Transit of Venus: The Other Half of the Longitude Story
While Harrison was reinventing the clock, astronomers turned the heavens into a celestial timepiece
Hi, this is Steven Cherry for IEEE Spectrum’s “Techwise Conversations.”
In 1707, four British ships mistook the ironically named Scilly Islands, 30 miles east of Cornwall, at the northern entrance to the English Channel, for Ushant, off the coast of Brittany, at the southern entrance to the English Channel. Fourteen hundred men died. It was just one of thousands and thousands of wrecks caused by a ship’s not knowing where it was.
In the history of mankind, there has probably been no scientific and technological answer more sought after than the 60-year search to solve the longitude problem.
The problem is this: To know where you are on Earth, you need to be able to define your position. Latitude—your position north or south of the equator—is as easy as knowing the length of the day. Longitude—your position east or west of the prime meridian, is a different thing entirely.
In her 1995 surprise best seller, Longitude, Dava Sobel says, “To learn one’s longitude at sea, one needs to know what time it is aboard a ship and also the time at…another place of known longitude—at that very same moment.”
Seven years after the Scilly Islands disaster, an act of Parliament offered £20 000—£5 million today, or about $8 million, for, quote, “a practical method of reliably ascertaining one’s longitude at sea.”
Hundreds of scientists in England and Europe pursued it, from two different directions: horology, or clock making, and astronomy. Over the course of six decades, the clock makers—specifically, John Harrison and his son—would eventually win most of the money. That’s the story told in Sobel’s book. The story of the astronomers has largely gone untold—until now. Next month, a new book, with the title The Day the World Discovered the Sun, will be published by Perseus Books.
Its author, Mark Anderson, is my guest today. He’s a freelance science and technology writer based in the decidedly inland town of Northampton, Mass., and although he also writes for the likes of Wired, Discover, New Scientist,and Harper's, we’re very pleased that he’s on our masthead as a contributing editor. He joins us by phone.
Mark, welcome to the podcast.
Mark Anderson: Thank you for having me, Steven.
Steven Cherry: Mark, the title of the book refers to something with the rather racy name of “the transit of Venus.” What is it?
Mark Anderson: The planet Venus passes directly in front of the sun very infrequently, less than two times per century, but when it does happen it happens in pairs, separated by eight years generally. So it turns out that it provides the perfect measuring stick for the entire solar system. You can essentially triangulate the distance to the sun using Kepler’s laws and a bit of creative geometry, and that number was really important in science, and it turns out to be highly relevant to this tremendously troubling problem of longitude and navigation at sea.
Steven Cherry: Yeah, so these really are erratic events. There were two in the 1760s, but the next one wouldn’t be for over a hundred years. And we have two in the space of a decade as well: 2004, and there’s one coming up in June. But after that there isn’t another one until 2017  again.
Mark Anderson: Yeah, 2117. Yeah, June 5th is the last Venus transit of this very young century.
Steven Cherry: So tell us exactly how it figures into the longitude problem.
Mark Anderson: The background is that the Venus transit was something that Edmond Halley—he is...[of] Halley’s comet fame—realized that this essential number in astronomy was the exact science of the 18th century. This essential number in astronomy that nobody had a clue how to really get a good fix on, he realized, well, when Venus passes in front of the sun next, that will be our measuring stick, and the Venus transit takes about 5 or 6 hours, and you’ve got to measure it down to the second to get a really good number. He said that you can get it to 99.2 percent accuracy. It turns out that the second time around, in 1769, people were able to do that. How does it relate to navigation? Well, turns out that the short answer is that there are basically two parts of a larger issue. Astronomy was in the 18th century the solution to the longitude problem. The longitude problem ultimately ended up being solved by the chronometer—by the John Harrison story, you know, the spring-wound clock. But that was so expensive and so inaccessible technologically in the 18th century that people just couldn’t do it cost-effectively. So what they had to do instead was to figure out how to make really exact predictions of where the moon would be at any given time, at any given day throughout a year, and so these Venus transit expeditions proved to be the perfect testing grounds for these fledgling technologies and techniques.
Steven Cherry: Yeah, and so among other things, they really wanted to know the exact size of the earth as well as the distance to the sun, and I guess the key to that was taking measurements from various points on the globe, especially at the equator, and then as far north as possible?
Mark Anderson: That’s right, yeah.
Steven Cherry: So it turns out—I lost count—but it turns out as many as six different countries sent out expeditions. Let’s start with the 1761 transit: Who went where, and why?
