This article is from the In-Depth Report The 2012 Transit of Venus
Science Talk

The Transit of Venus, Part 1

With a transit of Venus coming up on June 5th or 6th in different parts of the world, Mark Anderson, author of the book The Day The World Discovered the Sun, talks about the great efforts to track the transits of Venus in the 1760s and the science they would produce

Podcast Transcription

Steve:          Welcome to the Scientific American podcast Science Talk, posted on May 30th, 2012, I'm Steve Mirsky. On June 5th or 6th, depending on where you are, you will have the rare chance to see a transit of the planet Venus. Your next shot after this one is going to be the year 2117, so good luck with that. With the transit coming up I called Mark Andersen, he’s the author of the book, The Day the World Discovered the Sun, about the great efforts to track the transits of Venus in the 1760s. Back then, it was less celestial sight-seeing and more hard science. Here's part 1 of my phone conversation with Mark.

Mark Andersen, great to talk to you today.

Anderson:         Thank you for having me Steve.

Steve:          It's my pleasure. It's a fascinating book, The Day the World Discovered the Sun. It has a certain literary sensibility to it, which I think is great. You want to talk a little about your background? Because you bring a style that's maybe just a little unusual for a science book.

Anderson:         Well my book, first and foremost, I wanted it to be the story, this amazing adventure that is combining both the science, the cutting edge science, and the incredible stories—these odysseys—that these explorers and scientists, you know, underwent to really cross much of the planet to find the distance to the sun and many other things as well. But, so I was approaching that, kind of, from a little bit of, you know, a nonfiction angle, a little bit of a, kind of, Patrick O'Brian, if you will, adventure, you know, a novel kind of approach. So, my own background is I have a graduate degree in astrophysics and studied physics and astronomy as an undergraduate, and I've been a science and technology journalist for the last 12 or so years. I've written a book about Shakespeare, so I have not exclusively written about science and technology, but I've written for Scientific American, Wired and Discoverer and lot of familiar places. And so, I was bringing a little bit of, kind of, the story telling sensibility but also wanting to stay true to the science.

Steve:          And it really does read like one of those Patrick O'Brian books, and it's a lot of fun to read. But let's define some terms, let's go back. You talked about cutting edge science—this is cutting edge science of the 1760s. And first, explain to everybody what a transit of Venus is.

Anderson:         The transit of Venus has been called 'the rarest eclipse'. It happens less than twice per century and what it is, is the planet Venus briefly passing directly in front of the sun, and that takes about 6 hours.

Steve:          From our vantage point.

Anderson:         From our vantage point, yeah.

Steve:          What was it in the 1760s—and actually we go back to really Halley, who in the 20–30 years before that realized the wealth of information that this celestial event would be able to provide.

Anderson:         Yeah, I mean, in a sense it, kind of, gave dimension to the universe, I mean, the third dimension. There is no way of measuring physical distance scales, so there’s no immediate way of measuring these distance scales, you know, out there amongst the stars and the planets. So, this was a long-standing puzzle for centuries, millennia really. Not only how far away is the sun, but how far away are all the objects that we see? We can't measure the distance to  the stars. We haven't been able to do that, or we weren't able to do that till the 19th century. But as far as the near universe is concerned, finding the distance to the sun and the size scale of the planets, the solar system, this is a really big deal; because it meant, in some ways, understanding what our universe was made of, what it looked like and where everything could be found. So this was an essential problem in science and astronomy in the 18th century, was the precise, was the exact science of the age. So it was where a lot of the great, you know, scientific minds, both scientific and mathematical minds, had trained a lot of their fire power on this big question. And it was Edmond Halley who realized, "Wow, we could solve all these big problems in one fell swoop by triangulating all those distances from one measurement Well, okay, it's a measurement on one day, but you've got to take it from multiple locations on Earth, and sometimes those locations turned out to be really far flung places. The bottom line is, it's kind of somewhere up in the Arctic and somewhere down in the tropics; that's at the very least what you need.

Steve:          (laughter) Right, you have to go to the worst places in the world to do it.

Anderson:         Or maybe the best. (laughter)

Steve:          Or maybe the best, but either way it's going to be extreme conditions. And I want to talk about Halley for a second. It's a little bit ironic because Halley's comet is this event that happens regularly, and many people will live their entire lives between two visits of Halley's comet. And it was Halley who realized that the next transits of Venus would be able to provide all this information, and he also realized that there was no way he would be able to live to see that.

