Wisconsin Public Television Wednesday Nite at the Lab  The June 5, 2012 Transit of Venus  Sanjay Limaye April 4, 2012 UP713  cc >> Welcome, everybody, to Wednesday Nite at the Lab. I'm Tina Hauser and I produce Wisconsin Public Television's University Place. On behalf of Wisconsin Public Television and our partners, the Biotechnology Center, the Science Alliance, the Wisconsin Alumni Association and UW-Extension, thank you for coming to Wednesday Nite at the Lab. We do this every Wednesday night, 50 times a year. Tonight, it's my pleasure to introduce to you Dr. Sanjay Limaye. Dr. Limaye is the chair of NASA's Venus Exploration Analysis Group. His research interest is in the atmospheric circulation on planets and he began his career with research on the circulation of the Venus atmosphere from Mariner 10 observations. He joined the Space Science and Engineering Center at the University of Wisconsin-Madison in 1980 to continue research on planetary atmospheres. He is a participating scientist with the European Space Agency's Venus Express mission and is involved in coordinating the coverage of the 2012 transit. Dr. Limaye's topic tonight is the 2012 transit of Venus. 

A truly extraordinary event, as humanity will not see another transit of Venus for more than a century. Please join me in welcoming Sanjay Limaye to Wednesday Nite at the Lab. [applause]  >> Tina, it's a pleasure to be here to give this talk. Some of you heard a little music before we began. You may have thought it was the 4th of July, since the weather has been so warm in Madison. [laugher] Well, lilacs are blooming, the tulips are out, the birds are singing. It feels like July, but it's not. I played that march because, all of you recognize it I'm sure, John Philip Sousa. He composed a march to commemorate the transit of Venus. That's the Venus March. 

Now, I wasn't around for the last transit, obviously, one century ago, in Madison. But what is it about Venus transits that led to nations to start explorations to go around the world, force England and France to ignore the Seven Year War and collaborate and observe Venus? That led Sousa to compose a march. Not only that, but he actually went and composed another one, the Transit of Venus. There's actually another novel about Venus that was written in this century that documents the trials and jubilations of a French astronomer, Gentil, who had gone to India in 1761 to observe the transit. The question is, what is it about the transit that makes it so interesting? That we have so much excitement? Let's see if I can figure this out, yes. So a little bit about Venus, I think it's too bad it's a little bit cloudy outside right now, but hopefully by the time I finish this talk the skies might  be clear. You can go out and you can probably see something like this. This is a picture taken from Madison just a few weeks ago. You see the moon in the lower left corner. Then you see Jupiter, and up high, the bright star is Venus. 

Venus, as you know, has been seen both-- It has been seen in the morning sky, in the evening sky, because it is an inner planet. It's the brightest object in the sky, and people have claimed that they can see a shadow cast by Venus on a moonless night. I actually met a person who had seen his own shadow. I have not seen it, but maybe some of you have. If not, if you are in a dark spot, you can try. Venus was known to ancients as one of the five vindras and it obviously had a day named after him in almost all the calendars that we know of. I mention only Hindu here because I think, as far as I can find out, the observation of Venus and the planets goes back to several millennia before 2000. It goes back at least two of three millennia. So it has a day named after Venus in the calendar, Shurka. Shurkawar, today is named after Mercury, as all of you know. 

If you wonder way astronomy and the calendar are co-related, it's essentially because it's how we keep time. Times of the day, days of the week, the year, came from that. The Mayans also of course had been observing Venus. There are better experts than me to talk about the Mayan experience. But in the modern era, at least since "the Copernican Revolution," when the heliocentric theory was being proposed and excepted, Venus has been excepted, and why? Before we get into that, why explore Venus? We know it's the closest neighbor. It's the planet with the largest, the most significant greenhouse effect that we know of. What can we learn by finding out about Venus? Because now, as we have learned in the last fifty years-- I should point out that this is the 50th anniversary of the beginning of the space exploration, or planet exploration. Mariner 2 was launched in 2012, July, and got to Venus in December. So they're marking that anniversary. What can we learn by observing Venus? Here's a picture of Mariner 2, and on the right side is a picture of the Soviet space craft, which didn't make it. 