Mark Anderson: Who went where, and why? Well, the number is big, it’s more than 100 people reported into the—either the Royal Society or it’s equivalent in Paris. So you have a lot to choose from, but for the 1761 there are two transits: one in 1761 and one in 1769. And the way it worked out was, the 1761 transit was basically kind of the rough draft; people figured out how they had to do it right, but they weren’t able to get it all coordinated. So June 3, 1769, is the day of the book, the day the world discovered the sun. So in 1761, the French sent a big, high-profile mission into Siberia because that was predicted to be one of the best places in the world to see it. Mason and Dixon crucially were not Mason and Dixon before the 1761 Venus transit; they were paired together by the Royal Society to go and observe at another location in the Indian Ocean. They never made it to the Indian Ocean because their ship was attacked; England was at war at the time. But they did make it to South Africa, and they did such a tremendous job observing the transit from South Africa that when they got home, the Royal Society said, “Hey, you guys did such a great job we want you to settle a border dispute between Pennsylvania and Maryland.”
Steven Cherry: Yeah, and we’re actually going to have a follow-up later this week about the Mason-Dixon line itself because there’s some interesting technology there. So this 1761 set of expeditions was sort of—was in large part—a bust. I guess there was an expedition that the French sent to India, and they spent months and months trying to get there, and when they finally did, the colony that they were going to take the measurements from was taken over by the British, and they could do nothing. So they didn’t get all the measurements they needed. I guess I shouldn’t call it a bust, but it wasn’t a complete success. But fortunately, there would be another Venus transit in a mere eight years. Tell us about the 1769 expeditions.
Mark Anderson: Well, it became kind of the big science project of the age. We’re used to today, you know, these big things like the Hadron collider, the Genome Project, and the Hubble Space Telescope, where you get this massive international collaboration across borders, across fields, that was obviously new to the entire world in the 1760s, and this really was the first full-fledged, big science project in history. There was a group of astronomers in Paris that ended up coordinating it, and it was quite an undertaking. I mean, you look at the literature of the time, and it’s important to say that The Day the World Discovered the Sun is first and foremost a human story of incredible adventure: these people who are risking everything to go to the ends of the earth to find the sun. I do want to say that while we’re talking about the science and technology angle, the book too is—we’re always trying to keep the story right up at the forefront and just tell the adventure. But about the 1769 expedition: The French, the British, the Austrians, and the Russians, the Swedes, the Danes—those were some of the major players who were sending their own or collaborating—and the Spanish, too—who were collaborating or sending their own expeditions to various places. There ended up being more than 120 observations that were reported to the major scientific societies; we tell the story of three. We wanted to get the stories that did the most important science, that had the most interesting characters and had the best sort of stories to tell. And it turns out that you can tick all those boxes for these three major expeditions. One with Captain Cook and, just like with Mason and Dixon, he became a household name because he was sent out on a Venus transit observing mission. Captain Cook went to Tahiti. A joint French and Spanish expedition went to the far end of New Spain: present-day Mexico, at the Baja peninsula. And an Austrian-slash-Hungarian astronomer, a Jesuit priest with the unlikely name of Hell, Father Hell, at the invitation of the king of Denmark went to above the Arctic Circle to one of the northernmost towns in Norway, Vardo. So his expedition went there. So we’re telling these stories kind of in parallel as they all unfold, and everybody encounters a lot of surprises along the way.
Steven Cherry: Yeah, there’s some really wild stuff here. The French going to Baja California traveled thousands and thousands of miles and then took almost a month just to cross a 300-mile gulf, only to arrive at a place rife with epidemics, and this is the guy who eight years earlier had traveled through the Siberian winter and spring to make those measurements. And as you said, it’s just, like, one-third of the story. The Danish king who turned out to be this 20-something genius who had, like, dementia?
Mark Anderson: Right. Hah!
Steven Cherry: And the stories just go on and on. It’s really crazy. And we’ll come back to that in a minute. But this was really kind of an insane decade anyway, the 1760s. It was also the decisive decade for the clock makers, wasn’t it?
Mark Anderson: It sure was. Yeah, the story of longitude really figures so prominently into the story of the Venus transit that I was astonished that nobody had really connected those dots before. There are many, many books on the history of the Venus transit, and a number of them are very good: They tell the science, they tell the technology, they tell us who are the key scientists who did all this. But my agent and, in fact, my wife asked a really important question early on that initially I was not able to provide a good answer to. In one word, “Why?” Like, why did so many people care so much, and—crucially—why did people who weren’t scientists—namely, kings like King George III—why did they write such big checks? This is not motivated...certainly, however much we love the Age of Enlightenment and love to imagine that everybody just cared about enlightenment and learning and all that stuff, there was still a bottom line. And what I discovered was that navigation was the essential part of the story. So clocks—so people are perfecting clocks, and John Harrison made some of his most important sort of leaps forward in the 1760s in developing his own technology, so it’s really quite a race that’s going on behind the scenes concerning navigation...and these two competing methods, namely, astronomy—navigating by the moon—and navigating by clocks, at the same time that people are racing around the world to try to make the June 3, 1769, deadline.
