Venus viewed on the Sun
1. Venus's dark disc shows up against the searingly bright photospehere, the light-emitting surface of the Sun
Introduction | What is a transit? | The 2004 event
Transit Day | Reflections on the transit | Links and notes
On 6 June 2012, astronomers and other observers will have their last opportunity this century to see one of nature’s rarest occurrences – a transit of Venus. William Gould describes his experiences of viewing the last such event in 2004 and reflects upon the significance of a transit.
Introduction
We are used to seeing the planet Venus as the ancients did: as a brilliant morning or evening "star" glittering in the heavens just before dawn or just after sunset. But Venus is a solid world, not a star; it shines by reflected sunlight and, as the 17th-century physicist Galileo Galilei discovered, it shows phases like the moon as it travels around the Sun.
Venus's path lies inside that of the Earth and it takes 225 days to complete its orbit. When positioned between the Earth and the Sun, the planet has its dark side toward us and cannot be seen. But if the three bodies – Sun, Venus and Earth – line up exactly, then our view of Venus is transformed. For then Venus suddenly becomes visible to us as a tiny black disc moving slowly across the dazzling face of our star (see Ill. 1). This precise alignment doesn't happen very often – only once every century on average. But by a strange quirk of celestial mechanics, transits of Venus currently occur in pairs, the first exactly eight years before the second. Although a transit took place as recently as 2004, there had previously been no transit of Venus since December 1882. After 2012, there will not be another one until December 2117. The 2004 transit of Venus, the first of the current pair, took place on Tuesday 8 June, and I had the privilege of seeing it from no less a location than the grounds of the Royal Observatory, Greenwich.
As a child growing up in the 1950s, I had developed a great interest in astronomy mostly fired by the books and broadcasts of Patrick Moore, the doyen of British stargazers, now in his 80s. In describing the planet Venus, Patrick and the other authors I read on the subject would always include a description of a transit, finishing up with the information that the next one would be in 2004. I wanted to see it. Thus as 8 June of that year approached, it was a peculiar feeling to look back on those old books and realize that a lifelong dream was, weather permitting, about to come true.
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What is a transit?
A transit is somewhat like a solar eclipse, when the Moon blocks out the Sun's light for a few minutes. But Venus, though much larger than the Moon, is more than 100 times further away and appears as just a little black dot, hardly blocking out any sunlight at all. Also, unlike an eclipse, which may last up to two hours from start to finish, a transit of Venus takes much longer, about six hours, to complete. Not very dramatic, then, and easy to miss; and if you do observe it, you might think it's rather like watching paint dry. And how could it be observed properly anyway? Nobody could look at it directly – the brightness and heat of the Sun made that completely impossible. Even glimpsing the Sun without eye protection risked permanent damage to the retina. You could try using special solar viewing glasses made of Mylar to diminish solar glare, but prolonged use of these was not advisable either. A much better bet was to use a telescope to project the Sun's circular image onto a screen and watch the transit that way in complete safety. Of course, it might be on the television – but that wouldn't be the same at all.
2. Venus orbits closer to the Sun than the Earth does, but as this illustration tries to show, the plane of its orbit is tilted at an angle of more than 3° with respect to that of the Earth's. The planes intersect at the node points M and N. Transits can only occur when Venus is at or near M or N at the same time as Earth reaches m or n respectively.
If a transit of Venus is so unspectacular and so difficult to observe, why do astronomers get so excited about it? After all, Venus orbits closer to the Sun than the Earth does. Surely its path regularly takes it between us and the Sun. Indeed it does, once every 584 days. But a transit can't happen every time. The plane of Venus's orbit is tilted with respect to that of the Earth's by an angle of more than 3°, so most of the time Venus passes above or below our line of sight to the Sun and we see no transit. Only very occasionally, when both Venus and the Sun come into the same line of sight at one of the two points where the orbital planes of Venus and the Earth intersect, does a transit occur. At these points, called nodes, the alignment is just right for Venus to be seen passing across the Sun's face (see Ill. 2). Mercury, the innermost planet of the Solar System, can undergo transits too, but they are not as rare and attract less general interest because they are much harder to observe. It's the rarity of a transit of Venus, and its predictability, that makes it so fascinating.