Anderson:         Yeah, Halley was a brilliant man, and in some ways, kind of, overshadowed by Newton, who of course is one of the great geniuses of all times. But he was Newton's buddy, and Halley realized that once you take some of the mechanics that Newton and others had pioneered, and you run the clock forwards, you can forecast with astonishing precision—decades in advance—where everything is going to be. And Halley discovered that, you know, not only is the Venus transit perfect for solving these big questions in science, that there is going to be two of them coming in the summer of 1761 and in the summer of 1769. In the early 1700s, around 1715–1716, Halley gets on the bandwagon, he starts, you know, telling all of his colleagues and people across Europe, learned people across Europe, "You know, look, we've got to get on the ball here because we can solve one of the most fundamental problems, you know, in the physical sciences with this, you know, with this one day if we get it right." The other part of the story, I should say Steve, is that there are science histories of the Venus transit, and those are really important, for I mean, studying, kind of, the history of the science is important. But I felt that there was, kind of, an elephant in the room in these stories, and that was, kind of, part of the story, but that's the part that I wanted to also bring in; which is navigation, which is the problem of, you know, crossing all of the ocean and sending all your ships, not just, you know, to a port, you know, a few hundred miles away, but you know, maybe half way across the world. This was a tremendously, you know, a significant substantial problem both for militaries, for kings, for captains of industry, you could call it in those days. And astronomy was the way that appeared to be, you know, that appeared to be poised to solve that problem. In the 1760s, right in the middle of these two transits—there was one in 1761 and one in 1769—the astronomers, a lot of the characters in this book, had kind of cracked this puzzle wide open. And so it turns out that the Venus transit voyages are also this perfect test bed for these solutions to the navigation problems, finding longitude at sea. And so what the astronomers had developed is, you might call it an 18th century GPS system, using the moon as the global position satellite. You use the moon and some really clever tables in a book called the Nautical Almanac that was published every year from 1767 onwards. You use the moon's exact position in the sky, you figure that out with, you know, your navigator on your ship, wherever you are on the Earth; he finds the moon's exact position in the sky, and then looks it up his Nautical Almanac and that tells him Greenwich Time and then you compare Greenwich Time to local time, and you have your longitude. That is just a humungous deal. I mean, that really changes so much of the story. It's no coincidence that the great solution to the navigational problem coincides with the world really being opened up in substantial ways, to everything from world trade to world empires.

Steve:          The story has everything in it. It's got commerce, it's got politics, it's got science, it's got religion. Everything is crossing into everything else, and that's part of what makes it such a compelling read. Let's talk just briefly again about the science, and then we can go into some of the drama of the expeditions. But let's talk in a little bit of detail about what you're actually doing when you have these multiple expeditions in various parts of the globe, and they're all watching the transit as it's happening and then they take records of what they're seeing. And how does that altogether, when that information is all compiled, how does that let people figure out how far we are from the sun?

Anderson:         The fundamental measurement that needed to be performed was timing, down to the second if possible, timing the exact duration of the Venus transit from these different locations on the Earth. Because I said it's about 6 hours, but you know, it varies depending on where you are. And so if you're up in the Arctic, you measure that it might be 5 hours and 50 minutes; if you're down in Tahiti, where Captain Cook was in 1769, it might be more like, you know, 5 hours and I forgot the exact number, but something like 5 hours and, you know, 20 minutes. So those numbers, when you compare them you can do—and listeners should you don't have to, I mean I don't take people through that in the story itself.

Steve:          Right, that's in the appendix.

Anderson:         There is a technical appendix for people who, you know, who are interested in, kind of, the equations and some of that stuff. I do get into the technical stuff—there's a technical appendix where I discuss that in more detail. But suffice it to say that you can approach this as essentially a trigonometry problem—and then you have to add layers of complications if you want to get it exact—but what you're making is a big triangle, where you're, where one end of the triangle is Venus, another end of the triangle is your tropical location measuring the transit, and the third end of the triangle is your arctic location. So it looks like a tiny, tiny wedge of pie. And so if you know the distance between those two locations on Earth, and then you know the angle, then you can find the distance to Venus and once you know the distance to Venus, you can find the distance to every other planet and to the sun.

Steve:          And the time it takes Venus to cross the sun from your vantage point gives you that angle.

Anderson:         That's right.

Steve:          Now you say in the book how many slices of pie you could get at that angle. Do you remember the exact figure?

Anderson:         I didn't know this was going to be on the test (laughs). Let's see, I can look it up. It was something like 144,000, I believe.

Steve:          That's what it was. So if you could slice your pie into a 144,000 pieces, that's the kind of angle we're determining here with these measurements from around the globe, which is pretty mind boggling.

