In the 18th century one of the biggest mysteries facing astronomers was the size of the solar system. They knew the relative spacing of some of the planets that orbited the sun but had yet to establish the absolute distances between them.
A global project
The project, sponsored by the London Royal Society, saw expeditions dispatched around the world—including to North Cape in Norway, Hudson Bay in Canada and Tahiti in the South Pacific—to observe the transit of Venus.
The transit of Venus is a rare celestial event that occurs only four times every 243 years when Venus passes between the earth and the sun. During the transit, Venus is visible to the naked eye as a black dot approximately 1/32 the size of the sun. This 243-year pattern consists of pairs of transits eight years apart, separated by breaks of 121.5 and 105.5 years.
Viewing and recording the transit of Venus promised many benefits, including a better understanding of the size of the solar system and more accurate mapping and navigational techniques.
By observing the transit from multiple locations around the world the measurements would be more precise. It would also provide better odds against bad weather and bad luck interfering with the observation—both of which had derailed efforts to view the 1761 transit.
Fort Venus and the missing quadrant
The HMB Endeavour arrived in Tahiti on 13 April 1769, some seven weeks before the 3 June transit of Venus.
The ship and crew were largely well received, coming after British navigator Samuel Wallis’ five week visit in 1767 which had left a positive impression on the Tahitians.
A site was chosen for the main observatory on Matavai Bay on the north side of Tahiti. Two other observation posts were established on Irioa Island (Moorea) and Taaupiri Island (off the east coast of Tahiti).
The Royal Society and the Royal Observatory of England had provided the expedition with the most sophisticated instruments available including two Gregorian reflecting telescopes with wooden stands, one astronomical quadrant, an alarm clock, a brass Hadley’s Sextant, a barometer, two thermometers and a dipping needle.
The observatory comprised an assortment of large wood and canvas tents big enough to house the necessary scientific equipment and fit more than 40 men. The instruments were set up in wooden frames or on casks fixed firmly in the ground and filled with heavy wet sand for stability.
Known as “Fort Venus”, the observatory was intentionally positioned so it could be protected from interruptions during the transit. It was under constant guard and fortified with weapons from the ship. So, it came as a shock, in early May—just a month out from the transit—when one of the costly astronomical instruments disappeared overnight. The quadrant was essential to viewing and charting the transit and there was no possibility of sourcing a new one.
Cook recorded the discovery of the missing quadrant in his journal on 2 May 1769:
‘This morning about 9 oClock when Mr Green and I went to set up the Quadrant it was not to be found, it had never been taken out of the Packing case (which was about 18 Inches square), sence it came from Mr Bird the Maker, 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 together with several other Instruments, but none of them was missing but this.’
A bounty was offered and a search ensued to find the missing item. Eventually the stolen instrument was located but it had been dismantled and some small pieces were missing.
The quadrant was a complex instrument and it fell to HMB Endeavour’s third naturalist, Herman Diedrich Spöring to draw on his skills as a former watchmaker and instrument maker to reassemble and repair it.
Observing the transit of Venus
On 3 June, 1769, the weather was warm and clear allowing Cook, astronomer Charles Green and others to observe and record the transit of Venus from the three observation posts in the South Pacific.
Despite the ideal weather conditions, the team’s ability to record precise measurements was stymied by an odd phenomenon, which Cook recorded in his journal:
‘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.’
Cook and his team had anticipated they would see and be able to record the timing of a small black circle passing in front of a large bright one but instead they saw something that first resembled a drop of water, then tapered off into a tail before breaking free and moving as a circle across the sun.
In his Royal Society paper Cook explained the problem in more detail, describing how it was:
‘… very diﬃcult to judge precisely of the times that the internal contacts of the body of Venus happened, by reason of the darkness of the penumbra at the Sun’s limb, it being there nearly, if not quite, as dark as the planet. At this time a faint light, much weaker than the rest of the penumbra, appeared to converge towards the point of contact, but did not quite reach it … in like manner at the egress the thread of light was not broke oﬀ or diminished at once, but gradually, with the same uncertainty: the time noted was when the thread of light was wholly broke by the penumbra.’
This “black drop effect” threw out the measurements of Cook’s team as well as those observing the transit in Norway and Canada and would continue to puzzle astronomers and hamper efforts to accurately measure the solar system well into the 20th century.
• English clergyman-astronomer Jeremiah Horrocks (c. 1619–41) is said to have been the first to successfully record the transit of Venus in 1639.
• Venus takes nearly seven hours to cross the face of the sun, but the event is divided into four "contacts" that mark different phases of the transit.
• The next transit of Venus will happen on 10-11 December 2117 with the second of the “pair” of transits happening on 8 December 2125.