Question. [inaudible] Isn't it on? [inaudible] Well, I don't know how this audio system works. Is there somebody there who can help me?  I don't see a microphone here. [inaudible] All right. >> Is this enough? >> Okay, we'll try it. We'll find out. Thank you. In the upper left is Mariner 2 and in the lower right is one of the early Venera spacecraft, Venera 2. Actually, it reached Venus but it didn't quite survive. Then we jump to the recent days, this is 2006, a picture of Venus taken from a current spacecraft launched by the European Space Agency. 

And on the left it looks like a round planet with a dark part in the middle. That's because it's a single shot of Venus, it's a single picture. On the left side you see Venus in reflected daylight, and on the right side is the night side of Venus and we're seeing the heat emitted by Venus. That's how we know Venus is hot. Venus is so hot at the surface, 473 degrees Centigrade, or roughly 950 Fahrenheit, that it emits enough light that heat leaks through the clouds of Venus and we can detect it. The question is, are we likely to go down that path? Because we have learned in the last 50 years, that Venus did have liquid water at some point in it's history. To go back to the topic that I want to talk to you about today and explain what I've learned, I just want to give you an idea of how much the ancients knew about the planetary orbits. Here's a table that I just created today, and I was surprised. In the left most column you have sun, moon-- Oops, I've got one extra O there in the "moon." [laughter] Mercury, Venus-- "Mooon." Well, it's Wisconsin after all! Venus, Mars Jupiter and Saturn. The next column gives you the number of revolutions in a Mahayuga. 

A Mahayuga is a way of measuring time in the Puranic system, and it corresponds to that number, 1,577,917,828 days. So it's a real long period. How did they determined that many number of revolutions is something interesting. You can actually go and find English translations of Surya Siddhanta and find a description of that in that. But if you convert that into the days, that's the third column from the left, and you see the amount of time Earth takes to go around the sun, and the current value is in the next column. You can see that all of these values are extremely accurate. These were made before the advent of the telescope and before we had modern instruments, so it's quite remarkable.  Despite all these measurements, what they were able to figure out was the distance of the moon and the sun in terms of it's diameter. You might wonder how, but I think if you think about it, it sort of becomes obvious, if you find out when eclipses occur. The Indian astronomers had developed a complex system of predicting when the lunar eclipses and the solar eclipses were going to occur. 

From that they were able to determine that, for both the sun and the moon, the distance was 108 times the diameter. The modern values are 110.4 and 107.5. This is going back 2,000, or 3,000 years back. The question remained, however, how large are the sun and the moon? Unless you know their actual sizes you cannot really get an absolute measure of the distance. That where, basically, the problem arose. That problem remained until the 17th century. By that time they also had a good measure of the size of the Earth. This is given in terms of --, which is a unit they had used. It's roughly 7.2 kilometers, or about 6-1/2 miles. These are the values that they had determined. 

The ratio is about .9 so they had estimated them to be about 10% smaller. But nevertheless, remarkable. Now you fast forward to the 17th century, because the distance to the sun still wasn't known. By that time, you probably have learned or know already that, Copernicus had been proposing the heliocentric theory. Tycho Brahe was measuring the planetary positions from Europe. And Kepler had developed the laws of planetary motion, which is significant, because he showed that the planets moved in elliptical orbits so that the observations could be used to predict the future positions of the planets if you had prior knowledge of where the planets were. Sir Isaac Newton came along and he was able to tie it together from simple laws of gravitation. Using Kepler's laws, he could tell the relative distances of the planets which were known to the Hindu astronomers many, many centuries earlier, but not in the West. But nobody could know the distance between the Earth and the sun. So here's Copernicus and he had proposed the theory of planetary motion. 

Tycho Brahe, in the 16th century who had established an observatory. Mind you, this is still before the use of the telescope. The telescope had not invented yet until Galileo came along. Actually, Galileo didn't invent it, but he used it. Then Kepler, from 1571 to 1630, had formulated the equation from which he could predict the planet's position in the future. The trouble is that you still had to have accurate knowledge of the planetary positions, and to do that he worked with Tycho Brahe to create what are called the Rudolphine Tables. Why Rudolphine? Because they were named after King Rudolf, I believe, in Denmark. Emperor Rudolf was the one who was supporting these calculations. These tables were crucial in the future developments. Kepler was able to predict that by 1631 there would be transit of Mercury and people did actually observe it. 

