Marquesas Islands 2012 Venus Transit Expedition

Venus-TEx: the Venus Twilight Experiment

You will find here a detailed description of the experiment. The animation on the right is from observations made by A. and S. Rondi at the time of the 2004 Venus Transit.

Goal of the project

(Excerpt from the experiment website given above)
The goal of the Venus-TEx experiment is a detailed investigation of the dynamics and composition of the middle atmosphere of Venus by the June 2012 transit of Venus in front of the Sun, as seen by Earth-based observers.
On 5-6 June 2012, Venus will be transiting the Sun for the last time in this century. This unique opportunity, besides offering the opportunity of investigating the mesosphere of the planet, also provides a significant nearby analog of exoplanet transits.
Several studies using the transmission spectroscopy technique have provided significant insights into the atmospheric composition, structure, and dynamics of hot giant exoplanets. In this context, Venus is our closest model for a telluric exoplanet. Obtaining its transmission spectrum during its transit across the Sun will serve both as a comparison basis for transiting Earth-mass exoplanets now being discovered, and a proof of feasibility that such observations can effectively probe the atmospheres of exoplanets in this mass range. In addition, transit observations of Venus can bring precious information about how the atmosphere of a non-habitable world - observed as an exoplanet - differ from that of a habitable planet, the Earth.

Press Release from Williams College


1 June 2012


A Venus Twilight Experiment coordinated by scientists in France and in the United States has a round-the-world complexity resembling the 18th century expeditions that sent Captain James Cook to Tahiti in 1769. In the 18th and 19th centuries, transits of Venus were the only way to find the size and scale of the solar system, such as the distances among the planets. Hundreds of expeditions went as far north and as far south on Earth as possible to time how long Venus took transit the Sun's disk, taking advantage of giant triangles in the heavens to find distances.

The triangle involving Venus is long and skinny, with the north-south base nearly the diameter of Earth and the opposite point of the triangle is Venus. Since Venus projects against slightly different parts of the Sun when seen from different places on Earth, just as an old-fashioned speedometer needle read different speeds when seen by the driver and from the front passenger seat, the differences in the transit's timing from different locations gave the angle. From the north-south base on Earth and the angle in the sky, the actual distance, in miles or kilometers, can be calculated.

In the 21st century, it may be easier and quicker to get to the South Seas than it used to be hundreds of years ago, but the expeditions cover similarly widely scattered areas on Earth. The Venus Twilight Experiment consists mainly of 9 identical special telescopes, called coronagraphs, that mask the everyday solar disk in order to allow Venus to be visible for about 20 minutes before it entirely apparently enters the disk and, after a six-hour transit, for a similar length of time afterwards.

The experiment was conceived of by Thomas Widemann of the Observatory of Paris, located in Meudon, France, and by Paolo Tanga, of the Observatory of the Cote d'Azur in Nice, France, in collaboration with Jay Pasachoff of the Hopkins Observatory of Williams College in Williamstown, Massachusetts.

The places on Earth where coronagraphs have been sent are as far north as Svalbard in the Arctic, north of Scandinavia, and as far south as Australia. Pasachoff, Bryce Babcock, and undergraduate student Muzhou Lu from Williams College are in place at the 10,000-foot level of Haleakala, the giant volcano on Maui in Hawaii. Widemann is on Hokkaido, Japan, with local scientist T. Fukuhara. Tanga is at the Lowell Observatory in Flagstaff, Arizona, alongside historian-of-science William Sheehan, who will look with his eyes through an identical coronagraph to help understand whether or not 18th and 19th-century observers actually saw Venuss atmosphere. Pasachoff and Sheehan have published an article in the Journal of Astronomical History and Heritage that shows that Russian scientist Mikhail Lomonosov was not, contrary to widespread citing, the first detect to Venus's atmosphere, since he saw an artifact of observing instead, and falsely concluded that he had seen the atmosphere since he expected one on religious grounds.

Colleagues L. Fulham and F. Braga-Ribas are at the Moondara Observatory on Mount Isa in Queensland, Australia. French scientists F. Colas and F. Vachier are at the Tien Shan Observatory in Kazakhstan. Indian scientist Ashok Ambastha and P. Machado have a coronagraph at the Udaipur Observatory in Udaipur, India. French scientist Christian Veillet has the remaining coronagraph at Taiohae, Nuku Hiva, in the South Pacific's Marquesas Islands.

Three additional coronagraphs that were already in existence will also be trained toward the transit. Pasachoff has a coronagraph at Williams College that is temporarily on Mt. Wilson in California, operated by John Briggs. S. Bouley and L. Maquet have a coronagraph at the Khurel Togoot Observatory in Ulaanbaatar, Mongolia. And C. Sigismondi has a coronagraph at the Hauriou solar station, not far from Beijing, China.

From observations made from a NASA spacecraft of the 2004 transit of Venus, the first to be visible since the 19th century, Pasachoff, Glenn Schneider of the University of Arizona, and Widemann published their analysis of Venus's atmosphere. The Venus Twilight Experiment is designed to study that atmosphere in much more detail, as it appears as an arc around the trailing side of Venus as it enters the Sun, and around the leading side as Venus exits. The resulting data will be studied together with observations made at Venus itself with the European Space Agency's Venus Express spacecraft. The wide range of Venus's atmosphere seen at the transit complements the narrow range of observations made at the same time from the spacecraft, each set of data helping understand the middle level of Venus's atmosphere better than either alone.

Twenty-first-century astronomy is much more powerful than astronomy of the past, with its powerful telescopes and computers, as well as new theoretical ideas and a history of astronomical discovery. It may be easier to send teams of astronomers around the world than it was in the 18th-century, but the widespread dispersal of observing sites is a link to the past, recalling the Age of Exploration.