Chasing Venus

Andrea Wulf

As a young man, Edmund Halley traveled to St. Helena where he observed a transit of Mercury. At once he realized that if two observers were widely separated in latitude, they would see a transiting planet move along different chords as it traversed the Sun. If each observer timed the transit from beginning to end, the shift in the planet’s position — its parallax — could be calculated and used to determine the Earth-Sun distance. Although transits of Mercury are relatively frequent, the planet’s parallax is too small to be of any use. However, a transit of Venus is a different matter. But these transits are very rare, they occur in a pattern that generally repeats every 243 years, with pairs of transits eight years apart.

At that time, 18th century astronomers had a map of the solar system, but no idea about the true size. By using Halley’s method, they could determine the distance between the Sun and the Earth and therefore map out the true size of the solar system. Venus was they key to unlock this secret. The quest to use a transit of Venus to calculate the Earth-Sun distance became one of the great scientific obsessions of the 18th century.

For the first time in history, scientists needed to coordinate their observations with those from other countries. Nations locked in battle had to work together for the first time in the name of science for the first time ever. In many dozens of locations, many hundreds of astronomers would have to point their telescopes at the sky at exactly the same moment in order to see Venus’s progress across the burning disk of the sun and afterwards, share their findings. This was a time that Europe was in war, and the astronomers had to overcome warring armies, pirates, violent storms, tropic diseases and bitter cold. But they were literally prepared to die for science.

This book describes the efforts of those astronomers that were dispatched across the world to observe this rare celestial encounter. The book is divided in two parts, the first one detailing the efforts of various astronomers during the first transit on the 6th of June 1761, and the second part details the transit on 3-4 June 1769. It is a story of perseverance, disappointment and sometimes just bad luck. Imagine having traveled to the other side of the world, across hurricanes and pirates. French astronomer Guillaume Le Gentil, who never reached his destination due to the Seven Years War and had to observe the first transit from a rolling ship. Determined, he decided to stay in India for the next 8 years in order to observe the second transit, only to be the spectator of a fatal cloud. Disappointed, he decided to head for home, only to discover he had been declared legally dead, his wife had remarried, and all his relatives had “enthusiastically plundered his estate”. What a stark contrast with English astronomer Charles Green, who under captain James Cook was sent to tropical Tahiti, where he witnessed the transit in more pleasant circumstances where they celebrated the success by seducing some handsome girls who, as they were delighted to discover, agreed to sleep in their tents.

Both transits of Venus provided scientists with invaluable opportunities to narrow down the distance between the Sun and the Earth to agreeable ranges. It also was the first time that there was a remarkable international collaboration and competition among astronomers. The science of two nations may be at peace, while their politics are at war. It was this “Peace of the scientists” that proved to be of vital significance to the advancement of knowledge.

Wulf skillfully weaves together the historical backdrop, scientific discoveries, and personal narratives of the astronomers who embarked on these daring expeditions. The book effortlessly combines scientific knowledge with human stories, making it accessible and engaging for readers with varied backgrounds.

  • Andrea Wulf

    Chasing Venus: The Race to Measure the Heavens

    ISBN: 9780307744609 | Pages: 336 pages | Publication date: February 26, 2013

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the captivating story of how 18th-century astronomers, driven by Edmund Halley's groundbreaking insight, embarked on daring global expeditions to observe rare transits of Venus across the Sun

— Bill
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