He had missed the transit of Venus, however, and that's where a British astronomer, barely 24 years of age, was looking at the table and he realized some problems with these tables. At this point I just want to point out why the predictive astronomy and the planetary positions are significant. Essentially, they led to the discovery of Uranus and Neptune in the 18th century. How? Because as we started to track the planet's positions. People could find out that they were not entirely predictable using Kepler's laws. There were some small deviations in the positions. Uranus and Neptune are significant because their presence was predicted because people surmised that there had to be a large massive object beyond the orbit of Saturn. People looked for it and, first Uranus was found and later, Neptune was found. Until that time, or a few years after that, people kept making planetary position measurements. Errors in the future positions of planets still didn't completely go away. 

So another planet was a possibility, and that led to the discover by Clyde Tombaugh in 1930, of Pluto. That continued. People thought there had to be another planet beyond Pluto, because Pluto's positions could also not be predicted very accurately, and that led to something called Planet X in the '70s, as some of you may have heard. Then it turned out that, when the space era began, and we had good measurements of the masses of the planets and better observations of the planetary positions. Because Uranus, Neptune had been observed for a much longer period, we could get a much better orbit solution for them. The need for Planet X went away, and it turned out Pluto was also very small, so that to the IU to demote it. Now it's a dwarf planet, but it's still a planet. What is a transit? A transit is simply, think of it as an eclipse but the object is too small. It cannot cover the sun, because a planet is too small because of the distance as seen from Earth. It happens when the orbit of Venus, with respect to its location, is such that Venus goes in front of the sun. 

That's really what a transit is. Essentially, what you see in this diagram is, the inclined eclipse that you see, is the orbit of Venus which is at about 3.4 degrees, and that inclination was known for many, many, many centuries. The dark, shaded eclipse that you see is Earth's orbit. This is the perspective view. When Earth and Venus are lined up so that Venus is directly in front of the sun, that's essentially a transit. Now it so happens that there's kind of a resonance between the orbital periods of Venus and Earth. The amount of time, as we saw from the table earlier, Venus takes about 225 days to go around the sun. That means, that in roughly eight Earth years, Venus has completed 13 orbits around the sun. It's this synchrony that leads to a long period between when Venus and Earth are aligned directly in front of the sun, given the inclination. Here's an example of that. 

I'm getting slightly ahead of myself here. I wanted to introduce the concept of solar parallax. Basically, how far off the sun is angle if seen, say, from the north pole of Earth and the south pole of Earth. I hope you all understand what I mean by paralax. If not, hold out your index finger, and if you close one eye and try to find something behind that finger, then switch the eyes, then you'll see that the object has moved. That's basically parallax. That happens if you are looking at an object that's very close to you, and if you change your position, that object will appear to have moved even though it has not moved at all. That's the idea behind parallax. If you know the solar parallax, if you observe the apparent position of the sun from the north pole and the south pole, that gives you and angle. It's that angle that angle that people are trying to measure looking at transit of Venus. 

Why are those always rare? Well, the inclination is about 3.4 degrees and, essentially, because of the orbital periods of Earth and Venus, it takes roughly 105 and 120 years before they are lined up again. Eight years is just when Venus is at the opposite from Earth. Going back to Galileo. This is early 17th century. Telescope hadn't been invented. It was used in Holland. Essentially ships were using it to scope out territory, such as it was. Essentially, used as a spying tool. Galileo obtained a copy of it and here's, I don't know if you can see it in the picture, but in the lower left is a picture of Galileo's telescope with some of his drawings. In the middle you see a drawing of Earth and a schematic view of where Venus would be. 

You can see what Venus would appear like. This is essentially a sketch of what the drawings that he drew would look like. On the lower left-- I don't know how the mouse works here. Let's see, does this-- I don't see a mouse. Anyway, in the top you see an animation of Venus going around the sun and in the left you'll see what the illuminated portion of Venus would look like. On the bottom left, you'll see the drawings of the phases of Venus at the bottom. At the top are the other planets. Galileo noted that the apparent size of Venus changes over a few month as you keep observing it. If you're able to see Venus tonight through binoculars I would highly recommend that you go out and see it. You'll be surprised that, even though Venus looks very bright, you'd think you would see a round disk. 