Anderson:         Yeah, yeah. I should note, by the way, I just looked it up.  So, when Captain Cook, there were lots of these voyages and lots of these observations; and to bring the story, kind of, down to a human level, where people can appreciate, relate to the characters and you know, follow your story, I decided to just concentrate on three. And so, Captain Cook's is one of those expeditions, and he found that it was about 5 hours and 30 minutes, whereas the arctic expedition, this is led by an Austrian team—we can get to that in a moment—but they found 5 hours and 53 minutes and 14 seconds. So, anyway that gives a sense of, if you can get those numbers precisely then you can get the distance even better; but that gives you a sense that it's not the same for every observer.

Steve:          Let's talk about some of the expeditions. I mean, there were dozens and dozens, but as you say, you concentrated on just a handful; let's talk about those, briefly describe what the different ones were like and the dangers that they faced.

Anderson:         Yeah absolutely. Steve, the stories are quite, you know, it's quite incredible. One reviewer said “mimsy” scientists need not apply. To travel to these locations required, soft of, I don't know, mastering an obstacle course of some of the gravest perils of the age. And so I was looking at really three. Captain Cook is one of them; there's a French team led by this explorer named Jean-Baptiste Chappe d'Auteroche, and he was traveling to the Baja peninsula; and then there's an Austrian team, led by a Jesuit friar by the name of Hell, if you can believe it, and he is going up to one of the northernmost towns in Europe. And so each of these expeditions are facing just tremendous obstacles in getting there, in getting their data and coming back home. There are many others of, you know, there were many other observations, there were a number of other expeditions. But I felt to do justice to the story, I had to bring it down to a level that the reader could keep track of, so that, you know, you're not introducing 25 different characters.

Steve:          It's not an encyclopedia, it's a story about representative voyages and expeditions that these particular individuals make. And I'm sure the other ones that you don't talk about had equal or worse conditions that they had to deal with.

Anderson:         Well, I guess, I'd say this much; that is, I had three, criteria, really for finding those, the best representative expeditions. First of all, they had to have tremendous results; they had to be fascinating characters; and they had to have an amazing adventure, getting there and back. And so those three expeditions were Captain Cook's first voyage on the HMS Endeavor to Tahiti. And then this French and Spanish trip to Mexico, they faced, they didn't realize it when they were shipping out, but they, when they arrived in Mexico, they soon discovered that the place that they'd be observing from was experiencing an outbreak of typhus that was killing people left and right, and yet they decided to stay put. They decided, even though they were risking their very lives to gather this data, they decided to stay put. And then these Austrian and Hungarian explorers going up to above the Arctic circle, some of the people that they encountered on their way north up into the Arctic just figured they were crazy because to travel through the Norwegian sea at the late time of the year that they were traveling was practically suicidal, and I was actually able to bring into the English for the first time, their travel journals. And it's amazing, it is just so incredible Steve; I mean, every day is a new ordeal. And so I don't know which one of those you can talk about or…

Steve:          I'll tell you, well one of the interesting things is, as you say, they had to have good results or great results, so that leaves out a lot of people. You've got to keep in mind that when you prepare for your expedition in 1769 or 1761, maybe you prepare for a few years, you have one shot here, or one shot in 1761 and one more in 1769. If it's a cloudy day where you go…

Anderson:         You're screwed….

Steve:          You're screwed. That's it. And if it's 1769, there's not going to be another one. Maybe your grandkids can go and check out the next one, but you're not gonna make it. (laughter)

Anderson:         Yeah, yeah. No, there were people who also, you know, who went on these expeditions to these far flung places in the world and didn’t work out for them.

Steve:          Let's talk about Chappe, because I’ll tell you why—for me personally, in 1991, I sailed to the Baja to see a total eclipse of the sun; and I was on a luxury cruise ship with midnight buffets and very nice accommodations and an astronomy faculty. Why don't you contrast that with Chappe, who went to basically the same location to see this transit of Venus.

