The Evolution of Point Venus, Tahiti
In 1768, James Cook left England on the Endeavour bound for Tahiti. The first of his three famous voyages, one of Cook’s tasks was to observe the transit of Venus. The transit is one of few easily observable and predictable astronomical events and occurs in eight-year pairs, separated by first 101.5, then 121.5 years (Espenak). During the transit, Venus passes between the Earth and the Sun and is visible to the naked eye as a black dot approximately 1/32 the size of the Sun. The most recent transit occurred in 2012 and was the eight-year mate of the 2004 transit. With the next transit not set to occur until 2117, those who viewed both witnessed a rare and special event (Phillips, “Transit”). The transits of 1769 and 2012 provide a comparison in the evolution of science and of landscape. The scientific tools at the disposal of James Cook seem modest compared to modern solar instruments, and the landscape of Point Venus, where Cook observed the transit, has changed dramatically as well.
Captain James Cook lived in a time of scientific revolution called the Age of Enlightenment. An intelligent, self-made expert in mathematics, astronomy and hydrographic survey, he was selected by the Royal Society of England to make the voyage to Tahiti because of his ability to contribute to this surge in scientific discovery (Herdendorf, 40). The 1769 transit of Venus had an added significance in the scientific community because it was the key to understanding the size of the solar system (Orchiston, 67-71).
Scientists in the 18th century had an inaccurate understanding of the distance between the Earth and other planets. The relative spacing of the planets was understood, however no numbers were associated with these distances (Phillips, “Cook”). When Cook left England for Tahiti, Edmund Halley understood that accurate observations of the transit of Venus could be used to determine the size of the solar system. Halley found that by having two people view the transit at the same time on the same line of longitude but different lines of latitude, the calculations could be made to find the distance between the Earth and the Sun. In essence, the solar parallax, which is the angle formed by lines “drawn from the centre of the Earth and from the observer’s location on the circumference of the Earth,” can be converted into the astronomical unit, which is the actual distance between the Earth and the Sun, using the value of the radius of the Earth and simple trigonometry (Herdendorf, 41). Finally, using Johannes Kepler’s Third Law of Planetary Motion, the value of the astronomical unit can be extrapolated in order to understand the size of the solar system (Orchiston, 67-71).
The transit of 1769 was additionally important considering that the previous transit was a scientific failure. Bad weather and poor communication between nations resulted in a lack of data to calculate the solar parallax in 1761. The next transit would not occur until the 19th century, making 1769 the last chance any scientist alive had to make the calculation. Armed with this information, James Cook sailed for Tahiti, recently visited by Captain Samuel Wallis and deemed the perfect place to observe the transit because it was one of few locations where the entire event would be visible (Salmond, 124). According to his journal, Cook and his crew arrived in Tahiti in April of 1769 and began to build an observatory on the 18th. Known as Fort Venus (Figure 1), the structure served as a solid platform on which to make the necessary observations, and provided protection for the astronomical equipment from the Tahitians who, according to the log, had a penchant for stealing the instruments (Hawkesworth, Chap. IX). The fort was built on Matavai Bay on the north side of Tahiti, was fortified with weapons from the Endeavour and was spacious enough to fit over forty of Cook’s men (Salmond, 147). The instruments housed in the fort had improved significantly since 1761 and represented the best that was available for the time (Herdendorf, 43).
Cook was equipped for his journey by the Royal Society and the Royal Observatory of England with the most sophisticated instruments of the time (Orchiston, 56-57). At his disposal were two Gregorian reflecting telescopes with wooden stands, one astronomical quadrant, an alarum clock, a brass Hadley’s Sextant, a barometer, two thermometers and a dipping needle (Kaye, 8). Using these instruments, Cook needed to record the times of the four vital moments of the transit, known as contacts. These are the points at which (1) Venus just touches the edge of the Sun, (2) the planet’s opposite edge is at contact 1, (3) Venus has traversed the Sun and first touches its opposite edge, and finally (4) the last moment that Venus is in contact with the Sun (Espenak). Cook was unable to obtain precise measurements of the contacts due to the limitations of his instruments. He appeared frustrated and following the transit he recorded in his journal: “We all saw an atmosphere or dusky cloud round the body of the planet, which very much disturbed the times of contact…and we differed from each other in our accounts of the times of the contacts much more than might have been expected.” Cook refers to Charles Green and Dr. Solander, who were also members of the voyage of the Endeavour and observed the transit from another location on Tahiti and the neighboring island of Moorea (Hawkesworth, Chap. XIII). The “dusky cloud round the body of the planet” refers to the black drop effect, which is a phenomenon created by Venus’ atmosphere that causes the edge of the planet to appear in contact with the edge of the Sun longer than it actually is. This effect caused the data recorded by Cook, Green and Solander to differ by several seconds (Phillips, “Cook”).