You won't. What you would see is a crescent. No, not a crescent. It's almost at the position, so you'll see roughly a 90 degree slice. Roughly a gibbous, or a half-disk. That observation itself was significant, because that was one of the key pieces of evidence of the theory that planets go around the sun and not around the Earth. If Venus went around the Earth you wouldn't get a change in size. That was a key point. Here's a picture of, these are real images taken by armature astronomers in 2004 and they basically show you what the apparent size and shape would look like. They are to scale. 

The last one is significant, because it shows a ring. That's when Venus is directly in front of the sun, but not over the sun, slightly over or above. Then you can see the atmosphere of Venus. That discovery was made by a Russian astronomer, a scientist, Mikhail Lomonosov in 1761. That's the first planetary discovery ever made. That's the first time we learned anything about any planet. That Venus had an atmosphere. Until that time we knew nothing about any planet. That was roughly 250 years ago. Last year was the anniversary of that discovery. 

Going back to the Rudolphine Tables that Kepler and Tycho Brahe had created. They predicted that both Mercury and Venus would pass in front of the sun in 1631, but the accuracy was not high enough. One person, Pierre Gassendi, in Paris, actually tried to observe a transit of Venus in 1631, but the time was off by a few hours. It occurred at night, and he didn't see it. Kepler had missed the next transit. He was not able to predict. Or at least he didn't think that there would be a transit in 1639. This is when a young Jeremiah Horrocks, who had attended Cambridge University, but did not graduate. At least there is no record of his graduating. He was near Liverpool. 

He was teaching, and he had come into contact with another astronomer in Manchester named William Crabtree who was a cloth merchant. They had corresponded and exchanged information about the Rudolphine Tables, and they were observing Venus regularly, and finding that there were some errors in the tables. Interestingly, if you read his biography which was in print, it was re-printed some time back. He makes some nasty comments about a Belgian astronomer, Lansberg. Lansberg had also continued the effort of creating these observations. Unfortunately, they were more in error than Kepler. So in his biography, he actually talks about Lansberg in somewhat unfavorable terms. So they were able to use these observations and their own refinements. About a month before the actual event in November, they knew that a transit was about to happen. They were the only ones to actually see that transit. 

I mentioned about Gassendi. He had seen the Mercury transit in the 1631 transit. Then we come to Jeremiah Horrocks. This is a painting by Eyre Crow, painted many, many years after Horrocks had actually observed it. To observe the transit, he darkened his room and projected the image of the sun through a telescope onto a wall. He observed it on the wall. There's some interesting stories about how he had to leave the room, because he had to go to church and give a sermon. [inaudible] The transit is simply-- Have you seen an eclipse? Like the moon goes in front of the sun? Venus is going in front of the sun, so what you see is a dark spot in front of the sun. That's just what a transit is. So what he was observing, and they had a nicely raticulated piece of paper so they could take measurements, so they could observe exactly where Venus was on the surface of the sun. 

That's a drawing of how they observed it. Now, whether Horrocks actually looked like that is anybody's guess. There's another drawing of William Crabtree observing Venus. He also observed the transit in the same way, projecting it from a telescope onto a piece of paper in his room. And presumably, that's his family and children. That painting is in the Manchester City Hall. It was painted by a very famous English painter, Ford Maddox Brown, who lived 1821-1893. It was one of the few. I think he painted like 12 paintings for the city of Manchester, and this is one of the paintings in the town hall. Maybe some of you have seen it. 

I have not seen it myself, even though I've been to Manchester. Now, the 1639 transit was seen by just those many people. Jeremiah Horrocks, William Crabtree, and their family. There's no record of anyone else observing that transit. Luckily, the did make good drawings and records. On the right is an actual drawing, or a copy of it, from Jeremiah Horrocks, with the times, the duration, and the distance of the centers of Venus and the Sun, as percent of the Sun's diameter. They were able to deduce the parallax to be about 14, so that let to a slightly smaller value of the Astronomical Unit, although that was not his aim. I think at that point, he was simply trying to observe the transit and the position of Venus. So, after that transit was reported, Sir Halley took note of that, and was a third person who had also predicted that the transits could be used to measure the distance to the Sun, basically the Astronomical Unit, by measuring parallax. Parallax in the same way, that is you observe the transit from different points on Earth, and the apparent difference between the path of Venus across the Sun as seen from those two points, can give you the geometry to estimate the mean distance to the Sun. 

