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Captain James Cook 1728 - 1779
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Transit of Venus

King George’s Island, or Otaheiti (Tahiti), was discovered by Captain Wallis, commander of HMS Dolphin (1766-68).

A Map of King George’s Island
A Map of King George’s Island
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Wallis returned from the voyage in time for his findings to help the Royal Society decide that this was one of the best places from which to observe the Transit of Venus, the recording of the passing of the shadow of the planet across the disc of the sun. This was important to provide data from which the distance from the earth to the sun could be calculated and would serve as a unit for the measurement of the universe.

The Royal Society had persuaded the King to support a naval expedition to observe the Transit, arguing that this would not only help with scientific investigation but also navigation and therefore trade. HM Bark Endeavour was chosen to take the astronomers to Tahiti and James Cook, promoted to Lieutenant, appointed to command the vessel and to be one of the Transit observers.

A map of Royal or Matavie Bay
A map of Royal or Matavie Bay
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On arriving at Tahiti, Cook chose a location in Matavai Bay to establish his base. He had been warned in his Admiralty instructions:
…But as Captn. Wallis has represented the Island to be very populous, and the Natives (as well there as at the other Islands which he visited) to be rather treacherous than otherwise you are to be Cautious not to let your self be surprised by them, but to be Constantly on your guard against any accident.
Cook therefore built a fort to protect his men and equipment.

Fort Venus Otaheite
Fort Venus Otaheite
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Cook had built a banked, ditched and pallisaded enclosure to hold about 45 of his men, including the officers and gentlemen. It was defended not only by his men with small arms but also with some of the cannon and swivel guns from the Endeavour.

Cook wrote,
I now thought myself perfectly secure from anything that these people could attempt”, but in spite these precautions “…when Mr Green and I went to set up the Quadt. it was not to be found, it had never been taken out of the Packing case…and the whole was pretty heavy, so that it was a matter of astonishment to us all how it could be taken away, as a Centinal stood the whole night within 5 yards of the door of the Tent where it was put…
The quadrant was later recovered undamaged.

A plan of Fort Venus
A plan of Fort Venus
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Inside Fort Venus Cook established the observatory in preparation for the Transit, the armourer’s forge and a cook’s oven. Outside was a tent for the cooper and sailmaker.

The large wood and canvas tent observatory contained all the scientific instruments. The latter were set up in wooden frames or on casks fixed firmly in the ground and filled with heavy wet sand for stability.

Mr Banks shows the Indians the Planet Venus on the Sun
Mr Banks shows the Indians
the Planet Venus on the Sun
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On the 3rd June 1769 Cook, Charles Green the other official Transit observer, and two others recorded the six hour-long event using clocks, quadrants and telescopes. To avoid any mishaps native Tahitians were not allowed into the Fort. 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…

Observations Made at King George’s Island
Observations Made at King George’s Island
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In just over a century the Transit of Venus takes place only twice. It was important that accurate records were made of this rare event but Cook and his fellow observers noted discrepancies in what they saw, thus reducing the value of the exercise.

With the equipment available it was difficult to see the precise moment that the shadow of Venus started to pass over the face of the sun and left it at the end of the transit. This was due to the large 'Atmosphere or dusky shade'
(penumbra) around the circumference of the planet. Cook and Green's findings, along with the results of observations made across the World, were presented to the Royal Society after the Endeavour's return to England.

Appearances of Venus by Capt. Cook
Appearances of Venus by Capt. Cook
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Cook and Green recorded what they saw and produced diagrams to show the progress of the Transit. These illustrate the difficulties of their task and the discrepancies in their observations. What both observers agreed upon was that the planet was surrounded by the “dusky shade” or atmosphere.

 

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