Viewing great; timing difficult

I’ve never surfed a big ocean wave before, but I imagine it’s something like the transit of Venus. You prepare, you see it coming, you paddle franticly to catch the right spot. Then, for a fleeting moment you stand up, ride a mass of towering energy, delight in a surreal moment in a tunnel, then feel the watery bulk flow underneath you as you emerge on the wave’s backside, thankfully intact. For the last couple of weeks I’ve been floating on the backside of that transit of Venus wave, and it’s been quietly exhilarating.

Eight years of anticipation peaked with a gorgeous June 5 in northern Indiana. The day got underway early with site preparations, as a tent was raised and brought to life with with electricity, cable, internet, and sound. Of course, the phone was ringing as last minute seekers of solar shades and interviews dialed in to the day’s events. There was nothing left to do with the website, as that ship had set sail. I reset the Countdown Clock for the 2117 transit of Venus, some 38,000+ days in the future, and walked away from the computer.  Our Transit of Venus (TROVE) celebration was underway.

Irvin Stanley's tent

Irvin Stanley in his tent at USNO expedition

Shortly after noon a group of descendants of Irvin Stanley arrived, having come from Michigan, Indiana, Illinois, and California. Irvin Stanley was the Assistant Photographer for the US Naval Observatory (USNO) expeditions to Kerguelen Island (1874) and Patagonia (1882), and USNO Librarian Sally Bosken had generously provided many artifacts from the expedition archives. A highlight of the transit of Venus experience has been learning about and following this one person, Irvin Stanley, through the travails of exploration in a previous century. Now his family was touring regional transit of Venus highlights, with more than just the celestial sight on tap.

First up was an exhibit of transit of Venus art by high school students. Then I presented a talk centered on Irvin Stanley and the greater quest to find the Astronomical Unit from timed observations and photographs of the event from widely separated (hence, remote) locations. I wish I had had more time to peruse the family photo album that someone brought, but the family was ushered onto the motor coach and was off to the next stop–the Harris Branch Library, site of a transit of Venus display of artifacts and an art exhibit in pastel.

PHM Observing Site

Observing site in Mishawaka, Indiana

June 5 was the last day of school for the Penn-Harris-Madison (PHM) school district, and Art Klinger’s astronomy class students began setting up telescopes and viewing devices in the field near the tent. Over an hour before first contact, spectators were showing up as we secured solar filters to telescopes with bright orange duct tape. I gave one last frenzied lecture in the theater before returning to the scopes outside A bustle of activity ensued until suddenly hundreds of people had gathered and there was talk of the transit beginning imminently. Scott Potosky quieted the music and piped in a WWV time signal for anyone at a scope who wanted to capture the moments of contact by that audible method. Everybody had taken their first look at the sun with solar shades, which in itself yielded a lot of oohs and ahhs, before Venus even took the stage.

Venus entered near the one o’clock position on the sun, the planet not easily discernible to the naked eye as it worked its way across the limb. People waited patiently in lines to view through telescopes, a sight complemented by two significant bands of sunspot groups. I occasionally glanced in a telescope, but was strangely removed as I emceed the event for a growing audience. As second contact approached, I cut to the front of a line and planted myself. I felt like the narrator in Oliver Wendell Holmes, Sr.’s, The Flaneur. There I was, face to face with Venus, having a conversation with her at internal contact.

Chuck at scope

Trying to discern internal contact

My overriding impression of the moment was, “Damn, determining the time of contact is not easy.” I tried to imagine all of our predecessors trying to discern an exact moment when they felt convinced Venus was edge-to-edge with the sun. I understand Crabtree not recording the transit in writing. I understand Rittenhouse becoming overcome with emotion. I understand Cook’s time not being in synch with his shipmates’ times. I understand the expedition preparations that insist on total concentration and absence of distractions leading up to the transit, conditions that were lacking at the public event.

While the sight through a telescope was enthralling, I experienced an undeniable uncertainty and lack of confidence in my timing. Despite my familiarity with what to expect and my having witnessed the 2004 transit of Venus, I could easily have been off by 30 seconds. The sun was at about 30 degrees of altitude with no clouds obstructing the view, but Venus seemed to linger on the solar edge. I looked for the planet to separate clearly from the sun, but when that didn’t happen I tried to see if the horns of the sun had reconnected. Was that a minimalistic black drop effect I witnessed that suspended time? How did so many of those earlier explorers seemingly fare better?  I wasn’t at the eyepiece long enough to look for other phenomena, like the aureole effect.

