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RACHEL MARTIN, HOST:

This is WEEKEND EDITION from NPR News. I'm Rachel Martin.

It has been a good season for stargazers, a veritable meteor shower of astronomical goodies, from a supermoon to a solar eclipse just a couple of weeks ago. Next up, on Tuesday, witness the transit of Venus, one of the rarest astronomical events.

For more on this, I'm joined by Andrea Wulf. She's the author of a new book called "Chasing Venus: The Race to Measure the Heavens." Andrea, thanks for coming in.

ANDREA WULF: Thanks for having me.

MARTIN: So, first question. Can you explain the transit of Venus? What is this phenomenon? What happens?

WULF: It's actually quite simple. So, Venus moves between Earth and Sun. So instead of seeing Venus as the brightest object on the night sky, you see Venus as a tiny black dot crossing the burning disc of the Sun. So you see it during the day. It lasts for about six hours. And, as you said, it's one of the rarest astronomical events. It happens in pairs, eight years apart. And then it takes more than a century to occur again. And I think we will be the last living people to see one, because the next one is going to be in 2117.

(LAUGHTER)

MARTIN: It happens, as we said June 5th, and it's more of an object of beauty and curiosity than one of scientific significance these days. But that was not true back in the year 1761. Explain why was it so momentous to 18th century scientists.

WULF: In the 18th century, astronomers believed that they could measure the distance between Earth and Sun through the transit of Venus and, by extension, the size of the solar system. What they thought they could do is they would measure the time and duration of the transit, they could then collect the data they needed to find out about the distance.

The only problem was that they needed to have hundreds of observers looking at this from as far apart places as possible. Because if you were an observer in the Northern Hemisphere, you would see Venus marching across the Sun on a slightly different track to an observer in the Southern Hemisphere. And these differences in the tracks across the Sun, that's what they needed to calculate the distance between Earth and Sun.

So, this became the first global international scientific collaboration. I think that for me that's the amazing story about this, because you have hundreds of astronomers working together in the midst of the Seven Years' War.

MARTIN: And this is the thrust of your book, kind of chronicling this major international scientific project. A man by the name of Edmond Halley, a British scientist who was kind of at the center of this. Explain who he was and how does he end up predicting the transit.

WULF: So, we Know Halley really from Halley's Comet, which he predicted. But he also predicted something else. He, in 1716 - so almost 50 years before the transit of 1761 - he wrote a 10-page essay. And in this essay he predicted that there would be a transit on the 6th of June 1761, and that this transit could be used to calculate the size of the solar system. And he knew he's not going to be alive, and then because he was already quite old in 1716. So what he's doing, he's calling a future generation of scientists, saying, whatever happens, you know, in 1761, you will all have to work together to observe this spectacle.

MARTIN: And they answered the call.

WULF: Yes, this is a time when, for example, a letter from London to Philadelphia takes two to three months. There are no standardized measurements on Earth. And they're trying to, you know, all work together to come up with one common value, the distance between Earth and Sun. And so, they're sending out these expeditions to the remotest places in this world.

And these astronomers - they travel not lightly. So they travel with instruments which weigh about a half a ton, so they're like schlepping around these instruments.

(LAUGHTER)

WULF: There's a French astronomer, for example, who goes from Paris to Siberia. And he, for example, because his instruments are so heavy, he constantly crashes with his sleds through the frozen rivers.

MARTIN: And, as you reference, this is also a time of war. Did that impact the project at all?

WULF: Yeah, yes. The Seven Years' War, which started in 1756, is a war which tears apart Europe. So these astronomers - a lot of them get attacked so it makes it incredibly difficult.

MARTIN: So we don't want to spoil anything for readers. But how does this project work out?

WULF: Because there are two transits, so there's the 1761 that doesn't go very well. But they knew they had a second chance because there was the second transit eight years later in 1769. And the conditions for that transit were much better. The political conditions had much improved because the Seven Years' War had ended, and the astronomical conditions were also better.

So they come up with a value, which is very, very close to today's value. So today we know it's just under 93 million miles. And they come up with a range between 93 and 97 million miles - so pretty impressive.

MARTIN: And pretty remarkable considering the rudimentary state of their instruments and everything they had to go through to get that measurement.

WULF: Yeah. Also, I think in a funny sort of way, I think what's actually more impressive is that they worked together peacefully and successfully. And, you know, I would argue that they laid the foundation of modern science, how we understand it today. You know, all these kind of big science projects today, we take that for granted - international collaboration - but we tend to talk about this as if this is something rather recent. You know, the global village, all that. It all started in the transit decade. They laid the foundations. And all these connections they made with each other remained in place, and there were lots of other projects which kind of grew out of that later.

MARTIN: Andrea Wulf's new book is called "Chasing Venus: The Race to Measure the Heavens." Andrea, thanks so much for coming in.

Thank you for having me.

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