Phone app available in iTunes

Posted on February 22, 2012 by Steven van Roode

The Venus transit phone app is now available in iTunes for free download. While the Android version is still being worked on, users of an iPhone can already enjoy the phone app. This app will be of great help in your observation of the transit of Venus on June 5 and 6, when the planet Venus is seen in front of the Sun for the very last time this century.

The main aim of the app is to allow you to participate in our international outreach project determining the distance to the sun using the transit of Venus, but there are a couple of other features that come in handy for all observers.

Transits of Venus can be used to find the distance to the sun. It was this utilisation, proposed by Edmond Halley in 1716, that inspired the eighteenth and nineteenth century astronomers to travel all across the globe to watch the transit from far flung places. Halley described that the required observations could already be secured without

any other instruments (…) than common telescopes and clocks, only good of their kind; and in the observers, nothing more is needful than fidelity, diligence, and a moderate skill in Astronomy.

All they had to do was to measure the times the limb of Venus’ silhouette would touch the solar disk on the inside. When in 2004 the transit of Venus occurred again after an absence of a hundred and twenty-two years, this experiment was repeated by thousands of diligent amateurs like you, watching how the disk of Venus slowly approached the solar limb and timing the exact instant of contact. Submitting the data and letting a website do the math, all observations combined into an accurate distance to the sun, a distance that until the 1900s couldn’t be measured accurately otherwise. This historical re-enactment in 2004 was a great success, but there were also issues. That’s where the Venus Transit phone app comes in.

One problem was the determination of time: if you’re on a parking lot or on a sports field observing the transit, where do you get your time accurate to the second? Back in 2004, this was a problem for many one-time observers. Not anymore. The phone app makes use of a GPS clock, allowing you to make accurate time measurements wherever you are: if you have a an unobscured view of the sky, you’re okay.

Another hindrance was the determination of geographic location. In 2004, you had to resort to an atlas or a website which listed the coordinates of major places. A 2004 map of submitted data shows how things went wrong sometimes: a couple of European observers are found in the Atlantic! The phone app will find your location for you using GPS.

A third obstacle was the black drop effect that had troubled historical observers as well: the exact instant of contact between the limbs of Venus and the sun is obscured by a greyish fuzziness. In the eighteenth century there wasn’t really a workaround: every observer made judgements on his own. In the nineteenth century astronomers were trained on beforehand with a mechanical model representing a simulated transit. In 2004, we were back in the eighteenth century; nobody really knew how to deal with the black drop, resulting in times that differed up to 10 seconds for observers watching the transit side by side. The phone app features a realistic simulation of the transit, so you can practise the timing of the contacts over and over again, until you are a skilled transit observer!

And lastly, the phone app will take care of the submission of the data. In 2004, you had to go to a computer, fill in a digital form by hand and send your observations to a database. Now, all you have to do after you finished your observation is press the submit button, and you’re done!

Download the app for free and get actively engaged in the observation of the transit of Venus!

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Are transits and rotation linked?

Posted on February 21, 2012 by Nick Lomb

Lionel asks:

Congratulations on your Venus book. Excellent.
I notice that there is a 243 year cycle for Transits of Venus:

243 x 365.242 = 224.7 x 395

So far so good. The axial rotation period for Venus is 243.1 days. Is this a coincidence or is there some underlying geometrical fact that I cannot see?

An interesting and complex question that I address below.

Patterns in the transits of Venus
Let us first look at the patterns in the transits of Venus. We need to note that Venus and the Earth line up with the Sun every 583.92 days or 1.59872 years. This is called the synodic period.

If there was a transit, say the one in June 2004, for another transit to occur, the two planets must not only line up with each other and the Sun, but do so after an integer number of years so that they are back in the right places on each of their orbits.

Venus and Earth fulfil these requirements after five synodic periods = 7.9936 years as this is almost, though not quite, equal to the integer eight. Thus transits of Venus generally occur in pairs eight years apart. However, because of the slight inequality there is no third transit after another eight years.

