1769 transit of Venus observed from Tahiti

Choosing an island, a ship and a Captain

In June 1767 British navigator Samuel Wallis made the first European contact with Tahiti. Wallis returned from his voyage in time to help the Royal Society decide that it would be an ideal location to observe the Transit of Venus. A big advantage was that Tahiti was one of the few islands in the South Pacific that they knew the longitude and latitude of. The Admiralty was not interested in specifically where in the South Pacific the observation of the Venus transit would take place. They were more interested in the "secret" mission that would be revealed after the Venus transit observation: the search for the alleged southern continent. HM Bark Endeavour was chosen to take the astronomers and other scientists to Tahiti. James Cook was commissioned as Lieutenant and appointed to command the vessel. Cook was considered the obvious choice as he was an outstanding seaman with navigational qualifications, a capable astronomer, and had observed a 1766 annular eclipse in Newfoundland that was communicated to the Royal Society by John Bevis.

Preparation for the transit

Once the Endeavour arrived on the island, Cook decided to set up the Venus transit observatory on shore. He required a completely stable platform which the ship could not provide and plenty of space to work with. The location of the observatory would be known as "Fort Venus". A sandy spit on the northeast end of Matavai Bay, named Point Venus by Cook, was chosen for the site. They began building Fort Venus two days after they arrived. They marked a perimeter and construction began. It had earthworks on three of its sides adjacent to deep channels. Wood was gathered to construct palisades that topped the earthworks. Casks from the ship were filled with wet sand and used for stability. The east side of the fort faced the river. Mounted guns were brought in from the ship. A gateway was assembled and within this fortification, fifty-four tents were pitched which housed the crew, scientists, and officers as well as the observatory, blacksmith equipment and a kitchen. Cook sent a party led by Zachary Hickes to a point on the east coast of the island for additional observations. John Gore led another group of thirty-eight more on a neighbouring island of Eimeo (Mo’orea). Both parties were briefed and supplied with the needed equipment.

The day of the transit

The observers were ordered to record the transit in four phases of Venus's journey across of the sun. The first phase was when Venus began "touching" the outside rim of the sun. In the second phase, Venus was completely within the sun's disc, but was still "touching" the outer rim. In the third phase, Venus has crossed the sun, was still completely within the disc, but was "touching" the opposite rim. Finally in the fourth phase, Venus was completely off the sun, but was still "touching" its outer rim.

On the day of the transit, the sky was clear. Independent observations were made by James Cook, Green and Solander with their own telescopes. Because of the rarity of the event, it was important to take accurate records. The next transit would not occur for more than a century, in 1874.

In his journal, Cook wrote:

This day prov'd as favourable to our purpose as we could wish, not a Clowd was to be seen the whole day and the Air was perfectly clear, so that we had every advantage we could desire in Observing the whole of the passage of the Planet Venus over the Suns disk: we very distinctly saw an Atmosphere or dusky shade round the body of the Planet which very much disturbed the times of the Contacts particularly the two internal ones. Dr. Solander observed as well as Mr. Green and my self, and we differ'd from one another in observeing the times of the Contacts much more than could be expected. Mr Greens Telescope and mine were of the same Magnifying power but that of Dr was greater than ours.

Recording the exact moment of the phases proved to be impossible due to a phenomenon called the "Black drop effect". Originally, it was believed that the effect came from the thick atmosphere on Venus, but the haziness was too extensive for this to be the reason. It is now believed that turbulence in the Earth's atmosphere leads to the smearing of the image of Venus.

Results of the 1769 transit observations

The Royal Society was very disappointed in the results of data collected from the transit and Cook's report. The Tahiti observers had had trouble with the timing of the stages and their drawings were inconsistent. They later found out that this was also true with the observers at the other locations. Observers from all over noted a haze or "black drop" that seemed to follow Venus, making it very difficult to record the times of the entry point on the sun and the exit from the sun. Herdendorf explains:

The stages consist of contacts: first contact, when the dark dot (Venus) just touches the outside edge of the Sun; second contact, when the dot is just completely within the disc of the Sun near the point of entry; third contact, when the dot, having crossed the Sun's disc, just touches the opposite edge of the Sun; fourth contact, when the dot is completely outside the Sun but still just touching the outer edge. Just after the second contact the planet appears to draw a tiny area of blackness after it, the 'black drop' or haze that Cook noted in his journal. By the time the black drop has disappeared the planet has already separated from the Sun's inner edge and the moment for recording the time of the second contact has been lost.

For what they believed to be a failure in the observation, the Royal Society decided to blame Green, who had died on the voyage back to England. Green was not given an opportunity to personally present his own data, nor could he defend himself. Cook's rebuke was so sharp that it was struck from the official proceedings of the Society.

Scientific community

Halley's 1716 article called for observers to witness the transit at various places on the globe. The response from the scientific community was astounding. There were at least 120 observers at sixty-two individual posts for the 1761 transit. Observations took place not only in Europe, but also included Calcutta, Tobolsk, Siberia, the Cape of Good Hope, and St. John's in Newfoundland. The 1769 viewing also proved to be a vast international endeavor.

Modern results compared to results from the 1769 transit

Using the solar parallax values obtained from the 1769 transit, the astronomer Thomas Hornsby wrote in Philosophical Transitions December 1771 that "the mean distance from the Earth to the Sun (is) 93,726,900 English miles". The radar-based value used today for the astronomical unit is 92,955,000 miles (149,597,000 km). This is only a difference of eight-tenths of one percent. Their work was within the bounds of the aphelion and perihelion distances to the sun, ~95 million miles and ~91 million miles respectively. These results have been described as "absolutely remarkable" considering what the astronomers had to work with.