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The 2004 event
3. Tracking Venus. The 2004 transit of Venus began with first contact, as the leading edge of the disc of Venus touched the eastern (left-hand) limb of the Sun south of the solar equator at 05:13 Universal Tine (06:13 British Summer Time). About 20 minutes later, its trailing edge touched the solar limb at second contact. For six hours the tiny black dot of our sister planet tracked across the Sun's southern hemisphere, When its leading edge touched the Sun's western limb, third contact was made. Fourth and final contact, when Venus's trailing edge touched the south-southwestern part of the solar disc, was reached at 11:26 UT (12:26 BST). and Venus vanished from view to reappear as a morning star later in June.
In June 2004 people in the UK would be well placed to see the whole event from beginning to end (see Ill. 3). Hitherto, only five transits of Venus had been witnessed. None of them had been completely visible from western Europe. One, that of 9 December 1874, could not be viewed at all because it took place during the hours of darkness in our part of the world. The others could only be seen partially, ending well after sunset or beginning hours before dawn. When you take the geographical location of the observer into consideration, plus the local weather conditions, the rarity of a transit of Venus becomes even more pronounced. After 2004, Western Europe would not see another complete transit until 11 June 2247! Astronomers all over Britain prayed for a cloudless morning on 8 June.
I made the decision to view the event from Greenwich only a week beforehand. Actually, I could have watched it over the Internet (something that was undreamed of when astronomers last viewed the phenomenon in 1882), but having learned that members of the public would be allowed free access to the grounds of the Royal Observatory on the day and would be able to see the transit safely with telescopes set up by members of the Flamsteed Society, I decided that Greenwich was the place to be.
As the day approached, my excitement mounted. I anxiously kept an eye and ear on the weather forecasts as the days counted down.
4. The grounds of the Royal Observatory, Greenwich, offered an excellent vantage point for watching the transit. The place was not crowded, but quite a number of adults and children turned out. The Observatory provided eclipse glasses made of Mylar to give some protection when viewing the Sun directly. The metal strip you can see in the foreground famously marks the line of the Greenwich Meridian. So in taking this picture mid-morning I was standing in the Eastern Hemisphere of the Earth watching the scene in the Western.
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Transit Day
Tuesday 8 June 2004 was a perfect summer morning, bright and clear. The transit was due to start just before 6.15 a.m., about an hour and a half after sunrise, and end about 12.25 p.m. Reliant on public transport, I couldn't get to Greenwich before the start, but having risen at about 5.30 and breakfasted by 6.00, I turned on the TV to see how the occasion was being covered.
5. We were close to one of the Royal Observatory's landmarks. The famous time ball sits above the Octagon Room at Observatory House. At 1:00 p.m. precisely every day, having risen to its topmost position, it begins its drop to signal the hour, allowing mariners on the Thames to set their chronometers.
It wasn't, really. Although both BBC and ITV outside-broadcast teams were at Greenwich, breakfast-time television was preoccupied with other news. Prime minister Tony Blair was visiting the United States and Iraq continued to loom large in the headlines. The 6.30 BBC summary featured a fleeting shot of the start of Venus's passage, with the planet having already taken a little nick out of the rim of the Sun's disc, but that was all. Clearly you had to be there.
I got to Greenwich by 7.45. I made my way to Greenwich Park and climbed the hill to the Royal Observatory, Two journalists ahead of me were discussing the transit. "So what are we likely to see today?" one asked his colleague, "Is the Sun going to go dark, or what?" It wasn't, of course. Just as well, really.
But if the media seemed ill-informed on the subject and apparently showed a lack of excitement, it was not so with the general public at the Royal Observatory. Hundreds of people were already gathered in the Observatory grounds, including families with children (see Ill. 4 and 5). There was a spectacular view south over the park, and more to the point, an uninterrupted view of the Sun. The observatory provided solar viewing glasses, but I did not find them helpful. Having been operated on for congenital cataracts, my eyes have no accommodation, and the Sun's image was just too small for me to make out anything on it. Normally sighted people, however, assured me that they could see the tiny disc of Venus moving across it. Fortunately there were several telescopes equipped with suitable optical filters to allow safe viewing of the solar image through an eyepiece and others set up to project it on a screen. It was an immense thrill when I caught my first sight of the Sun's bright disc with Venus's much smaller disc showing starkly black against it. A little dream half a century old was fulfilled.
