For Scientists of the 18th Century, the Transit of Venus Was Their Final Chance to Measure the Solar System

The story of an 18th-century voyage to a remote island in the Pacific, in the hopes of pinning down the distance between the Earth and the sun

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James Cook's measurements of the 1769 transit of Venus. (NASA)

Later today, as the planet Venus glides across the face of the sun, people all around the world will gather to watch. For 21st-century observers, the event -- known as the transit -- is a chance to see the orbits of the solar system at work as they come into a momentary alignment. Another transit will not occur until 2117. 

Unless you are very young and plan to live for a very, very long time, this is it for you.

So perhaps you feel a bit of pressure to get out your solar shades and catch the historic event. But that pressure is nothing -- nothing -- compared with the pressure that was on British Captain James Cook and the crew of the HMB Endeavour when they set sail in 1768 to reach Tahiti in time to watch the 1769 transit. It was their hope that accurate measurements of the transit could finally give scientists the data they needed to answer one of the big questions in astronomy at that time: Just how far away was the Earth from the sun? If they could answer that, they could calculate the size of the solar system as a whole.

The ship set sail in August of 1768 in anticipation of the transit that would occur the following June. Their destination was Tahiti, a tiny island of about 400 square miles that had become known to Europeans only one year earlier. It was a dangerous mission: "We ... took our leave of Europe for heaven alone knows how long, perhaps for Ever," wrote Joseph Banks, a young botanist on board the ship.

At the time, astronomers knew six of the planets in orbit around the sun (Mercury, Venus, Earth, Mars, Jupiter, and Saturn) and they knew, basically, the relative distances among them -- for example, that Jupiter was five times farther than the Earth from the sun. But absolute distances -- kilometers or miles -- were as yet unknown. A century earlier, Edmund Halley (of Halley's Comet fame) had developed a trigonometric method for getting the absolute distance, a formula that was simplified by French astronomer Joseph-Nicolas Delisle. The only catch was, they needed data from a transit, and there wasn't going to be one until 1761.

The transit of 1761 came and ... went. The data collected during the transit wasn't very good, due to weather, logistics, and observers' inexperience. Thankfully, they only needed to wait eight years until another transit, and this time they would be ready. They had to be, because the next one wasn't going to happen for another 105 years -- meaning, all of the astronomers alive in 1769 would go to their graves not knowing the answer to one of the biggest questions in their field.

And so they sailed; they sailed for eight months, making landfall in April of 1769, leaving them two months to set up their observatory. Other teams were making similar preparations in Philadelphia, St. Petersburg, the Hudson Bay, and Baja, California (two solid observation points were required for Halley and Delisle's formula). Cook's team had lost six of its 94 men en route, but, the rest had made it in time. "At this time we had but very few men upon the Sick list ... the Ships compney had in general been very healthy owing in a great measure to the Sour krout," Cook wrote.

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Rebecca J. Rosen is a senior editor at The Atlantic, where she oversees the Business Channel. She was previously an associate editor at The Wilson Quarterly.

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