Lou Sandock at scope

Scoping out Venus

Four monitors in the tent showed other perspectives. Feeds were coming in from NASA’s webcast in Hawaii, news channels, and websites like Steven van Roode’s transitofvenus.nl and my own transitofvenus.org. David Wyatt tracked Venus through a video camera that was hooked into a telescope and broadcast on a monitor. Another person projected a vast image of the transit onto a screen. Some people saw the transit through white light filters and a hydrogen-alpha telescope provided by AstroCamp of YMCA Camp Eberhart in Three Rivers, MI. Of course, a couple of telescopes were equipped with Sun Funnels, which yielded a projected image of the sunspots and of Venus in silhouette.

A few clouds moved in, thankfully after first and second contact, so during breaks in the action I demonstrated with a paper plate why transits come in 8-year pairs separated by over a century. I solicited and answered questions, pleased by the inquiries while realizing what I had previously left unsaid while narrating the event. The best memories are from the quiet moments when I shut up.

Observing from Indiana

Observers look up in unison

For example, after one extended cloud was ready to give the sun back to us, everyone realized when the sun was about to emerge from the well defined edge. I watched the crowd as they lifted their heads in unison and donned solar shades just as the sun brightened their faces.

During the whole event, a few local businesses that were supportive of science hawked their wares. Victorian Pantry served its Black Drop Effect Coffee, a smooth dark roast that was packaged with a design borrowed from the stained glass windows of St. Michael’s Church in Hoole, England. Pizza Transit (yes, its pre-existing name) had both a Transit of Venus pizza and a Black Drop Effect pizza for sale. Taylor Design sold commemorative t-shirts, one of which has Worlds on Tour printed on the backside with the dates of past and future transits of Venus. Rock on.

Nitzschke illustration

Nitzschke illustration of a transit

People had been coming and going from the PHM site for the duration of the transit, and I joined the early departures. My heart was set on watching the transit underway at sunset from Warren Dunes State Park in Sawyer, Michigan, overlooking Lake Michigan. I had to see the Nitzschke-like view, what I consider an iconic transit image.

While I was driving to that lakeside destination, I realized other local observing sites with telescopes were drawing large crowds–New Carlisle Public Library (160 people), Andrews University (500 people), LaSalle Intermediate Academy (400 people), University of Notre Dame (~2,000 people).  Each was staffed by dedicated transit of Venus enthusiasts.

Young observer

A young observer tries solar shades

We had distributed about 6,000 pairs of solar shades to people ranging from church pastors to juveniles at the detention center, so hopefully the shades were pressed into service. Venus was readily apparent naked eye to most people when it got away from the edge of the sun, albeit just a small dot on the sun.

As we arrived at Warren Dunes, a chilly breeze kept a red warning flag blowing stiffly. The Kalamazoo Astronomical Society had stationed itself on a large concrete pad overlooking the beach and lake, with a fabulous assortment of telescopes and observing devices, from grand to humble. Over a thousand people had gotten a glimpse of the transit through their gear that day, and sunset was approaching. I can’t emphasize enough how beautiful the sunset was, with only the slightest of wispy clouds near the horizon that added character to the spectacle rather than obscured it. Seen in filtered telescopes, Venus contorted into a variety of shapes as her blackness or absence of light flickered through the thickest atmosphere.

Sunset at Warren Dunes

Sunset over Lake Michigan

The time came to abandon telescopes and solar shades. With the orangish sun plunging into the water, Venus was visible to the naked eye. I saw Venus at times appear to be nearly square, like four pixels on a monitor, and then it would momentarily disappear completely before re-emerging as an obvious spot on the sun. At one point the black blemish was above and right of a thin cloud line across the sun; soon it was below the thin line. Eventually Venus was gone, set below the horizon. The spectators watched the remainder of the sunset and cheered, sending well wishes to Venus for the next 105.5 years.