6. Top. NEVER LOOK AT THE SUN THROUGH AN UNPROTECTED TELESCOPE. Watching the transit of Venus safely requires telescopes fitted with special filters to eliminate harmful solar glare or show the event in light of a specific wavelength. Bottom. A telescope set up outside the Observatory building was fitted with a TV camera that transmitted an image to a rather dusty screen in a nearby tent.
7. Astronomers speak of "shooting the Sun" – that is, projecting the solar image through a telescope onto a screen. The Solarscope offers a compact way of doing this and showed the transit well.
I watched the last four hours of the transit, moving from one telescope to another, chatting with astronomy enthusiasts and taking pictures of some of the screen projections and other sights (see Ill 6 and 7). One instrument showed the Sun in hydrogen-alpha light, in which it appeared as a true ball of fire. (To gain some idea of what the transit looked like in H-alpha light, see Ill 10.) Another telescope allowed us to compare the round *dot" of Venus with splodgy sunspots, Earth-sized cooler regions of magnetic disturbance on the solar surface. I looked in on a BBC/Open University presentation hosted by Adam Hart-Davies (see Ill 8). As the transit reached its climax(see ill 9), with first the leading edge of Venus, then its trailing edge, moving off the solar disc, the excitement was palpable. Schoolchildren were encouraged to time these two events as accurately as they could in an effort to re-create scientific experiments from earlier transits.
8. Adam Hart-Davies (centre, in front of large screen) presented a BBC/Open University programme in the "Stardate" series on the transit event. The programme was done in segments throughout the morning and shown at 11:30p.m. that night. Here he and the rest of the crew take a break between takes.
9. The beginning of the end. As the disc of Venus approached third contact, when its leading edge appeared to coincide with the Sun's limb, excitement mounted. Over the next 20 minutes the planet moved inexorably off the Sun's face. At Greenwich Observatory, children and adults counted down the seconds to final contact. Elsewhere schoolchildren all over the country sought to time the moment as accurately as they could. They sent in their timings to Greenwich to try to re-create the old experiment to measure the distance to the Sun.
10. The transit as seen in hydrogen-alpha light. The Sun appears as a huge restless ball of fire, a raging furnace using up its hydrogen fuel in nuclear fusion reactions at a rate of over half a billion tonnes per second. In the process, the Sun is also losing about 4 million tonnes of mass, which is converted to energy (the heat, light and other radiation that sustains life on Earth but can also threaten our existence.
Reflections on the transit
It reminded me that not long after a transit of Venus had been observed for the first time by Jeremiah Horrocks and William Crabtree in 1639, the phenomenon had been hailed as a practical aid to measuring the mean distance between the Earth and the Sun. Astronomers already recognized this distance as a celestial yardstick, the so-called astronomical unit, but they had no clear idea of its physical value. The English astronomers James Gregory and, more particularly, Edmond Halley, who had witnessed a transit of Mercury in 1677 from St Helena, championed the idea of using a Venus transit to measure the astronomical unit. In two papers placed before the Royal Society (1691 and 1716), Halley suggested a way of doing it. He knew from Kepler's laws of planetary motion that the square of a planet's orbital period around the Sun is proportional to the cube of its mean distance from it. So he had a rough idea of the ratio of Venus's Sun-distance to the Earth's Sun-distance and once he knew ore distance he could work out the other. In fact knowing just one distance would allow him to find the sun-distances of all the known planets by simply applying Kepler's third law.
