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The Venus Twilight Experiment – what we learned from the last transit of Venus

5 Apr 2016, 17:00 UTC
The Venus Twilight Experiment – what we learned from the last transit of Venus

In the run-up to the Transit of Mercury on 9th May, the Europlanet Outreach website is featuring a series of guest articles by European scientists that study the innermost planet or whose research relates to transits.  
Our second post is by Paolo Tanga, Senior Astronomer and planetary scientist at Observatoire de la Côte d’Azur, Nice, about the last transit of Venus in 2012. This article has been published to coincide with the premiere of a new film about the Venus Twilight Experiment by Lightcurve Films. 

The Venus Twilight Experiment – what we learned from the last transit of Venus
Faint aureole at Venus’s south pole as the planet approaches the solar limb. Credit: Venus Twilight Experiment
In 2012, people on Earth had their last opportunity to view a transit of Venus for more than a century. Transits of Venus are even less frequent than transits of Mercury – the next time Venus passes directly between the Earth and the Sun will be in 2117. And unlike the atmosphere-free Mercury, the thick clouds surrounding Venus make its transits fascinating opportunities for science.
Venus transits usually occur in pairs, 8 years apart, once per century. These rare events have been historically important for astronomers in trying to determine the distance of the Sun from the Earth. In theory, this distance can be calculated by observing a transit from different locations around the globe and using the principal of parallax. By extrapolating this measurement to the other planets, transits could therefore be a tool in putting a distance scale on the Solar System as a whole.
This method was first suggested in 1677 by the English Astronomer Royal, Edmond Halley. For the next opportunity of observing a transit of Venus (in 1761, after Halley’s death), challenging overseas expeditions were organised. The same year, a thin arc of light – encircling the planet at the beginning and end of the transit, on the portion projected outside the Solar limb – was detected for the first time, and correctly interpreted as the presence of an atmosphere.
In fact, it turned out that using transits of Venus for distance determination was not to be as precise as expected. Subsequent transits were approached with curiosity, rather than with a real scientific interest.
 
Aureole at ingress during the transit of Venus on 5 June 2012. Credit: P Tanga, Venus Twilight Experiment
However, for the first “modern” event in 2004, planetary scientists realised that the “aureole” created by the halo of atmosphere around Venus was a good target for astronomical cameras and that its brightness could be accurately measured. What’s more, the brightness of the aureole, and its evolution over time while Venus was entering and leaving the solar disk, could be reproduced by modeling the refraction (bending) of light in the planet’s atmosphere. This refraction is directly related to the physical properties of the upper atmospheric layers (the mesosphere at more than 80 km of altitude above the planet’s surface), and above the thick cloud deck that surrounds Venus.
Thus, interest in a transit of Venus revived from two new angles. Firstly, it provided an opportunity to increase our understanding of Venus. The European Space Agency’s Venus Express satellite was orbiting around the planet (from 2006-2014), collecting close-up data. However, it was clear that the transit gave us the opportunity to study Venus from a wider perspective and probe the whole planet (all latitudes) at the same time.
Secondly, the Venus transits enabled us to collect detailed data on planetary transits and use them as an analogue for exoplanets transiting distant stars. We have discovered more than 2000 exoplanets to date, but too far away to see any detail. The events in 2004 and 2012 represented a lifetime’s opportunity to study a planet’s atmosphere during a transit in our own Solar System.
 
Thomas Widemann constructing a corongraph for the Venus Twilight Experiment. Credit: Venus Twilight Experiment
For this reason, Thomas Widemann (of LESIA, Paris Observatory, France) and I, with the participation of an international group of astronomers, set up the “Venus Twilight Experiment” to try and obtain useful data.
This “modern” expedition covered half of the Earth’s globe and used instruments designed and built especially for the 2012 event. Technically, it was very challenging – from the observer’s perspective, probing the aureole of Venus is like looking toward a sunset or sunrise sky.
The instruments used to make the observations were small portable telescopes, equipped with a “coronagraph”, i.e. a device that hides the solar disk to enhance the contrast of the faint aureole. I should say, to us on Earth, the aureole looked faint, since it is thin and far away. But for an observer close to the planet it would have the same surface brightness as the Sun – a glorious sight!
A film about the adventures of our team of observers, who travelled halfway around the world to observe the transit, has been produced by Lightcurve Films and is going to be premiered at the International Venus Conference 2016 in Oxford on 5th April. You can watch the film here:
 

 
Setting up equipment on the eve of the 2012 Transit of Venus. Credit: Venus Twilight Experiment
We collected a huge volume of data during the Venus Twilight Experiment, including hundreds of gigabytes of movies, and the analysis is still underway. Some results will be presented at the conference in Oxford this week, showing that our coronographs (which were never tested in real “transit” conditions before the actual event) performed well, even beyond our expectations. In some images, Venus and its aureole are even seen before the beginning of the transit, with the Venus disk silhouetted against a faint background of solar corona.
Also, our measurements complement other data, such as a vertical temperature profile obtained by Venus Express when performing a similar “solar sunset” measurement (by using the spacecraft’s SOIR experiment) at the transit time – but for a single latitude. This Venus Express/SOIR data demonstrates that the detailed structure of the mesosphere matches our observations of the aureole. We were able to extend the measurements to all other latitudes, as expected. Some known features, such as lower-altitude cloud at Venus’s poles, show up nicely in our data as brighter aureole regions.
The expedition was an exciting challenge, involving several teams and observing sites, but the rewards have been huge. The Venus Twilight Experiment will remain a heritage that we hope will assist future transits observers, in a little more than a century from now…
 
 
The Venus Twilight Experiment would like to thank:
Optical design: Sylvain and Alain Rondi (Societé Astronomique de France)Coronagraph realization: Alain Roussel, Christian Baccelli, Serge Bonhomme – S2M Observatoire de la Côte d’Azur (mechanical workshop)
Venus Twilight Experiment Teams   Mobile Station, Svalbard, Norway : Thomas Widemann, Jérôme Berthier   Lowell Observatory, Flagstaff, USA: Paolo Tanga, Klaus Brasch, William Sheehan   Mees Solar Observatory, Haleakala, Hawai’i, USA: Jay Pasachoff, Bryce Babcock   Mobile station, Hokkaido, Japan: Tetsuya Fukuhara, Nicolas Thouvenin   Mobile station, Taiohae, Marqueses Islands: Christian Veillet   Moondara Observatory, Mount-Isa, Australia: Felipe Braga Ribas, Len & Sandra Fulham   Udaipur Solar Observatory, Udaipur, India: Pedro Machado, Ashok Ambastha, João Retrê   Tein Shan Observatory, Kazakhstan: François Colas, Frédéric Vachier
Paolo Tanga is Senior Astronomer and planetary scientist at Observatoire de la Côte d’Azur, Nice.
https://www-n.oca.eu/tanga/
Follow Paolo on Twitter: @ziggypao
The Venus Twilight Experiment film was co-funded by Europlanet RI under FP7.

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