One of the rarest of all predictable astronomical phenomena, a transit of Venus, will take place this week and if you miss it, you won’t see another until 2117. Nick Leech speaks to astronomer Dr Ilias Fernini and historian Andrea Wulf to find out more.
On Wednesday, Dr Ilias Fernini will look up wistfully at the night sky. If the assistant professor of astronomy at the UAE University in Al Ain could have his own way, dawn would occur at seven minutes past two in the morning; this would allow him to observe a transit of Venus, one of the rarest of all predictable astronomical phenomena, in its entirety.
As it is, Dr Fernini will only be able to start his observations here in Abu Dhabi at 05.36, by which time Venus’s journey will already be well under way. “We will not see the beginning and it will only be observable for around three hours until about 08.35,” he says. This won’t detract from the awe of seeing a planet almost as large as Earth, dwarfed and silhouetted by the immensity of our nearest star. Dr Fernini is understandably excited. “For me as an astronomer, the transit is special because we are observing an event that will not happen again in our lifetimes.”
Transits of Venus occur in a pattern that only repeats itself every 243 years. They happen in pairs, eight years apart, separated by gaps of 121.5 years and 105.5 years. The last transit, only the sixth to have occurred since the first recorded observation in 1639, was in June 2004. After Tuesday, another will not be another visible until 2117.
Despite their rarity, transits of Venus are now well-understood and predictable events that can be viewed in comfort and safety but this wasn’t always the case. In 1716, the British astronomer Edmond Halley threw down a gauntlet to future generations by calling on the world’s scientific community to unite in an unprecedented global experiment to measure the transit that was predicted for June 6th 1761. According to Halley, if enough astronomers were stationed widely enough around the globe and enough data was collated, it should be theoretically possible to answer the most pressing astronomical question of the age, the size of the solar system, by mathematically comparing their results.
In 1760, hundreds of astronomers rose to the challenge and risked their lives by travelling thousands of miles, braving the elements, war, pestilence, and the unknown to make accurate observations of the event and it is their remarkable story that historian Andrea Wulf recounts in her latest book, Chasing Venus, the race to measure the heavens. Wulf is unstinting in her praise of the astronomer’s audacity, bravery, and dedication to the pursuit of knowledge. She describes the logistics of the Venus missions as ‘mind-boggling’.
“Imagine a time when there wasn’t even a standard unit of measurement on Earth,” she tells me. “When it took three months for a letter to get from Philadelphia to London, and these men travelled to the furthest reaches of the globe simply because they thought the project was so important.”
The attempts to map the transits of 1761 and 1769 saw expeditions launched by governments and the leading scientific societies of the day. The British dispatched Captain James Cook to Tahiti and Charles Mason and Jeremiah Dixon (later of the eponymous line) to Sumatra. France sent astronomers to India, California, and Siberia while Catherine the Great sponsored eight expeditions that set out across the icy vastness of the Russian empire.
Some of the motives for this Herculean effort were noble than others. While the astronomers may have seen themselves as part of some international brotherhood of science, monarchs, governments, and commercial ventures like the British East India Company were more interested in the commercial and strategic advantages that an understanding of the size of the solar system would bring, particularly in the field of navigation.
Venus had exercised the imaginations of priests, artists, and thinkers since prehistory but it was during the 17th and 18th centuries that the planet played a fundamental role in defining our current understanding of the universe and our place within it. In 1610, Galileo’s telescopic observations of the phases of Venus finally overturned the older geocentric theories of the solar system that had existed since before Aristotle, by showing that the planets orbited the Sun, not the Earth. In the seventeenth century, astronomers had understood the relative distances between the Earth, Sun, and other planets but what they didn’t understand was how far apart the planets actually were. As Wulf writes:
“Eighteenth-century astronomers had a map of the solar system, but no idea of its true size. Without knowing how far the earth really was from the sun, such a map was all but useless. Venus … was the key to unlocking this secret. As the brightest star in the sky, she also became the perfect metaphor for the light of reason that would illuminate this new world and extinguish the last vestiges of the Dark Ages.”
It took a long while for the data from each transit to be collated and the results from the 1761 observations were riddled with irreconcilable mistakes. The data from the transit of 1769 proved more reliable, and allowed astronomers to determine the distance between Earth and Sun within a range of 92,900,000 to 96,900,000 miles – very close to today’s value of 92,960,000 miles.
Like subsequent attempts to split the atom, reach the moon, or map the human genome, the 18th century efforts to observe the transits of Venus were the pre-eminent scientific endeavours of their time. For Wulf they occupy “a pivotal moment” in an era in which man tried to understand nature through the application of reason and, just like Diderot’s Encyclopédie, Samuel Johnson’s dictionary, and Linnaeus’s sexual classification of plants, they are the product of a very particular historical moment. “The period of the 18th century transits is a wonderful moment when two things come together. Not only are the transits able to be accurately predicted, but technological innovations in clocks and telescopes mean that they can be accurately observed and measured as well.”
For Wulf, the real value of the Venus missions lies not in their historical status as the world’s first truly international scientific experiment, but in the fact that they continue to have an impact on contemporary science, culture, and society. “We tend to talk about a global village as a twentieth century phenomenon, but the foundations were laid in the transit decade. It’s the beginning of modern science as we understand it and the model for the international scientific cooperation, communication, experimentation, and funding that continues to this day,” she says.
For Dr. Fernini at UAEU, observations of the transit of Venus also have a currency that extends beyond their rarity. “The transit is an important way to present astronomy, physics, and science to a wider public.” He hopes that other students, faculty members and even teachers and pupils from local schools will accept his open invitation to attend his observations on the UAEU campus, despite the early start.
A version of this article originally appeared in The National, Abu Dhabi.
Chasing Venus: the race to measure the heavens by Andrea Wulf is available from all good bookshops.