Put very simply, Halley in essence decided to find the distance from the Earth to the Sun by first finding the distance from the Earth to Venus. He reasoned that if the position of Venus's dark disc against the Sun's face were observed from two or more widely separated locations on the Earth, the apparent angular displacement of the planet's image – what astronomers call its parallax – could, by the application of some simple trigonometry, yield its distance from us. Knowing that, Halley would be able to work out from Kepler's third law the distance between the Earth and the Sun. The problem was that Venus was so far away, its parallax was vanishingly small and impossible to measure directly. Halley hit on the idea of getting astronomers situated in various parts of the world to time the observation of certain stages of the transit (specifically the points where the edges of Venus entered and left the Sun's disc). The time differences would in theory be measurable and could be translated into a parallax displacement (see Ill. 11). The more observations that could be made, the more accurate the result would be. Thus it would be possible to work out how far away from us Venus is and let Kepler's laws do the rest.
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Halley's method for discovering the exact distance from the Earth to the Sun and thus assigning a physical value to the astronomical unit. |
11. During a transit, Venus's track across the solar disc does not appear to be in the same position for two Earth-bound observers widely separated in latitude. The above drawings are the front and side views of a transit observation from two points on the Earth. The observer at point P in the right-hand drawing will see the lower track AB. Simultaneously, the observer at point P' will see the upper, somewhat longer track A'B'. The two tracks shown in comparison to each other in the left-hand drawing illustrate the angular displacement known as parallax. The drawing exaggerates the scale, since in reality the parallax is barely measurable directly. However, Halley believed that timing the transit accurately through its various stages, namely its four contact points, would yield measurably different results for our two observers (that is, Venus's passage for observer P' along track A'B' would last minutes longer than its passage along AB for observer P). Duplicating timings at a large number of locations throughout the world and collating and combining the results would lead to an accurate figure for the parallax of Venus and would therefore help supply a precise value for the distance from the Earth to the Sun.
Halley exhorted his astronomical colleagues worldwide to take advantage of the next pair of transits to make the measurements, but, dying in 1742, he never lived to see them himself. In 1761 and 1769 astronomical expeditions from many countries were dispatched to far-flung locations to take timings that would assist in arriving at a correct figure for the Sun-Earth distance. Indeed, viewing the transit of Venus from Tahiti was a prime objective of Captain James Cook's first voyage aboard HMS Endeavour.
Unfortunately none of the timings taken then or later, by expeditions in 1874 and 1882, were accurate enough to provide a satisfactory figure. But they did at least give us our first impressions of how vast the Solar System and the Universe beyond it really are. Today scientists can bounce radar signals off the moon, Venus and other solar system bodies to determine their distances from us and use the data to get a very precise value for the astronomical unit. The mean Sun-Earth distance as determined by radar is currently given as 149 597 870.619 kilometres, give or take about 30 metres. That's about 92 975 681 miles.
With such measuring devices as radar at their disposal, astronomers find little practical value in transits anymore, except to test old theories. Scientific progress marches on. I pondered this as I looked around the small Transit of Venus historical exhibition at the Royal Observatory, which included photographs taken in 1882 with unwieldy plate cameras and Cook's quite crude drawings made in 1769. This time, digital imaging would be the order of the day. How great are the changes that take place in science and technology between one pair of transits and the next! As I walked back down the hill through the park to the station enjoying the beauty of a flower-scented summer afternoon, I recalled the words of an American astronomer, William Harkness. In August 1882, heralding the transit of that year, he said to a meeting of the American Association for the Advancement of Science:
We are now on the eve of the second transit of a pair, after which there will be no other till the twenty-first century of our era has dawned upon the earth, and the June flowers are blooming in 2004. When the last transit season occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows. Not even our children's children will live to take part in the astronomy of that day.
It is a sentiment we can ponder today. Transit pairs are so far apart in time that we can hardly imagine how the world will look when the next pair comes along. Modern mechanization was in its infancy in Europe when the transits of the 1760s took place. When the next pair occurred in the late 19th century, the industrial revolution was well into its stride. Today we live in a hi-tech, computerized post-industrial society. But who knows what our science and society will be like when the next pair of transits comes around in 2117 and 2125?
As I have already said, rarity and predictability are the things that make transits interesting. But here's something else to consider: an article I read recently suggested that transits of Venus encourage overseas travel. This could well be true, bearing in mind the importance of location and weather conditions. If you want to see the next transit of Venus in full in June 2012, you will have to go to the Far East, eastern Australia, New Zealand, or – dare I say it – Tahiti. Now there's a thought.