Anderson:         Wow, yeah, well it's nothing like that, I'll tell you that much. So the craft that they crossed the Atlantic ocean in—well first of all (laughs), now they're traveling to Mexico, and it is not just a matter of getting your passports in order. First of all, they're dealing with the king of Spain, who has invited these French explorers to come observe the transit of Venus from one of his colonies; and that involved a major diplomatic, it was a major diplomatic deal to get that in place. And this king of Spain was potentially going to invite the Brits and there was like a, I think it was an English Jesuit in Italy who was going to make it all happen; in fact if that English Jesuit has gotten, had secured that expedition, there might not be a Captain Cook (laughs), you know, immortalized in legend because the English would have had their expedition to the tropics. As history would have it, that deal fell apart with the English, and so Spain turned to France and said, "Hey, you want this?" And France said, "You bet." And so these French explorers traveled to Spain, and they wind up in this crazy bureaucratic nightmare, just for starters—this is before they've even really launched—and they are stuck in Cadiz and then stuck at a way station; and it looks for a while like they are not even going to make it across the Atlantic Ocean, let alone cross Mexico to get to the Baja Peninsula. Nevertheless, after weeks and weeks of just this horrendous ordeal, they finally get to Vera Cruz on the east coast of Mexico. And this was no small accomplishment, because the ships that were available for them to cross by that point, it was getting so late in the game that the French, these explorers, had to take anything they could find. They get this tiny little craft that gets bounced around by every little wave, and it's this astonishing (laughs) transatlantic crossing. And then they get up to Vera Cruz, and there's a hurricane that hits, and it practically ends everything. The hurricane, I think, there was a very smart pilot on board this little brigantine that moves the ship into the lee of this big fort, and that probably saved the expedition right there. Then they offload after the hurricane, and they start crossing Mexico. And by the end of that, they have to go through Mexico City, and it's wild—just going through Mexico City is a wild part of the story. And then going west from Mexico City, they are going, traveling these paths that are known for these roughneck bandits who regularly, you know, rob and accost travelers.

Steve:          And they have all this precious astronomical equipment.

Anderson:         Yeah, so they're risking being held up by, you know, banditos all along the way. And then they get to the final stretch, which is the Gulf of California, the body of water between mainland Mexico and the Baja peninsula. And then they get stuck in these doldrums that, it looks like they're going to be sitting in the middle of the Gulf of California on June 3rd, 1769. It looks like they're going to miss it after all of this ordeal. And yet there was this fortuitous breeze that pushed them across, and they landed with just, I don't know how many days to spare, but it was precious few. And then they find out, when they get there, there is a deadly epidemic that is laying waste to both the indigenous population and to the missionaries—it was mostly Jesuit and Franciscan missionaries who were at the Baja, you know, who were the Europeans who were residing in that region—and they decide, well this astronomer decides, we've come too far, we have to risk it. All the people in the expedition were concerned, and for good reason, because most of them did not get out there alive. Now that's one third of it. There's another third, which is the English expedition, and I mentioned that the English might have had their opportunity in observing from Mexico. And if that had happened, they wouldn't have needed to outfit, you know the ship, and pull this guy out of obscurity, this Lieutenant James Cook, who becomes of course the Captain Cook of… .

Steve:          Seafaring lore.

Anderson: But nevertheless that the deal falls through, and so they need to find something. They don't have much, the English don't have much. The Royal Society really doesn't know where they're going to send their ship. And then just a year and a half before the transit itself, another English ship lands and says, "Hey we found this new island out in the South Pacific." It was called King George's Island; they named it King George's Island at that time, today it's called Tahiti. One of the things that (laughter)—this ship was called the HMS Dolphin—one of the things that the HMS Dolphin discovered was that, they found the same thing that Paul Gauguin found when he traveled to Tahiti more than a century later; that the, how to put it, the mores of Tahiti were very different, and there's a kind of the…

Steve:          I was going to quote Animal House, actually.

Anderson:         Yeah okay, quote Animal House, that might be good.

Steve:          Early in the movie, they're trying to set up one of the guys, and he says, "Well she should be pretty, but we're willing to trade looks for a certain morally casual attitude."

Anderson:  (laughter) Well put, well put. Well, so these explorers on the HMS Dolphin, who had landed there—they were the first, you know, English ship to land there—what they found is that the native tribe, the Tahitians were very intrigued with the iron that these explorers were bringing them, like the nails and the bullets and really anything, made of iron.

Steve:          Like the instruments—the astronomical instruments. (laughter)

Anderson:         Yeah, and so because there really was no prohibition or very little prohibition, you know, against what we'd called prostitution, that iron quickly became the currency for sex. And so it was said that the HMS Dolphin almost fell apart in the harbor, because there were so many randy sailors who were pulling nails out of the ship; and you know, pulling like hammock hooks and things like that out of the ship. Captain Cook, wanted you know, did not want this want this kind of anarchy, and he was not going to put up with this kind of anarchy. Nevertheless, he stocked a few extra barrels of nails, just in case.

Steve:          We'll be back with part 2 of our conversation with Mark Andersen, author of the book The Day the World Discovered the Sun on May 31st. Check out our Web site, for lots of info about the next transit of Venus coming your way on June 5th or 6th, depending on where you are. And Mark’s Web site is For Scientific American's Science Talk, I'm Steve Mirsky. Thanks for clicking on us.

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