Steven Cherry: Yeah. So there was a lot of money, and there was this sort of practical angle to it as well as the pure science, just as we see in, as you say, big science projects today. So do you think that these kings and emperors and empresses got their money’s worth?
Mark Anderson: Absolutely. Yeah, I think the Venus transit expeditions are a perfect case study of the importance of investment in, you know, sort of primary research because what they got…I mean, first of all, just look at Captain Cook alone. His first voyage, in 1758 to 1761—not only did he go to Tahiti, but in a sense he really kind of opened up much of the Pacific to exploration. Not that other people hadn’t ventured there before, but he was the first mariner to not lose anybody to scurvy. Now, people died of other things along the way, but scurvy was an incredibly big deal. There were ships at the time that staffed half again or twice again as many crew as they needed because they expected to lose that many people to scurvy. And Cook cracked that code, too. But as I said, it was also a crucial testing ground for these sort of lunar navigation techniques which became the standard throughout the 18th century; it was the way that most people navigated, and reliably so. And one of the sort of footnotes to the Age of Empire in the 1800s, the 17th century, and into the early 19th century, is this quiet little story of The Nautical Almanac, which was...its whole birth, The Nautical Almanac. It was an annual production, and it came out once a year, and it’s been published ever since from Greenwich, England. It was the centerpiece in longitude and in navigation for a long time, for generations, and it remained the essential sort of backup for when the clocks didn’t work, even in the 19th century.
Steven Cherry: Yeah, and I guess it took another 30, 40, 50 years before clocks would be reliable and inexpensive enough to become the main method of calculating longitude. So these almanacs, which I guess used the configurations of the moons of Jupiter to actually tell the time, were the main method. But that—so that kind of was independent of this business of knowing the distance between the earth and the sun, and so I’m kind of curious whether the transits of Venus really…I mean, they certainly inspired a number of things, for example, Cook, who opened up, as you say, the South Pacific, you know, like, Australia had not been discovered yet. But with these transit of Venus expeditions, did everyone get their money’s worth out of them?
Mark Anderson: I think—again, I think the short answer is yes. Because you look at the science and technology that came out of it that—essentially, perfecting navigation at sea—it’s not just Captain Cook who did it. I mean, Jean-Baptiste Chappe, who is one of my main characters, he’s the French explorer who goes to the Baja peninsula. Now, he did a lot of the testing of the French techniques of navigation at sea, both by clocks and by the lunar method, and so he helped to crack the code for the French as well. Interestingly, you look at the world’s first information network, the world’s first instantaneous information network—that was the visual telegraph system that was set up in the early 19th century. It relayed the results of the Napoleonic wars, various battles and things like that. There were hundreds of stations posted around Europe, and it spanned thousands and thousands of miles across Europe. It was essentially a network of sort of semaphore stations, and so someone could send a message in Paris and it could be received in Geneva in relatively short order, certainly much faster than a horse could carry it. Well, these visual telegraph systems were invented by Jean-Baptiste Chappe d’Auteroche’s nephew, and his nephew, as he reports in his own memoirs, he says that the reason that he turned to science and technology, the reason that he did all the inventing that he did, was because of the inspiration of his uncle. So that’s one of those knock-on effects, that law of unintended consequences, that sometimes come up that you see that you can never predict these things. But there are a number of examples beyond that that I get into in the epilogue of my book, where the quest to find this number is almost as important as the actual science itself.
Steven Cherry: Very good. Mark, this book—we already alluded to it—has more ups and downs than the Alps. Besides ambitious astronomers and fortune-seeking horologists, it’s got murderous banditos in New Spain, suspicious Russian villagers, Tahitian women trading views of their breasts for anything made of iron, and at least two actually mad, as in insane, European kings. Did you know when you went into this research what a wild ride this story would be?
Mark Anderson: I had a hunch, but I had no idea it would be this much fun.
Steven Cherry: Very good. Well, Dava Sobel’s book kicked off an entire trend of books about natural phenomena with one-word titles. Two of my personal favorites were Coal and Salt (not to be confused with the Angelina Jolie movie of the same name). I think you missed a bet, having a title that has so many words in it—maybe just Venus or Transit would do better—but anyway, I wish you great success with a great book.
Mark Anderson: Thank you so much, Steven. I appreciate it. I love the podcast. I’m glad to be a guest here today.
Steven Cherry: We’ve been speaking with freelance science writer Mark Anderson about his new book, The Day the World Discovered the Sun, about the greatest tech contest ever, the Longitude Prize, and what we might learn from it today.
For IEEE Spectrum’s “Techwise Conversations,” I’m Steven Cherry.
NOTE: Transcripts are created for the convenience of our readers and listeners and may not perfectly match their associated interviews and narratives. The authoritative record of IEEE Spectrum’s audio programming is the audio version.