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Links and notes
This article does not go very deeply into the technicalities of a transit. It is first and foremost my own account of my visit to Greenwich to watch it. There are, however, several sites where you can delve into the scientific background and also into the history of our observations of the phenomenon, which has been witnessed now only six times. These sites may well tell you more about such things as the "black drop" effect or explain the difference between ascending node and descending node transits. Here are just a few. There are many more.
For a comprehensive and highly readable account of the historical and scientific background to the subject, read David Sellers's book The Transit of Venus and the Quest to find the True Distance of the Sun, available to buy at Amazon.co.uk or to read online.
For a minute description of the statistics of transits, visit Fred Espenak's NASA Eclipse site.
For a wide-ranging treatment in German, with some fun Java applets, visit Jürgen Giesen's site.
One of the clearest explanatory sites is the Transit of Venus page of the Orpington Astronomical Society.
For some spectacular computerized animations visit NASA's Sun-Earth Day 2004 Venus transit site.
Check out further Transit of Venus links by clicking www.transitofvenus.com.
Here are some educational sites:
Transit of Venus, which looks forward to the 2012 event as well as covering that of 2004.
Observing, Photographing and Evaluating the Transit of Venus, June 8th, 2004, an Internet project run from Germany with a range of goals, including promoting contact and collaboration between schools, universities, and amateur astronomers, collating information about this and earlier transits, learning how to photograph the Sun and accurately measure the position of an object, and using the 2004 transit to measure the Sun-Earth distance.
Venus Transit 2004 (VT-2004), another Internet project aimed at transforming curiosity into knowledge and interest in science through a broad set of actions. It was launched by the European Southern Observatory.
NASA's Pioneer Venus probe took this photo of Venus's perpetual cloud layer in 1978. With a diameter of more than 12,100 kilometres, Venus is a planet similar in size to the Earth. But its cloud cover is the top of a dense. noxious atmosphere of mostly carbon dioxide with clouds that rain sulphuric acid down upon the rocky, catered, mountainous, volcanic surface. The atmospheric pressure is more than 90 times that of Earth. Venus is in the grip of a runaway greenhouse effect. Its surface temperature is 470 °C, hot enough to melt lead. Venus's day is the longest of any planet in the Solar System, lasting 243 Earth days, and for reasons we don't know Venus rotates in a clockwise direction instead of anticlockwise, like all the other planets.
To learn more about Venus itself, start by checking out these sites:
Venus at the Nine Planets site
BBC Science and Nature – Space: Venus
Venus Introduction at solarview,com
NASA's Solar System Exploration: Planets: Venus: Overview
See also the Wikipedia article on Venus
Illustrations
All but one of the photographs at the Royal Obervatory were taken by me on the day of the transit. The following astronomical photographs taken from various observatories around the world have been added to illustrate various points in the text. I acknowledge the intellectual property rights of all the photographers/artists whose material is represented here.
The masthead incorporates a picture of Venus as a morning or evening star (source: VT-2004 Project Website: Venus in mythology, where the image is credited to the British Astronomical Association). The image of the Capitoline Venus was taken from this site. The image is part of the MythMedia Project, a collection of art images relating to classical mythology held at the Library of Haifa University, Israel.
1. Photo by Jamie Cooper, Northampton, England, UK (source: VT-2004 Photo Archive)
2. Illustration adapted from Colin A. Ronan, Edmond Halley, Genius in Eclipse. Macdonald & Co. 1969, p. 109
3. Photo by Giovanni Paglioli, Centro Astronomico Neil Armstrong, Salerno, Italy. (Source: VT-2004 Photo archive.
6. (first picture) Agency picture published in the Daily Telegraph (9 June 2004).
9. Three pictures from a montage of transit shots. Many sites that produced a wealth of photographic images of the 2004 transit were taken down shortly after it. I cannot now trace where I got these pictures from. If anyone can identify them, please email me.
10. Photo from the European Southern Observatory (ESO) (Source: Astronomy Magazine Website).
11. Drawing from the Transits of Venus page at phy6.org, a group of educational Websites on astronomy, physics, spaceflight and the Earth's magnetism.
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