From there a modest crowd and the bus tour went to The Livery microbrewery in Benton Harbor, MI, where musician Venitia Sekema was performing an excellent musical set. We propped up the Transit of Venus Time Keg for people to write any last comments to denizens of the 22nd century. Shortly after 11:00 p.m. EDT we broadcast live via Slooh, showing some more transit of Venus art on display at the microbrewery. And, of course, we quaffed some Venusian ale, which had been crafted specifically for the celestial phenomenon. To see the half hour of our live broadcast, watch from about 0:52 to 1:23 of the Slooh Space Camera Transit of Venus Part 3.

Shortly before midnight, everybody inside the pub went out to the back patio and watched the International Space Station pass nearly overhead at magnitude -3.3. Onboard the ISS, astronaut Don Pettit would have been between the second and third contacts, though we were in the dark below. Again we cheered, wishing Don well in his photographing of the transit from the ISS cupola.

Time Keg

Transit of Venus Time Keg

Everyone was clearly getting tired from a long day. While I had intended to seal the Time Keg for its long storage, we opted not to do so that evening. I figured there may be a couple of items to add before we hastily closed the lid for good. If you have any contents that you wish to include, email me (timekeg@transitofvenus.org) to see if there is room and time. I’d like to wrap it up soon.

The week after the transit of Venus has been a time to decompress and re-connect with family and friends. From my perspective, much more has passed by than just 6+ hours of Venus on the sun. It’s been a long, fun ride, and I’m grateful to many people for their respective roles in this adventure. There is still much more to do, of course, but on a different level of urgency. Thanks for being a part of the 20th century transit of Venus experience, especially to everyone who contributed to the outreach effort.

What a sight. What a day. Surf was definitely up.

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How did your observations go?

How did your observations of the 2012 transit of Venus go? Whatever disastrous things may have happened during your observations, they are most probably dwarfed by what Le Gentil had to go through between 1761 and 1769. The following animation tells his story:

Use our Facebook page to tell your story!

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Among coyotes and rattlesnakes

“I observed the transit from southern Saskatchewan, and lived to tell the tale!” – would look good on a t-shirt, but it does describe my ToV experience.

(As the last of Steven’s blogging team to report I must offer readers my apologies – shades of Fr. Hell in 1769? – but I don’t have his excuse…).

I had agreed to go out to Canada’s west from Toronto to observe the ToV because the window for observation would be longer than in the east, and an invitation from the Regina Centre of the Royal Astronomical Society of Canada to deliver a ToV lecture at the Regina Science Centre would nicely cover the expenses.

The appointed meteorologist for our ToV équipe, Chris Beckett, was obsessive about the weather – as any astronomical expedition meteorologist ought to be. He was fully prepared to drag us 400 km in pursuit of clear skies on the day of the ToV, come Hell or high water (curiously enough, both the landscape of Hell, and much high water can be encountered in Saskatchewan with its fourteen major drainage basins, the world’s biggest Møns Klint glaciotectonic hills, and the world’s largest active sand dunes).

Chris’ favoured potential ToV observing site featured the following amenities:

  • wood ticks
  • black widow spiders
  • rattlesnakes
  • inviting quicksand pockets
  • cheerfully undomesticated packs of Coyotes
  • friendly lone Grey wolves
  • wolverines with attitude
  • BIG steely-eyed irate Bison
  • ghost towns, isolated graves, and smiley bleached skulls of indeterminate species
  • authentic locals with guns but little patience
  • relics of Capone
  • remote location far from succour
  • and the clearest (and at night darkest) skies one could wish for

According to the weather data Chris had constantly feeding into his MAC, on the day before the ToV his favoured site was forecast to have good to very good weather for the event. We went out that night to a much closer observing location to test some of our equipment (but obviously not the full-aperture solar filters and Herschel wedges), and when I remarked on the “interesting” animal noises, he said: “Ah, the coyotes. Don’t worry. They’re small, almost like domestic dogs – well, except that they’re not domestic. I’ll tell you what; pay no attention to them till you hear them crunching the detritus underfoot, then you’ll know they’re close, and have surrounded the observing site. Still don’t worry, just run fast for the car. We can collect the gear in the morning.”