This was so significant, because at that time, I remember Harrison had already developed a good nautical clock. I think some of you may have read the book "Longitude" by Dava Sobel, who had given a talk here some years back. So to really use these transits, to understand them, you really need to know the time really well. So if you're going to send expeditions, you had to have a way of keeping good time. So these clocks, the development of the clock was very significant. King George III was so impressed that he built himself an observatory at Kew, specifically for that purpose. Okay, so the 1761 transit led to a discovery by Mikhail Lomonosov, as I pointed out earlier. He discoverd the atmosphere of Venus. He also noticed something that he found it difficult to tell exactly when Venus was touching, or came into contact with the edge of the Sun. This is something that will become more important later on. 

That's the Black Drop Effect. To give you an idea of what the Black Drop Effect is, if you bring your thumb and index finger, and without touching, simply hold it in front of a light, you can see that they appear to touch each other before they actually physically touch. This is something that happens when Venus moves in front of the Sun. This effect essentially was the main problem. So King George had sent off James Cook to observe the transit, which could only be seen from the Pacific. So he went straight down and sailed around the southern tip of South America, and then went west. That's his statue in Tahiti, in the area where he parked his ships. That's called Point Venus, Tahiti. He and his captain drew drawings of Venus. They differed in their estimates of exactly when Venus came into contact with Sun because of this Black Drop Effect. 

Several people had observed it, but they could not agree on the precise time, and it actually led to some action against a his first officer. They had to be separated. But that transit of 1769, also later on, created a lot of stir in the public. Here's a drawing. I have no idea what that character in the garden is doing there, or who that is. But it led obviously, to several postage stamps. Here's a drawing of how Captain Cook observed it. He himself did observe it, and then the drawings. It's pretty fun. So, there are actually four methods to measure the distance, or to get an idea of what an astromical unit is. 

One is simply by looking at the transit of Venus across the face of the Sun from different locations on Earth, using the separation of the parallax. Another is measuring the rate at which Venus crosses the disc of the Sun. So for this, you need accurate times. As you can see, there are actually four separate times possible, when Venus, the leading edge of Venus first touches the sun; when the trailing edge touches the disc of the Sun for the first time; and the same thing when it's exiting the disc of Venus. So there are four separate times that you can time. You can go through a lot of complicated geometry and calculus and try to figure out the parallax. I'm not going to through that, but the result of estimates of the 1761 and 1769 transits differed by a lot. I mean, the estimate was like 109 million miles from the 1761 transit. The current value is about 93 million, so it was way off. Measurements in 1769 led to a much lower value, 88 million, which is somewhat closer to the actual value of 93 million. 

The main problem was the Black Drop Effect. People thought that maybe their eyes were not really suited for making that observation. So now we turn to the next transit. By that time, photography had been invented. The next transits were in 1874 and 1882. People thought, gee, if they took a picture, you wouldn't see the Black Drop Effect. We have a picture from the US Naval Observatory. If you go onto the Internet, you can probably see an animation of several of these photographs. You can see Venus going across the disc of the Sun. Many years later, after these observations were made, they came up with an estimate of what the Astronomical Unit was. 

If you go to 1862-1874, the line, you can see that the different estimates are about 91.7, 92.7, and if you take all the previous transits and allow for errors, they came up with 92.797 million, which is close to the modern value. So the effort to measure the Astronomical Unit did indeed succeed by 1882, but it took a long time, almost more than 150 years, to actually get to that value. By that time, people had surmised that some of the smaller asteroids that had been discovered, Eros, they could be used because they're smaller, and they wouldn't create so much of a Black Drop Effect. So Eros actually gave a very good value of 91.5. If you take all of these measurements, they still arrive at about 92.67 million miles. That's where things stood until 1961, when something spectacular happened. As you recall, during the World War, to protect enemy planes, the radar was being worked on at MIT in Lincoln Labs. There was a team, which was working on radar. They decided to find out what would happen if they turned the radar toward Venus. So in 1961, they obtained the first echo from a planet, a celestial object, by sending radar beams to Venus. 

