S.J. Gale
Transits of Venus are amongst the rarest of predictable celestial phenomena. They occur in pairs eight years apart, with gaps of over a century between each pair. The last transit took place in June 2004. The event of 6th June 2012 completed this century’s couplet and the next pair will not occur until December 2117 and December 2125.
Transits are also amongst the most stunning of astronomical spectacles. Over a period of around six hours, the black dot of Venus follows a path across the face of the Sun. Fiji lay within that part of the Earth from which the entire sequence could be viewed, and with a bright sunny day and plenty of gaps between the clouds we were able to witness an event that, even with massive injections of monkey glands and copious amounts of prayer, none of us will ever see again.
Figure 1. The 5th June 2012 Transit of Venus across the face of the Sun (Source: NASA Solar Dynamics Observatory Atmospheric Imaging Assembly).
But this is not just an event of astronomical significance for the Pacific. It marks a significant historical anniversary, for it was the Transit of Venus that brought James Cook into this ocean and changed the Pacific world forever. If we think of it at all, most of us probably believe that Cook’s first voyage was one of geographical discovery. But, as detailed in the instructions he received from the British Admiralty, the main aim of his expedition was to sail to Tahiti to observe the Transit of June 1769. It wasn’t that the Admiralty had suddenly been imbued with high scientific ideals in sending Cook to do this, although the Royal Society, which originally proposed the trip and petitioned King George III for his support, was committed to the scientific results of this work. But none of this would have persuaded the Admiralty to fund and equip an expedition to the 18th century equivalent of the dark side of the Moon if the measurements hadn’t held the key to successful blue water navigation for the largest fighting machine in the world, the British Navy.
The fundamental part of the entire enterprise was the measurement of the distance between the Earth and the Sun. One way to do this involves determining the distance to other planets in the Solar System and using this information to calculate the distance to the Sun. By the early 18th century, astronomers knew enough about the motions of the planets to be able to predict when Venus would lie between the Earth and the Sun, and thus when a Transit of Venus would occur. The Transit provides a means of calculating the planet’s distance from the Earth, but only if it is possible to observe the spectacle from a number of distantly located points on the Earth’s surface. June 1769 offered a chance to do this, the last opportunity for the next 105 years. The timing and the locations on the Earth at which the Transit could be seen were known and it was a matter of finding suitable locations from which to make the measurements. The last piece of the jigsaw fell into place only a year before the event, when Samuel Wallis discovered (from the European point of view, at least) Tahiti. This offered the ideal location from which to complete the global network of observations and the massive enterprise began. Ships were commissioned and men with the talents to sail them to the remotest corners of the planet were sought. The expeditions were equipped with the razor edge of 18th century scientific technology. The vessels left Britain’s shores at intervals with the aim of simultaneously reaching their destinations in time to observe and record the event.
Figure 2. James Cook’s and Charles Green’s observations of the appearance of the start of the Transit of Venus of 3rd June 1769 as seen from Point Venus, Tahiti (Source: Green, C. and Cook, J. 1771. Observations made, by appointment of the Royal Society, at King George’s Island in the South Sea; by Mr. Charles Green, formerly Assistant at the Royal Observatory at Greenwich, and Lieut. James Cook, of His Majesty’s Ship the Endeavour. Philosophical Transactions 61, 397–421).
Cook, navigating for 230 days to a fly speck in the biggest ocean on Earth, reached Tahiti with almost two months to spare. His scientists carefully constructed an observatory of stone on what is now known as Point Venus. They awaited the 3rd of June with trepidation. An overcast day could wipe out years of planning and investment, but conditions were ideal and three observers were each able to make careful measurements of the passage of Venus across the Sun’s disc. At this point the tale of geographical and scientific triumph is rather spoilt, for the three sets of observations were more variable than was consistent with simple measurement uncertainty alone. It turns out that the measurements were much more challenging to make than was realised at the time (for reasons that were not fully understood until 1999). The range of distances to the Sun was refined down to between 148 million and 156 million kilometres, much better than previous estimates, but not really good enough to be used for reliable navigation.
The critical part of Cook’s voyage from the Pacific point of view came next, but that, as the cliché goes, is another story.