Transit morning things had changed dramatically weather wise. According to satellite imagery a weather system coming up from Montana and  s t r e t c h i n g  far to the south but heading north was going to make his preferred site not so preferable. Regina, in fact right outside where I’d lectured earlier in the week, was looking better. We did a couple of early morning press interviews for the ToV from the site, which gave us the chance to check the sight lines to the west, and available wind shelter. And this indeed became our transit station.

When we set up at about 15:30 CST there were still clouds obscuring the Sun, but in the 10 minutes or so before 1st contact they cleared – one is almost tempted to say miraculously cleared – and we were able to see 1st and 2nd contact clearly, including the black drop and aureole effects. We also did some education and public outreach, but only after we’d done our critical observations – as did John Winthrop in 1761, showing his hosts the ToV in the observing lulls, as it were.

What of the experimental archaeology of the ToV? We did manage to run several experiments, the chief of which was to answer the question “with how small an aperture (O.G.) can the aureole effect be observed?”. Our experiment was qualified by the equipment available. We sought to employ 18th-century OTAs with doublet and singlet objectives, and single-element eyepieces, alongside Short-style Gregorians. Likely instruments were located, but unfortunately permission could not be obtained to use them for the experiment. The experiment was altered to use modern hand-made high-quality doublet refractors, several of the 80 mm O.G. class, and one 125 mm O.G. OTA.  We had prepared simple aperture stops for the latter, and our preliminary result is that we could discern the aureole intermittently in the period embracing 1st and 2nd contact with a 50.8 mm O.G. of f/14.56, a surprisingly modest system. We are presently writing up the results for publication. By way of a very preliminary conclusion we can venture to say that if an 18th-century observer was viewing the ToV through the best quality doublet or triplet of the day, and using the on-axis area of best definition and least aberrations, it is possible that a system as modest as 50.8 mm O.G. of f/14.56 would have sufficed for the aureole. In the nature of such experiments it is important to state that no stronger or more robust conclusion is possible.


Aperture stops for aureole experiment

As it turned out I did get attacked by one of the dangerous creatures of the wilderness listed above. Fortunately that didn’t happen during the ToV. I’ve mostly recovered…

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The transit in HD

The following images of the transit are taken by the NASA’s Solar Dynamics Observatory.

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The transit at Siding Spring

Wonderful views of the transit were seen from Siding Spring Observatory, the home of Australia’s largest telescopes, near the town of Coonabarabran in NSW.

A parfocal image of the transit through a 70-mm lens telescope near mid-transit. Nick Lomb

I was with a coach load of 50 people organised by the Australian Museum and co-led by one of the country’s best-known and most popular astronomers, Professor Fred Watson. The group had spent three days visiting Australia’s main astronomical establishments, starting with the historic Sydney Observatory and then followed by Tidbinbilla Deep Space Tracking Station, Mt Stromlo Observatory and the Parkes Radio Telescope. There was even a stop at Goulburn to see the site of one of the observing stations set up for the 1874 transit by Henry Chamberlain Russell of Sydney Observatory.

Observing the transit from Siding Spring Observatory. Nick Lomb

The weather was cloudy with occasional light rain during the first two days and everyone on board the coach was on tenterhooks about the weather for the transit. Fortunately, on Wednesday morning we woke in Coonabarabran to a beautiful clear sky and it stayed that way during the half an hour trip up to the top of Siding Spring Mountain.

There Fred had chosen an observing site behind the UK Schmidt Telescope and had two telescopes set up ready for the group. I quickly set up my own little 70 mm refractor and all was readiness for the start of the transit. The slight indentation of Venus on the edge of the Sun was seen at first contact followed 18 minutes later by second contact. With the Sun low in the sky both Fred and myself thought that there was a slight touch of dark haziness between Venus and the edge of the Sun at second contact – a manifestation of the infamous ‘black drop effect’.

Toasting the transit. Nick Lomb

To add to the feeling of occasion one of the group played the bagpipes that he had brought along with him. Not only was there the sound of bagpipes, but the tour organiser Marnie thoughtfully provided champagne so that we could toast the transit and some of the many people associated with the transit during its long history.

