Date: June 5, 2011

SolarEclipseTitle: Transit of Venus

Podcaster: Jay Pasachoff

Organization: Williams College –

Links: Transit of Venus website. Working Group on Eclipses of the International Astronomical Union –

Description: A year from today, on June 5, 2012, in the United States and June 6, 2012, in Europe, a transit of Venus across the face of the sun will be visible. It is a very unusual and exciting event and one that almost all the people of the world can see since only a few parts of the world, mainly the eastern part of South America and the western part of Africa are excluded from seeing any part of the transit and all the other people of the world will be able to see it just by looking up!

Bio: Jay Pasachoff, Chair of the International Astronomical Union’s Working Group on Eclipses, is Field Memorial Professor of Astronomy at Williams College and a Visitor at Caltech. He has viewed 64 solar eclipses, and is an expert on both their use for scientific observations and their use for public education. Pasachoff is a former president of the International Astronomical Union’s Commission on Education and Development and Chair of the Historical Astronomy Division of the American Astronomical Society. He received the Education Prize of the American Astronomical Society, the Janssen Prize of the Société Astronomique de France, and this year’s Richtmyer Memorial Lecture Award from the American Association of Physics Teachers. Pasachoff is the author or co-author of The Cosmos: Astronomy in the New Millennium, the Peterson Field Guide to the Stars and Planets, and Nearest Star: The Surprising Science of Our Sun plus, on a more technical level, The Solar Corona, as well as a new, 2017 book, The Sun, for the Science Museum, London.

Sponsor: This episode of “365 Days of Astronomy” is sponsored by an in-kind donation from Universe Today. Space and astronomy news from around the Universe at


Jay M. Pasachoff, Williams College

A year from today, on June 5, 2012, in the United States and June 6, 2012, in Europe, a transit of Venus across the face of the sun will be visible. It is a very unusual and exciting event and one that almost all the people of the world can see since only a few parts of the world, mainly the eastern part of South America and the western part of Africa are excluded from seeing any part of the transit and all the other people of the world will be able to see it just by looking up!

If you look up, and of course whenever you look at the sun you need a special solar filter to project the sun’s image, but by looking up you would be able to see, even with your unaided eye, a tiny dot go across the face of the sun. It is very unusual to be able to see that. As Copernicus discovered hundreds of years ago, only Mercury and Venus have orbits smaller than the earth’s and only Mercury and Venus can therefore go in transit across the face of the sun. The transits of Venus take place in pairs separated by only 8 years but then there is a gap of over 105 years between the pairs. When the first of the current pair was visible in 2004, nobody alive on earth had seen a transit of Venus since the preceding one had been in 1882. But now, only 8 years later we have a chance to see it again and we scientists and (I hope) the general public are all going to watch it and be very excited by it. Of course, though with your eye, you’ll just see a tiny dot, with our telescopes, we can see a greatly enlarged image and follow the silhouette of Venus as it goes in front of the sun.

In 2004, we used a spacecraft of NASA called the Transition Region and Coronal Explorer to see this in great detail. That spacecraft has now been superseded by the Solar Dynamics Observatory. SDO, the Solar Dynamics Observatory, takes 8 pictures of the sun at high resolution through special filters every 12 seconds 24 hours a day. So it will automatically capture this event at a high cadence and we will be able to see, as Venus gets halfway on to the sun at the sun’s edge, the atmosphere of Venus appear on the side of Venus away from the solar disk. Then for 20 minutes or so we will see this atmosphere change. We know from 2004 it looked asymmetric and we’ve been correlating what it looks like with the spacecraft observations from the European space agency’s Venus Express spacecraft. Eventually, Venus will go on to the sun. It will take about 6 hours to cross the face of the sun and then we’ll see the same thing in reverse. First, the atmosphere appearing and then disappearing as Venus entirely leaves the sun not to be seen across the face of the sun again until the next transit of Venus in the year 2117. If you don’t want to wait until 2117 or you don’t think you can wait till 2117, then you better see the one in 2012.