Despite Cook’s difficulties in obtaining accurate values of the four contacts, mathematicians used his observations to find a value for the solar parallax and thus the astronomical unit. The presently accepted value for the distance between the Earth and the Sun is 149,597,870 kilometers, defined as one astronomical unit (“Glossary”). Cook’s observations were accurate to within 5% of this value, which is impressive considering the difficulties overcome to make this observation (Orchiston, 67-71).
Following the 1769 transit, technology improved the accuracy of the astronomical unit. During the 1874 and 1882 transits, photography allowed scientists to eliminate human error and the black drop effect (Orchiston, 67-71). The invention of radar in the 1960s allowed for the most precise measurement and the calculation of the presently accepted value of the astronomical unit (Phillips, “Cook”). Solar telescopes used for the 2004 transit produced stunning images (Figure 3) that are quite a contrast to the drawings of Cook and Green. The most recent transits were therefore not observed for the purpose of determining the size of the solar system, but were nevertheless scientifically important.
The 2012 transit featured high-tech observatories around the world ready to watch the tiny black dot of Venus traverse the Sun. The solar telescopes of NASA’s Solar Dynamics Observatory are notable in the Hubble-quality images they produced (Phillips, “Cook”). The National Solar Observatory (NSO) and the associated NSO Integrated Synoptic Program (NISP) were other notable contributors, and projects associated with this group analyzed the atmosphere of Venus (“Success!”). By studying Venus’ atmosphere, scientists hoped to develop techniques to measure the atmospheres around planets outside of our solar system (Klotz). The Venus Twilight Experiment also studied the 2012 transit with goals to understand the atmosphere of Venus and to use this information to determine the habitability of extrasolar planets. The Venus Twilight Experiment used an instrument called a Cytherograph, which uses at least eight lenses and filters in order to analyze the actual atmospheric composition of Venus (Tanga). In comparison, James Cook’s reflecting telescopes offered views of Venus using two concave mirrors, which provided only 140 times magnification (Herdendorf, 46). This difference highlights the dramatic advances in technology that have changed the way scientists study the transit of Venus.
As an event that requires observations around the globe, the transit of Venus creates an international sense of community. The 1769 transit was the first major international scientific effort, and the observations of the 2012 transit crossed borders as well (Herdendorf, 43). Live feeds of the event were posted on the Internet and any amateur sky-watcher could report their observations to a database run by NASA (Espenak). Those adventurous enough to travel to Tahiti had a front row seat to the entire transit, just as Cook did over 200 years ago (Veillet).
Just as the instruments have changed, the location from which Cook observed the transit has changed as well. Today Point Venus looks much different than when James Cook visited almost 250 years ago, with surfers, swimmers, sunbathers and fishermen lining the location where Fort Venus once stood. As shown in the modern day image of Point Venus (Figure 2), no evidence remains of Cook’s observatory. There is, however, one small white monument with a bronze plaque that welcomes visitors and includes information about Captain Cook. Although no physical evidence of Cook’s visit remains, visitors are now able to learn about his purpose in travelling to Tahiti, and his efforts in observing the transit of Venus.
Hannah Aichelman, University of North Carolina at Chapel Hill
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Figure 1. A drawing of Fort Venus by Samuel Parkinson, an artist who travelled on Cook’s first voyage. The inscription reads: “Venus Fort, Erected by the Endeavour’s People, to secure themselves during the Observation of the Transit of Venus, at Otaheiti.” (Wikimedia Commons)
Figure 2. A photo of Point Venus taken on a beautiful day in January 2011. Courtesy of Mary Malloy.
Figure 3. Venus beginning its transit across the face of the Sun during the 2012 transit. This image was taken by a solar telescope that is part of NASA’s Solar Dynamics Observatory (Wikimedia Commons).
How to cite this page:
Hannah Aichelman. “The Evolution of Point Venus, Tahiti,” Atlas for Sustainability in Polynesian Island Cultures and Ecosystems, Sea Education Association, Woods Hole, MA. 2013. Web. [Date accessed] <html>