Later there was time for a tour of the mountain that is now crowded with telescopes including the SkyMapper which is an automated survey telescope. Fred took the group through the 3.9 metre Anglo-Australian Telescope, of which he is Astronomer-in-Charge. This was followed by a tour of the UK Schmidt Telescope that I missed as I wanted to spend some time observing and reflecting on the transit on my own. At mid-transit light cloud started to come in. This was actually useful for me as it allowed my compact digital camera to cope with the brightness of the Sun and finally allowed a few useable photographs.

Nick Lomb rugged up for the conditions at Siding Spring Observatory with the dome of the Anglo-Australian Telescope in the background. Nick Lomb

The coach left before the final two contacts as the plan was to observe them from a picnic spot on the way back to Sydney. We arrived at the designated spot in time, but there cloud covered the entire sky and there was no sign of the Sun. Still the bagpipes played a lament at 2:44 pm AEST as Venus left the Sun for the last time this century.

We had seen most of the transit in near perfect conditions from Siding Spring Observatory and I cannot think of a better place to have watched the rare event. I feel privileged and very happy to have successfully observed two transits of Venus.

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A spectacle on Mount Wilson

We had perfect success from on top of Mt Wilson in southern California.

My wife and I attended a gathering of the Antique Telescope Society, I gave a presentation on old and new transit maps, we received an excellent tour of the 150′ solar telescope, and a group of us enjoyed a night with the historical 60″ telescope (the view of Saturn was jaw-dropping), and shared an elegant dinner on the floor of the 100″ telescope!

Some small wispy clouds teased us between 1st and 2nd contacts, but they were no serious threat. After 2nd contact, it remained perfectly clear and there were clouds below us over Los Angeles.

The transit of Venus from Mt Wilson was an incredible experience because of the mountaintop setting, the adjacent 150′ tall solar telescope, the live webcast from Astronomers Without Borders, the chance to meet astro luminaries such as Owen Gingerich, Nick Kanas, Sarah Schechner, and Dava Sobel, and the assemblage of many antique telescopes. I watched the transit through several Alvan Clark telescopes and we had fun posing for pictures with a projection from an intriguing helioscope comprised of both antique and modern components.

I carefully watched 2nd contact with my Meade ETX and I think I saw the black-drop effect. Not a slender filament, but there definitely was some schmutz between the solar limb and Venus as they separated. The seeing was excellent until the sun was low. An airplane transited the transit near sunset, I hope someone got a photograph of that.

Ironically, I was not able to view the transit of Venus web app until near sunset because of lack of wifi or cellphone access. (the leading GIS company Esri, Astronomers without Borders, and the Dutch team behind the VenusTransit app collaborated to build a live web map showing observations in real-time.) When we at last saw the web app with live observations late in the transit, we gasped! It worked and several thousand contact timing observations were registered on the map. This effort in participatory citizen science demonstrated innovations in sharing astronomical experiences among amateur astronomers and the public.

If you used the VenusTransit smartphone app, be sure to look for your observation at tov2012.esri.com. This web app will be up for another week or so. On this web app, click the Transit Times Map, and select Observations on the bottom. You’ll see a nice world-wide distribution of several thousand contact timings. Curiously, there are a few observations reported from places like South America where none of the transit was seen!

Also of interest on tov2012.esri.com is the social media map overlay. You’ll still see many tweets, pictures, and videos related to the transit on the map. It’s a great way for the public to share their experiences with each other.

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Fabulous Transit from Hawaii

Well, it’s come and gone for another 105.5 years. And we couldn’t have asked for the day to have worked out any better. Well, we could have, but that we’d have had to be daft!


I’ve just seen 1st and 2nd contacts. You might guess I’m having a good time!

Even with a stiff breeze at the Mauna Kea Visitor Information Station, Venus and the limb of the Sun were crisp and vivid. Those of us with white-light filters got a bit of an advanced alert on

First Contact – the Hydrogen-Alpha observers saw Venus enter the chromosphere half a minute before we did. An intensely scientific poll of a well selected group (i.e. everyone in earshot who gave a damn) revealed that most of us saw some kind of mild black drop effect.