An interesting part of the transit now is that we can observe the total amount of energy coming from the sun every second. It’s called the total solar irradiance. It used to be called the solar constant until it turned out, by measuring with great precision from space, that it isn’t really isn’t constant. Now that we know it varies, you can take account of that variation on a slow scale but for the 6 hours or so that Venus takes to go across the face of the sun, we can actually see a dip in the total energy that reaches the earth of about a tenth of a percent. That corresponds to the geometrical area of Venus with respect to the geometrical area of the disk of the sun but we can also see details of how that silhouette appears on the sun. It goes from the edge or limb of the sun onto the center of the sun and the sun’s center is brighter than its edge. That’s called limb darkening.

We like to know the details of this because scientists are now using the Kepler spacecraft and other spacecraft to study planets around other stars by what happened to the light from those stars as the planets go in front of their parent stars. The Kepler spacecraft has reported over 1,000 of these so-called exoplanets now and we provide, by observing the transit of Venus, an analogue in our solar system to what is being seen far out by the Kepler spacecraft and the scientists analyzing that data. So to understand all these exoplanet observations better, it’s very good to understand just what happens when Venus goes in front of the sun next June 5th and 6th, 2012.

The story of observing the transit of Venus is a very interesting one. Johannes Kepler in his Rudolphine Tables from 1627, predicted a transit of Venus and a transit of Mercury four years later. The transit of Venus would have been visible only from California and of course there were no telescopes in California in 1627. The transit of Venus was detected in Europe and that really verified the calculations and tables of Kepler. The first person to see a transit of Venus was a young man, Jeremiah Horrox. Eight years later, in 1639, he had told two friends that this event might happen and one of them had good weather, so only two people, Horrax and his friend, saw that transit of Venus in 1639.

Now, some years later, Edmund Halley, of comet fame, predicted that you could tell how big the solar system was by observing transits of Venus from various places on the surface of the earth. Finding out how big the solar system was if the solar system was the universe, it was finding out how big the universe is was a major problem in astronomy or maybe THE major problem in astronomy for hundreds of years so transits of Venus had a very important scientific purpose. As a result, for the transits of 1761 and 1769, hundreds of expeditions were sent by countries all over the world to distant locations; far north and far south and everywhere in between.

Among the most famous of these expeditions was Captain Cook’s to go to Tahiti in 1769 to see the transit of Venus from there. Point Venus is still a location in Tahiti and Captain Cook had secret orders to go on and explore what was further south after the transit and he did go and map New Zealand and the coast of Australia. Another interesting case was Le Gentil, a French astronomer who tried to go to Pondicherry, India, but was denied landing in 1761 because the British had taken it and he had to watch from sea. So he saw the transit but his boat was rocking, of course, and the pendulum clock he was trying to use to time the transit for scientific purposes didn’t work. Now, since he was all the way there in India in 1761, he decided to wait the 8 years until 1769 and he had an interesting set of events and finally in 1769 the day was clear. He was watching and the clouds came and he missed it again. By the time he got back to France, he had been declared dead and he had lost his seat in the French Academy, which had been given to someone else. There’s a wonderful play called The Transit of Venus that shows his fiancée marrying someone else and in any case, the true story is that Le Gentil did have these deprivations but he did recover. He did marry happily, had a family, lived to a ripe old age, got his seat in the academy back, but nonetheless, he did have 11 years of travails over this transit of Venus.

Nowadays, it is much easier for us to go to India to see a transit or go to Hawaii where I hope to see the transit next year. But wherever you are, you can see it if you’re within the zones from which it will be visible. In the whole continental United States the transit will start in the late afternoon and the transit will still be in progress, that means Venus will still be silhouetted against the sun, when the sun sets. In Europe and in India and in the western part of Russia and China, the transit will be in progress when the sun rises. And then there’s a whole zone in the eastern part of China and Russia and in Japan and Hawaii and Alaska and the western part of Canada when the whole transit will be visible (all 6 hours worth) including the entry and the exit of Venus and then you can hope to see something known as the Black Drop Effect. Sometimes people say the Dreaded Black Drop Effect when Venus did not separate clearly from the edge of the sun. My colleague Glen Schneider and I have shown that to be a combination of details of the darkening near the sun’s edge and a pattern of blurriness that comes from the telescope itself.

So we have been studying all kinds of aspects of the transits of Venus either scientifically or just as a phenomenon to see, a natural phenomenon. I recommend that everyone pay attention a year from today when we have the last transit of Venus until 2117.

End of podcast:

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