had waned. Despite this, Mr Russell set up equipment to record the transit, but clouds covered the sky above Sydney and much of New South Wales on the day, frustrating his efforts.
The transit of 2004
The transit of Venus on 8 June 2004 was one of the most viewed in history.
People around the world watched the transit on the Internet or at observatories. Despite the lack of any scientific need to view the transit, many astronomers attempted to observe it. Whilst many were successful in obtaining similar results to previous transits, no one observed the famous black drop effect.
CSIRO provided a webcast of the event, which resulted in more than 55 000 simultaneous live web video streams, an Australian record at the time.
Discover what are Transits of Mercury and Venus.