Mid-Transit as seen by a video team on the summit of Mauna Kea

I’m going to state that I saw the famous aureole around the space-side of Venus. It was tiny and hard to see, and it wavered in and out of view, but I tried every trick I’ve come up with to undo the effects of creative eyesight. When I heard another observer I hadn’t even met describe something very similar to my observations, I felt reasonably sure I was seeing something real.

After a trip to the Mauna Kea summit, clouds wrapping around from the low-pressure system that passed just north of the Big Island started to close in on the Visitor Information Station, so our group organizers started checking the alternate sites on our contingency list. The most likely site for observing egress -the beach behind our hotel. Ya gotta do what ya gotta do, so we toughed it out and went back to a tropical climate zone.


Left: Observing the Transit on a summery Hawaiian day. Right: We endured great hardships to observe the 3rd and 4th contacts.

And there you have it, folks. The 2012 Transit of Venus as viewed over 11 (or so) climate zones. Incredible!

Check out a few more photos.

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Transit at Kitt Peak, AZ

Just a quick transit report from the Kitt Peak National Observatory in Arizona.

Wow, what a day it was. I had come from Washington DC to Tucson on the day before – so we only had to drive from Tucson to the observatory where we arrived at noon. Lots of telescopes were set up but also binoculars and projecting devices. For the beginning of the transit the wonderful Bob Martino had set up a 10–inch Dobson telescope for me – so that I would be able to see external and internal entry.

Just after 3pm everybody was at tenterhooks. As we stared into the telescopes, someone shouted ‘there she is, entering at about 1pm’ – well, on my telescope Venus entered from the other side between 7 and 8pm  … so, I think , I might have missed the very second Venus was pushing in, because I was looking at the wrong side of the sun. Haha. But I didn’t faint like David Rittenhouse.

And then we just enjoyed Venus’ majestic march. At 4pm, my talk started at the Visitor Centre and I didn’t think that I would be able to compete with Venus … but most people came for the talk (maybe it was a good excuse to get out of the heat). It was truly extraordinary to be able to talk about the 18th century astronomers and how they risk their lives to catch a glimpse of Venus while Venus was doing her dance at the same time.

After that we all went back to our observations, sharing telescopes and binoculars – until sun set with Venus on her. I’m kind of speechless – which is bad for a writer but sometimes good for the soul.

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Vardø: overcast but fun

In the days preceding the transit, weather forecasts were giving different predictions for Vardø, Norway: one said it would be partly clouded, another predicted an overcast sky and yet another added some drizzle and snow. I was planning to observe from this historical site, where in 1769 the transit of Venus had been observed by Maximilian Hell. Sunday we had already explored Vardø, looking for the perfect observing spot, examining sight lines and available space. The weather was beautiful: not a cloud to see. Tuesday afternoon we drove again to Vardø from Vadsø, which is only 74 km eastward. The sky was still blue with some sparce clouds, but during our trip things changed dramatically. Coming from the east, a grey blanket soon covered the entire sky, depriving our view of the sun. For hours we waited in vain, until an hour before midnight we thought it wiser to abandon our observing site and head towards the party tent erected for the occasion next to Vardø’s town hall.

Here, all kinds of activities were going on, including a projection of the direct coverage by the Norwegian television. A third plaque was revealed, giving Maximilian Hell a “new” Hungarian first name: Miksa (even though he is known to have used the Latin and the German forms of his name only. Hot snacks, a transit of Venus cake, a band playing Sousa’s Transit of Venus March, a quiz (of which feel I a bit abashed to disclose the winner’s name), songs and dances formed the joyful entourage for us to process the great disappointment of not being able to see the transit of Venus. It was wonderful to see how the local community had put everything together, in which students had an important role. Despite the dreadful sky, the Norwegian television even had direct views from Vardø, showing interviews, the band playing and salutes at the fort at the start and end of the transit.

So, all I got to see of the transit was by looking at the television screen, either in the party tent in the centre of the village, or the room of the attendents of the transit of Venus conference in Tromsø, who had travelled to Vardø the previous day.