A more accurate relationship occurs after 152 synodic periods = 243.00544 years or ~395 Venus years (this is the cycle quoted by Lionel in his question above). The pattern of Venus transits thus repeats at 243 year intervals. For example, the first pair of June transits after 8 June 2004 begins on 11 June 2247. Of course, in the meantime there is also a pair of December transits beginning in 2117.

The rotation of Venus
Scientists using radar observations from the 1960s onwards discovered that Venus spins backwards, that is in the opposite direction to its motion around the Sun, at the slow rate of 243.02 days.

They soon realised that means that Venus, almost but not quite, shows the same face towards the Earth each time the planets are lined up with each other and the Sun. Somehow there is a resonance between the motion of the Earth around the Sun and Venus’ spin around its axis. Scientists are unsure why this is the case, but one suggestion is that Venus is more massive on the face turned towards the Earth at those times and consequently it was gravitationally captured by the Earth.

How is it worked out that Venus shows the same face towards the Earth each time they line up? The quoted value of 243.02 days is with respect to distant stars. With a little arithmetic (taking inverses) we can easily convert that value to the rotation period with respect to the Sun or, in other words, to the day on Venus. It is 116.75 (Earth) days. Five of those periods equal 583.75 days, which is almost the same as the 583.92 day synodic period. So each time the planets line up Venus shows almost the same face to the Sun and hence the same face to the Earth, which is always on those occasions on the opposite side of Venus.

Coincidence or not
As Lionel points out it is interesting that transits of Venus repeat in a cycle of 243 years while the rotation period of Venus with respect to the stars is 243 days, The above detailed discussion indicates that there is no obvious connection that gives rise to the same number in each case. However, the calculations all depend on many of the same factors such as the orbital periods of Venus and the Earth so maybe there was a chance that the same number should recur.

Note: the values quoted above are from the NASA Venus Fact Sheet.

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The transit isn’t going to wait

Posted on February 20, 2012 by Steven van Roode

On October 31 of last year, Huw James gave an interview to BBC Radio Wales about the transit of Venus and his Venus Transit Expedition. Huw explains that by driving all the way from England to the West Pacific he runs the risk of not arriving on time. “The transit isn’t going to wait for anyone.” In that respect, his Venus Transit Expedition is just like the transit expeditions of the past and therefore a real re-enactment of the historical journeys by Chappe d’Auteroche, Maximilian Hell and James Cook: the voyage is hazardous and you never know whether you will actually reach your destination before the day of the transit. On the other hand, we now have tools to our disposal the Enlightened explores did not have: GPS, Google Maps and phones. Huw’s undertaking shows an appealing combination of eighteenth century dauntlessness and twenty-first century comfort. Listen to the full interview:

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Projecting an image of the sun

Posted on February 18, 2012 by Steven van Roode

How are you going to observe the transit of Venus? Many a amateur-astronomer is already exploring the options: buying a solar filter for a telescope, getting a card board solar scope or even acquiring a special telescope with Hα filter. For a concise guide, see Chuck Bueter’s Six Ways to See the Transit.
One of the methods promoted here is something you can always fall back on if special equipment is out of your league. It’s simple and effective, and allows you to see the actual transit: pinhole projection. In Popular Science Monthly 4 (April 1874), pp. 669-670 this technique is described by Wiliam Jay Youmans in connection with the transit of Venus:

When the sun shines through a small orifice into a darkened room, a cone of rays is produced, as everybody has observed, by lighting up the particles of dust which are scattered in its course; for, if the air were quite clear, the track of the rays would be invisible. In this case an image of the sun is formed upon the floor or opposite wall by the crossing of the rays through the aperture. The best condition for the formation of such an image is when the sun is low, and there is a white wall opposite to receive it; the image is then perfectly circular. But if the light falls upon the floor, as represented in Fig. 4, the cone of rays produces an oblong or elliptical image; the deviation from an exact circle depends upon the angle which the cone of rays makes with the floor. Such an image may often be made instructive in observing solar phenomena. By closely examining it, it is sometimes possible to detect the presence of spots on the solar surface. Solar eclipses may be watched in the same way. As the moon gradually encroaches upon the sun’s disk its progress can be traced by a corresponding change in the form of the image, which resembles that of the uneclipsed portion of the solar surface. In such observations, however, it will be remembered that the course of the movement is always reversed. It was in this way that the transit of Venus was first observed by Jeremiah Horrocks, November 24, 1639. He had calculated the time, which fell upon Sunday morning. He arranged his room for the observation, and then went to church, as he did not wish any secular interests to interfere with religious devotion. It is very probable, however, that Venus was mixed up with his devout meditations, for he hurried back from service, and was delighted to find that his calculations were verified, as the planet was already far advanced in its passage across the sun’s face.