Now, back in those days, I think I don't remember what the distance to Venus was at that point, but it was around 4.5 or 5 minutes, I believe. They did get their echo back. When they tried it again, they failed. At the same time, there was s student, Goldstein, who was working at Cal Tech. They were also developing the radar. So he talked to his advisor, if he could use Venus as a target for the radar. Basically, he was told, go ahead, but no echo, no thesis. [laughter] Fortunately for him, he did get the echo back, and they were able to repeat it. There was a difference in the radars. The MIT radar was a pulse radar, which sent a series of short bursts, and they had to wait for the echo back, whereas the GTS was a dual system. 

They used two large radio telescopes. They broadcast a tone from one dish continuously, and then looked for the echo from the other dish. They worked differently, but they were able to get the delay, the transit time of the radio waves from Earth to Venus and back. At exactly the expected time, they received the signal, and it disappeared when they stopped the transmissions. So that led to the currently accepted value of the Astronomical Unit which is 92.955820 million miles. So, if we have determined the Astronomical Unit, why do we care about the Venus transits? That's the question. Well, since 1960, what has happened, is we launched the lunar spacecraft. Now we can observe the transits from space. This is a view of the transit of Mercury from TRACE, one of the American satellites. If you observe those dark spots, that's an actual image of Mercury against the disk of the Sun. 

The Sun looks modeled, because it's not taken in white light, it's taken in a special wavelength. I forget the exact wavelength, but I believe it's-- So it's showing you the activity going on, the connective activity going on, on the surface of the Sun. But notice that it's not a straight line. Mercury doesn't bob along like that. So what's going on? This is actually an illustration of the parallax measurement. Remember, this is from an orbiting satellite. The satellite is going around the Earth at roughly 600 miles up above the surface of the Earth. It's doing exactly what the parallex is supposed to do, when you are at a high point in orbit, you see Mercury is like lower, and when you are at the low point in the orbit, that is at the South Pole, you see Mercury is higher up, just as the parallax principle. That's what leads to this undulating signature of Mercury. These observations are a certain time apart, and you can actually get the orbit of the satellite, which works out to 90 minutes. 

So we know it works well. On the lower right is a diagram that shows where the spacecraft is. The red line shows the orbit of the satellite.  Well, the same thing should apply to Venus. This is the Venus transit. Notice the difference between the two pictures. Notice the size of the black dots. That's Mercury. Venus is much thicker. That just shows you the difference in size of Mercury and Venus. 

The same thing again, it shows the up and down motion of the parallax, because the satellite is going around. If you actually count the individual spots, the images, which are taken every 450 seconds, you can get the satellite, which is 97.5 minutes. So now we come to the current transit coming up in 2012. This is, I don't know if it's possible to dim the lights, because this is an absolutely stunning image. It's a composite taken by an amateur astronomer, Tunc Tezel, in Turkey. Actually, I wrote to him this week, when I saw this image, and asked him if I could use this image in the presentation, because I knew Public Television was going to be here, and his images are copyrighted. So I asked permission. He wrote me a page-long legal list on how we could use this image. This is taken on film, on a particular film. There are about 27 images, I believe. 

This is the actual path of Venus in 2004. It's exactly the same path that it is gracing this year, to the day. So today, I think on the upper right is Venus next to Pleiades. Yeah, that's Venus next to the Pleiades. That's what Venus would've looked like yesterday. The exact same date, April 3, 2004, and yesterday, April 3, 2012. Because Venus repeats that path in the sky, every eight years. This is a wonderful, wonderful demonstration of that. Because this is a photographic composite, you can't go by the absolute brightness of the planet. But please notice that in the middle is the Sun. 

If I could blow up this image, enlarge it, you could actually see a tiny dot, which is Venus, on this image. That's what Venus is doing in the sky this year. So, this is another of his images that he sent me. This is Venus this year, taken just six weeks ago, February 25, by Tunc Tezel. This is from Central Europe somewhere, in one of those central European-- No, not European, in one of the formerly Soviet nations, in Azerbaijan, I think. So we have seen that we can now learn a lot more about transits from spacecraft. The TRACE instrument, the TRACE satellite is not operating anymore, but we have two other satellites, the Solar Dynamic Satellite, and STEREO, spacecraft, which is actually two. They are going to observe the coming transit in June this year. But what about Venus? You know, I mean, so far, all of these transits have focused on how well we can measure the Astronomical Unit, but very little about Venus itself. I think something that happened in 2004 is going to change that. 

