Book review: Chasing Venus

Like many astronomy buffs, we’ve been putting a great deal of thought into deciding where we’ll go to try to see the total solar eclipse that will cross the United States next August 21. Seattle Astronomy has done 13 articles and a dozen podcasts on the topic. As with the 2012 Venus Transit or this year’s Mercury Transit, the key is figuring out where you’ll have the best odds for clear skies, and how to get to an alternate site if the clouds beat those odds on eclipse day.

With such thinking fresh in mind, I eagerly snapped up a copy of Andrea Wulf’s book Chasing Venus: The Race to Measure the Heavens (Vintage Books, 2012) when I spotted it in the astronomy section of Powell’s Books during a recent trip to Portland.

Chasing Venus is the story of the Venus transits of 1761 and 1769, and the international scientific effort to accurately observe the transit from many spots around the globe and use the solar parallax between those observations to calculate the distance between the Sun and the Earth, and thus get a true grasp for the size of the solar system.

The whole project was the brainchild of British astronomer Edmund Halley, who predicted the 1761 transit and wrote an essay in 1716 that urged scientists to spread out across the globe to make these vital observations. Halley was 60 at the time of the writing and would have to live to be 104 to see it himself; he died in 1742.

Scientists and nations answered the call with enthusiasm. This was, Wulf writes, “a century in which science was worshipped, and myth at last conquered by rational thought.” One is tempted to think that we’ve regressed in the intervening 247 years.

Technology was a challenge for the observers. They had good telescopes, but had to transport them long distances, in many cases, and set up observatories in remote locations. A bigger challenge was the actual timing of the transit. Clocks were not yet reliably accurate, and precise determination of longitude was still a challenge. The greater difficulty was actually getting to the observation sites. It was easy for those in the cities, but for the calculations to work observations had to be made from points on Earth as far apart as possible. Thus for every astronomer observing from the relative comfort of Paris, London, or Madrid another team was on a treacherous expedition to the far-flung corners of the world in an era when it took several months to get a letter from the American colonies to the European capitals. For the 1761 transit, the journey of French astronomer Jean-Baptiste Chappe d’Auteroche to Siberia was particularly harrowing, and those traveling by sea had to navigate not only the sea but the politics of the day, lest their efforts be defeated by heavy waters or hostile navies.

Imagine the mindset of astronomers who, at great expense, had to travel for months, and in many cases more than a year, in order to set up and prepare for an event that, if it was cloudy during the wrong six hours, would be a total bust. It makes our deliberations about where to go to see the 2017 total solar eclipse seem trivial by comparison.

Measurements of the first transit proved largely unsuccessful. Weather foiled many of the expeditions, and a variety of problems caused great variance in the accuracy of the data collected. But they learned from the effort and improved their approaches, and by the time of the 1769 transit, the combined observations narrowed down the distance to the Sun to within four million miles, which was quite an improvement.

Wulf spins a great tale of scientific inquiry, daring (and not-so-daring) adventurers, political intrigue, and fascinating personalities involved in what was arguably the biggest collaborative international scientific event up to that time. It’s a marvelous read and highly recommended.