The method by which Horrocks observed the transit isn’t quite accurately described by Youmans, as Horrocks used a telescope to project the sun’s image. But yet, if you lack any other way to observe the transit of Venus safely, you can just project the solar image and see Venus advance on the sun’s disk. Here’s a simple guide to build your own pinhole camera, complete with questions for use in the classroom. When the sun shines on June 5 or 6, you should go out and see the transit yourself, and not confine yourself to watching a webcast on your computer screen!

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Pattern on a paper plate

Posted on February 16, 2012 by Chuck Bueter

ToV frequencyTransits of Venus are predictable, but the pattern seems odd. Two transits occur within eight years, then there is a lapse of 105 1/2 years, then another pair within eight years, then wait 121 1/2 years. How do you explain this frequency?

Kathryn Williamson, a graduate student at Montana State University, has adapted and improved with her illustrations an activity I have done that uses a simple paper plate to explain the conundrum.

Imagine daughter Venus and mother Earth are racing around a central object, starting from the bottom of the circle with Earth-Venus-Sun aligned for a transit of Venus. Speedy daughter laps mother after 1.6 earth orbits (2.6 Venus orbits), or 1.6 earth years. Continuing the race, the fifth time (shown in colors) that daughter catches up to mother–eight years later–they are again near the starting line. So why don’t transits happen every 1.6 years?  Or every eight years?

Five alignments Transit of Venus pentagon

Incline Venus’ orbit a few degrees, and Venus is usually either a little above or below the Sun-Earth line. Only at the nodes, or the hinge of the paper plate orbits, are they near an alignment enough for Venus to transit the half-degree sun.

There’s one more problem. After the fifth conjunction, the two planets are actually just a wee bit short of the starting line. Each fifth alignment is shifted clockwise. Eight years after the race begins at the 2004 transit, they’re still close enough to the node for a second alignment in 2012. But by the 2020 conjunction,Venus has moved up the orbital incline so it would be just above the sun, as seen from earth–no transit.

In drawing the pentagon from the five sets of conjunctions, you can see how one point of the star (“1st pass”) needs to regress to the opposite node before the next 8-year pair of transits occurs. That explains the “half” of 105 1/2 and 121 1/2. Hence, the 8-year transit pairs shift from June to December each century-plus.

Try it for yourself on a paper plate using some colored markers, scissors, and Williamson’s instructions (PDF file). Thanks, Kathryn, for the clarity.

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Out of Diaries: 14 February 1761

Posted on February 14, 2012 by Andrea Wulf

On 14 February, after an arduous and exhausting journey from Paris, Chappe d’Auteroche arrived in St Petersburg. He had endured freezing temperatures and much of the way through the Russian mountains he and his team had walked – slipping and falling – soon covered in bruises. In Riga he had switched to sledges only to discover half a mile outside the city that the snow had disappeared. Sometimes strong winds blasted clouds of snow high up into the air, whipping the flakes into frozen pellets.

In St Petersburg Chappe was introduced to his colleagues at the Imperial Academy of Sciences but also discovered during the meeting that he had astronomical competition. Two and a half months previously the Russians had decided to mount their own expeditions. Seemingly, the letter which the Académie des Sciences in Paris had written, informing the Russians that they were sending Chappe to Siberia, had never arrived.

With no news from France, the president of the Academy had announced that Russia needed to participate. They had procured the necessary instruments and had trained two young observers for the task. In January one observer had left for Irkutsk near Lake Baikal in Siberia, and another for Nerschinsk near the Chinese border. With their own observers on the way, the Russian Academy now regarded Chappe’s expedition as pointless.