A pair of French amateur astronomers, Alain and Sylvain Rondi, built a conograph. A conograph is just an instrument, a little device, that blocks the bright sun so you can see what's around it. People use it frequently. In a sense, what happens during a total solar eclipse is what a conograph does. The Moon blocks the Sun, so you can see the photosphere and all the beautiful rays and -- emitting from the Sun. They did that, and this is what their picture looks like. Actually, this is a composite, but I have three of their animations here that you can see moving. Perhaps the one at low right is the most useful one. The black curved disc is the edge of the Sun. The Sun has been blocked off, so that's why it appears dark. 

You can just see the edge of the Sun in bright light. In the middle, you'll see a little circular arc. That's Venus' atmosphere. The same view is shown in the colored portions in this red and green illuminated disc. But if you observe it carefully, you'll notice that the peak brightness changes location around atmosphere on Venus. Now that was a chance observation that really was not possible even from the spacecraft observations from space. So these data, just in the last two years, were used to find out something about Venus itself, for the first time since we began looking at the Venus transits. That is, that in one instant, we can get a measure of the temperature structure all the way around Venus, along that boundary. That's something we cannot obtain any other way, by any spacecraft or anything that we know of. This is why the coming transit has excited us immensely. 

In the upper left, you see two transits. You see the International Space Station and Venus. After the amateurs discovered the change in the atmosphere brightness, people went back and looked at -- Venus and that's shown at the right. I don't know if you can see it on the bright screen, but at least I can see it on my screen that there is an arc, and you do see the difference in the brightness. The difficult thing about TRACE observations is that the instrument is designed to observe the Sun, it's not designed to observe Venus. So you have to fight the immense brightness of the Sun and the refracted light, the much diminshed light that gets refracted through, that atmosphere of Venus. So that's the beauty of the conograph. So last month, there was a group of scientists who met in the Paris Observatory, in the very same room where the were in 1874 and 1882 transits were --, where the expeditions were --. And then in the main council room in the Paris Observatory, it's a round room, and if you look up, you'll see that painting. You'll see Venus and you'll see the Sun, the chariot to the left. 

At the bottom, you'll see a telescope on Venus. And then up near the edge on the right, you'll see the transit of Venus, 1882. We line this years observations under the watchful eye of Louis XVI, who is in the middle bottom picture. In the uppper left, is the group of scientists that had gathered from Japan, from Europe, from the US. So we are planning these conographs to take around to various sites around the world. There'll be one in Norway, one in Japan, at least one in India, hopefully one in Mount Wilson. If we can find enough people or travel support, we'll send one to Hawaii. Then we'll make these observations of Venus --. These observations actually are not during the the transit. They actually get all of the information before Venus enters the disc of the Sun and after it exits the disc of the Sun. 

So you have all the light refracted through the atmosphere of Venus from the Sun. So here's a little list of the resources that you can use on the web to learn more about the transit of Venus and what they have told us. If you come across John Philip Sousa's march, "Transit of Venus," you can enjoy that. There's also an opera on Gentil's life. The poor fellow had a terrible experience. He did not know if I'd have enough time to show it all. But he had taken off in 1761 from France. He traveled all the way to India, because the French had a fort there. By the time he arrived, that fort had been taken over by the British, so he could not land. So he had to observe the transit  from a bobbing ship, so he could not make any useful observations. 

Instead of going back to France, he decided to explore the territory and sailed around for eight more years, until 1769, when the French had re-taken the territory, and then he observed the transit from India in 1769.  The next transit in 1882, an Italian team had also traveled to India, but they did something different that had not been done before. The Black Drop Effect was the main problem, as I mentioned earlier. The Italian team thought that if they used special filters, they might be able to get away from the Black Drop Effect. So they actually used a spectograph. They imaged Venus in different light using the spectograph, but they were not able to get away from the Black Drop Effect. In a sense, that Black Drop Effect happens because of the -- Sun, and the instrumenal aberrations, so it really messed up the precise measurement of the Astronomical Unit until this century. Thank you all for coming. [applause]