Only at 4.30 p.m., the cloud cover at Vardø got thinner and Venus became finally visible in front of the Sun. However, a dramatic “fight” between the Sun and the clouds continued until the fourth contact. Reinhard Neul from Stuttgart and Thomas Posch from Vienna were lucky to acquire the following (and many more) images:


Left: image by Reinhard Neul. Right: image by Thomas Posch

Among the local people, who had celebrated the overcast beginning of the transit with so much enthusiasm, a few brave ones stayed up all night. Among them was Kai-Egil Evjen, who took this picture of the final phase:

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The Twilight Experiment

The aureole of Venus, showing up during the egress in 2004, as imaged by a coronagraph built by A. and S. Rondi

Three extra-large suitcases in a high speed train from Nice to Paris, France, have attracted some attention, especially at the end of the travel in the passenger crowd pressed to leave the cars. It happened on May 14. Later that day, they were transported to the observatory in Meudon, opened and inspected. The six precious instruments had done a good travel – the first leg of a worldwide adventure.

This is just a recent step of the “Venus Twilight Experiment”, born from the joint effort of Thomas Widemann (Paris Observatory), the author of this note – later supported by a network of several researchers.

Basically, the idea is to get the best possible data on the aureole from a homogeneous set of identical instruments, placed in the region of visibility of the transit.

The core of the coronagraph: on the left, the cone for occulting the Sun.

By hiding the light of the Sun behind an occulting disk, the contrast obtained on the aureole is the best possible. Also, by accurately placing appropriate stops in the optical path, light scattered by the telescope objective can be considerably reduced. These are the principle of the classic “coronagraph”, the instrument fist conceived and built by B. Lyot for enhancing the visibility of solar prominences.

In our case, we modified the design for taking into account that we are not interested in imaging prominences all around the solar disk, but only a small area centered on the contact points between the Sun and Venus, at the ingress and egress. As a consequence, the optical axis is not at the Sun’s center, but on its limb. Also, the magnification is such that a convenient image scale of the Venus aureole can be obtained. André and Sylvain Rondi, two exceptional French amateurs, helped in the design of the instrument.

This concept was just on paper last autumn, when it became clear that the effort required to design from scratch a campaign for the aureole was well worth the effort. In fact, the scientific interest for the transit – and for the aureole in particular – began to grow attracting much attention. Why? First, we already knew that it is possible to better study the mesosphere of Venus itself (80-110 km above the surface), by a careful characterization of the refraction producing the aureole. The principle was tested by the data collected in 2004, but for that transit no systematic campaign had been organized. The 2012 event is our last opportunity for a targeted, well-planned experiment. Also, in the meantime the interest for the transit of telluric exoplanets in front of their stars has grown, thanks to the discovery of several candidate planets in this category. Is this motivation not sufficient for proposing to study the transit of Venus as the nearby prototype of an exoplanet, much easier to exploit?

Several researchers began focusing their efforts on existing facilities, large solar telescopes in particular. They will certainly offer high-quality data of the best possible resolution, but with the constraints dictated by being built for other kinds of observations.

One of our coronagraphs (the longest tube) set up in Australia, ready to observe (hoping that the clouds will go away!)

Having a dedicated telescope, to place at strategic sites, presented several advantages in itself. These motivations were strong enough to imagine the construction not of one, but several coronagraphs. We thus asked financial support for a small set, and in the end we have built 9 of them, at the workshops of the Observatoire de la Côte d’Azur, in Nice (one of the main supporting institutes along with LESIA, Obs. de Paris, Laboratoire Lagrange, Observatoire de la Côte d’Azur, Paris VII Univ.).

With time passing and the transit approaching, the interest around the experiment has grown beyond expectations. Several observers offered their expertise or proposed complementary observations to be done at different facilities, mainly solar telescopes but also from space (the Picard satellite). Different sites declared their interest in providing logistic support for the observation. The Venus Twilight Experiment was born!

The sites of the Venus Twilight Experiment. In blue, the sites where the coronagraphs are situated. In red, other talescopes participating to the campaign.

We thus identified the sites where the instrument should go, over a wide area, each one with a filter chosen among 4 possible wave bands. This way, we hope to have color information and good data from at least a part of the sites, in case the others are clouded out.

Right now, all the 9 coronagraph are at the chosen sites, and observers are practicing with the procedures of observation. It is the first time that such instruments are used, and several tricky details must be carefully mastered for an appropriate imaging.

I am at Lowell observatory, and I’ll let you know how things will go!

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