The academicians argued that Chappe should view the transit somewhere more convenient and closer to St Petersburg, but the Frenchman was not having any of it. Highly competitive and keen to provide the most important data of all, Chappe began to rally support for his Tobolsk expedition, arguing that there was no other part of the globe where the transit could be observed ‘to so much advantage’. He had not endured the hardships of his long journey to be ouflanked by the Russian scientists.

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The astronomer of Rousdon

Posted on February 12, 2012 by Steven van Roode

It must have been sometime in 2003, while preparing a contribution for the French book Vénus devant le Soleil edited by Arkan Simaan, when I stumbled upon a couple of wonderful sepia prints of an 1882 British transit of Venus expedition stationed at Jimbour, Queensland. The collection consisted of people posing on the veranda of a large mansion, and astronomers posing with the long equatorial telescopes in their observing huts. One illustrious name was among the people depicted: Lieutenant Leonard Darwin, son of Charles Darwin. He was a member of the expedition headed by Captain William Morris. Two other astronomers joined the expedition on their own expense: Cuthbert Peek and his assistant Charles Grover.

Doing more research on this particular expedition, I came into contact with Jerry Grover, a great-great-grandson of Charles. Jerry not only happened to have the prints of the very same photographs in his possession, but he also could provide full accounts of the expedition written down by Charles and Cuthbert. Their stories turned out to be real page turners: the entire trip to Australia proved to be an extraordinary journey and a great adventure for the two. All these factors added up to make Charles Grover one of my favourite figures in the history of the transit of Venus.

In 2005 Barbara Slater published The Astronomer of Rousdon, telling the fascinating story about Charles Grover, who worked as an astronomer in this small village in Devon. Charles is an example of the many self-educated working-class astronomers from the Victorian era. Born into a poor family of shoe makers and starting his career as a brush maker, he rose to a respected position in the household of Sir Cuthbert Peek, heir to a baronetcy, as assistant in his private observatory and later as observer. Charles had specialised in observing variable stars and gained the admiration of quite a few well-connected contemporaries.

Barbara’s story expands on every stage in Charles’ life, with a special focus on the journey to Australia in 1882 to observe the transit of Venus. Using quotes from his own writings and illustrating the events with sketches by his hand she brings the person of Charles to life. Thus, we not only get an intimate image of Charles himself, but also of the generally little-known nineteenth century amateur astronomer he represents. Interestingly, at the end of her book, Barbara investigates if the main character Swithin St Cleve of Thomas Hardy’s novel Two on a Tower might have been modelled after Charles Grover. It’s pure speculation, but still a compelling thought.

Barbara Slater manages to give us a lively insight in the fateful events of a nineteenth century transit expedition as seen through the eyes of an assistant, and by doing so she pictures a typical Victorian amateur astronomer at the same time.

Barbara Slater, The Astronomer of Rousdon – Charles Grover 1842-1921 (Steam Mill Publishing, 2005), 276 p
ISBN 1 898 737 30 4
Price £9.95
www.steammill.phlegethon.org

 

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Tracing historical sites

Posted on February 10, 2012 by Steven van Roode

The wealth of scanned nineteenth century reports and accounts of transit of Venus expeditions provide some nice educational science activities. One of these activities involves retracing the sites of these expeditions using the trigonometrical measurements performed by the astronomers in order to secure the geographical location of their stations. And this may, in turn, allow us to relocate historical sites which were long lost.

For example, in Hartford, CT, a German expedition observed the 1882 transit of Venus in the grounds of Trinity College. The chief astronomer, Gustav Müller, left an inscribed stone marker on the heliometer pier, so that the location of the main instrument would be known to future generations. But in 1959 this marker was relocated to its present location, just outside the entrance to McCook Hall and the Gallows Hill Book Store, to make way for the construction of Mather Hall. As a result, the original location of the German station is now only roughly known.

Fortunately, two City Engineers, C.H. Bruce and H.G. Loomis, determined the geographical position of the heliometer in 1882 by measuring the angles between the meridian and the directions of a couple of buildings in Hartford. Three of the four buildings used still exist today, and this enabled me to relocate the position of the heliometer pier. The following angles, measured from a point 497.32 feet north of the heliometer, were used to trace the position back in Google Earth (see the image at the top of this post):

Windsor Avenue Church: 20˚36′30″ (Bruce) and 20˚36′30″ (Loomis)
Old State House: 39˚41′ (Bruce) and 39˚41′30″ (Loomis)
South Church: 47˚55′30″ (Bruce) and 47˚55′ (Loomis)

Even though Müller complained that the instruments of the two engineers had an accuracy of only one minute of arc, that it remained unclear how the direction of the meridian was established, and that the points chosen were alleast from the station, the location found in Google Earth is remarkably close to where the site is thought to have been: right over Mather Hall!

I invite you to join our Mark that site! project and have your students play around with the data from the linked reports and accounts on our history pages for the 1874 and 1882 transits of Venus.

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Transit acts as regular in sitcom

Posted on February 8, 2012 by Steven van Roode

The other day I showed you how the movie Sunshine starred a transit of Mercury. Now I have a still for you from the intro of a well-known American sitcom, in which a glimpse of a planetary transit can be seen. While the images shown in rapid succession in second part of the intro are described in great detail, what we get to see of the Universe in the first part of the intro is not. Yet, it might be a good discussion starter for a high school astronomy class.

The question is: will Sheldon, Leonard, Howard and Rajesh see the 2012 transit of Venus too? I can’t imagine these characters wouldn’t want to. On our website you can already find out at what time they will be able to see the transit, and what part of it will be visibile from what is thought to be the location of Sheldon’s apartment. Let’s wait and see if the last episodes of this season’s The Big Bang Theory will give us a definitive answer.

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Out of Diaries: 6 February 1761

Posted on February 6, 2012 by Andrea Wulf

On 6 February 1761, the French astronomer Le Gentil wrote a letter from Mauritius (then called Île de France) to a friend in France, updating him with his progress (or to be precise his lack of progress). Le Gentil had been the first in the race when he had sailed from France on 26 March 1760 to view the transit from the French trading port Pondicherry at the south-eastern coast of India.

His journey seemed doomed from the beginning. They had encountered storms and zigzagged the ocean in order to escape from the British who all too often loomed on the horizon. On 11 July 1760 he had finally arrived in Mauritius from where, so Le Gentil had been told, he would be able to find a ship bound for Pondicherry. Only two days later a vessel arrived from India with the devastating news that what was left of the French possessions in India was crumbling under British attacks. Pondicherry lay under siege.

To make matters worse, much of the French fleet that had been stationed at the naval base in Mauritius and that was to sail as reinforcement to Pondicherry had been destroyed by a hurricane earlier that year. ‘I do not know when I will be able to leave’, Le Gentil wrote despairingly to Paris. For the time being he was stuck on Mauritius. With Pondicherry besieged, no supplies were arriving from India and the dishonest officials of the Compagnie des Indes in Mauritius sold the goods left in their stores at ridiculously inflated prices. ‘Life is horribly expensive’, Le Gentil wrote to his friend, moaning in particular about the rising cost of wine. To add to his misery Le Gentil was also struck down with debilitating attacks of dysentery. His illness, he was certain, resulted from frustration. His ‘mortification and concern’ about the transit observations had made him sick.

But Le Gentil was not going to give up that easily and decided to look for alternative locations from where to view the transit. Tenaciously he tried to concoct a plan, he wrote to his friend, but felt that he was wasting his time on ‘chimerical projects’. He first picked Batavia (today’s Jakarta) as an alternative to Pondicherry but gave up on the idea. The only option, so Le Gentil decided, was to sail on a small local ship to Rodrigues, an island not too far from Mauritius and known mainly for its turtles.

But with his seemingly unwavering talent for attracting problems, Le Gentil had managed to pick out of the vastness of the ocean the one small speck of land that the academicians had chosen for another French observer, Alexandre-Gui Pingré who had already sailed past the Cape Verde islands and was now nearing the equator.

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