Many people around the world were thrilled to see a transit of Venus in June (June 5 in the United States and June 6 in Asia, on the other side of the International Dateline), the dark silhouette of Venus passing in front of the Sun. Jay Pasachoff of Williams College (Williamstown, MA) and the California Institute of Technology (Pasadena, CA) and Glenn Schneider (Steward Observatory, University of Arizona) organized extensive observations of the transit. With support from the Committee for Research and Exploration of the National Geographic Society, they observed the six-hours of the transit from the 10,000-foot mountaintop of Haleakala on Maui, Hawaii.
The event was a rare opportunity to see close up, in our own solar system, the type of transit that is being studied by many scientists around the world using NASA's Kepler spacecraft and ground-based telescopes. Some thousands of these exoplanet transits have been seen. Though it was widely and correctly said that the 2012 transit of Venus was the last to be seen before the year 2117, that restriction applies only if you are limited to observing from Earth. The Pasachoff-Schneider team subsequently organized observations of a transit of Venus that was occurring if you were on Jupiter on September 20, using NASA's and ESA's Hubble Space Telescope to observe the reflection of sunlight off Jupiter's clouds. Subsequently, they used NASA's Cassini mission, in orbit around Saturn, to try to observe the December 21, 2012, transit of Venus that was occurring if you were on Saturn. The data are still under study for the Jupiter and Saturn events, and it cannot yet be said whether the attempts to detect the transit succeeded.
Pasachoff, with his colleagues as co-authors, is presenting his team's observations of transits of Venus as an analog to exoplanet transits at the 221st meeting of the American Astronomical Society being held on Sunday, January 6th, through Thursday, January 10th, in Long Beach, California. The scientific paper is scheduled for Wednesday morning, and Pasachoff will participate in a press conference related to exoplanets on Tuesday morning.
The observations using 14 orbits of the Hubble Space Telescope to study Jupiter, at both ultraviolet and infrared wavelengths, in September were joint with David Ehrenreich of the Observatory of Geneva, Switzerland, and Alfred Vidal-Madjar, of the Institut d'Astrophysique in Paris, France. The observations will be studied with the help of a grant from NASA through the Space Telescope Science Institute. The observations using the Cassini spacecraft in December were headed by Philip Nicholson of Cornell, who is on the Cassini team. Each of those transits, at Jupiter and Saturn, lasted about 10 hours, with a drop in the solar intensity of only a hundredth of a percent.
Based on their study of the previous transit of Venus visible from Earth, in 2004, using a NASA spacecraft as well as their own ground-based observations, Pasachoff and Schneider concentrated their efforts on studying Venus's atmosphere. It becomes visible as a bright whisker or arc on the edge of Venus for several minutes before Venus entirely enters the solar disk and after Venus entirely leaves the Sun.
As part of the team, Kevin Reardon of the National Solar Observatory, then also at the Arcetri Observatory, Florence, Italy, used the Dunn Solar Telescope on Sacramento Peak, New Mexico, to observe Venus's atmosphere at high resolution as Venus entered the Sun. From sites in the continental United States, Venus and the Sun set partway through the transit, so only the opening phases were observable. These observations were also supported by Pasachoff's grant from the Committee for Research and Exploration of the National Geographic Society.
Because of the predicted visibility of Venus's atmosphere, Paolo Tanga of the Observatory in Nice, France, and Thomas Widemann of the Observatory of Paris in Meudon, France, prepared nine identical telescopes especially to observe Venus's atmosphere at the June 2012 transit. These telescopes are called coronagraphs, since they block out the everyday solar surface, as do specialized telescopes for studying the solar corona. One of them was with Pasachoff and Schneider at Haleakala, where it was operated by Bryce Babcock and undergraduate Muzhou Lu '13 of Williams College. It had a blue filter, and its results will be compared with those from the 7 other coronagraphs used photographically, with two each having blue, yellow, red, and infrared filters, respectively. The ninth coronagraph was used visually by Minnesota historian of science William Sheehan to detect Venus's atmosphere; Pasachoff and Sheehan have written articles showing that the scientist long credited with discovering Venus's atmosphere, at the 1761 transit, in all likelihood did not actually see it. Determining just what could be seen visually of Venus's atmosphere, with both modern and ancient telescopes, helps researchers today to interpret the old situation. On Haleakala, Schneider had brought a small telescope; he and Pasachoff were able to see Venus's atmosphere with their own eyes looking through it, with a suitable safe solar filter, of course, always necessary for looking at the Sun except during totality of a total solar eclipse (of which Pasachoff and Schneider have each seen 30, most recently in Queensland, Australia, on November 13/14).
Also at the site with telescopes and electronic cameras as part of the Pasachoff-Schneider team was Ronald Dantowitz of the Clay Center Observatory (Brookline, MA). His equipment took hundreds of thousands of short exposures, the rapidity of the images helping counteract the 40 mph winds at the Haleakala site. The images clearly show the transit and Venus's atmosphere. During the summer of 2012, undergraduate student Eric Edelman of Wesleyan University, sponsored by the NSF's student research support for the Keck Northeast Astronomy Consortium, worked at Williams College with Pasachoff on these and other transit-of-Venus images.
Another major US site for observation was the New Jersey Institute of Technology's Big Bear Solar Observatory in California. Vasyl Yurchyshyn of their staff led the observations; Joseph Gangestad, a Williams College alumnus now at The Aerospace Corporation, represented the Pasachoff-Schneider group there. The observations with Big Bear's New Solar Telescope showed not only the atmosphere of Venus very well but also the so-called black-drop effect that delayed scientists' discovery of the size and scale of the universe for centuries. Pasachoff, Schneider, and Leon Golub of the Harvard-Smithsonian Center for Astrophysics explained the cause of the black-drop effect fully for the first time based on a previous spacecraft observation of a transit of Mercury, and then observed it at the 2004 transit of Venus. The extensive observations of the black drop in 2012 will lead to further analysis and confirmation.
The observations of the details of the transit, and the visibility of Venus's atmosphere, showed the moving parts that go into any transit observation, for planets in our solar system or outside it. Only about 90% of the dips in stellar brightness observed with the Kepler spacecraft are actually from exoplanet transits, so it is especially important to understand all the workings of transits. The closest analog in our own solar system to exoplanet transit observations are made by two NASA spacecraft that study the Total Solar Irradiance, the total amount of energy received from the Sun each second on a unit of area at the Earth's average distance from the Sun. Collaborating scientists who run their respective spacecraft that measure TSI are Richard Willson of ACRIMsat for his Active Cavity Radiometer Irradiance Monitor 3 instrument, and Greg Kopp of the University of Colorado for SORCE/TIM (Solar Radiation and Climate Experiment/Total Irradiance Monitor). They have provided graphs, which are being shown by Pasachoff in his talk, that show the 0.1% dip in solar intensity that occurred because of Venus's blocking that tiny bit of the Sun from being visible from Earth's vantage point.
Coordinated observations during all three transits were made from all possible spacecraft aloft to study the Sun, so that variations on the Sun itself could be monitored and separated from the effect of the transit as seen from Earth, Jupiter, and Saturn, respectively. The spacecraft-related scientists who collaborated, coauthors on the paper, include Alphonse Sterling of NASA's Marshall Space Flight Center and Murray Silverstone of the University of Alabama at Tuscaloosa for the Japanese-American Hinode spacecraft's Solar Optical Telescope; Philip Scherrer and Jesper Schou of Stanford University for their Helioseismic Imager on NASA's Solar Dynamics Observatory; and Leon Golub, Patrick McCauley, and Kathy Reeves for data processed from their XRT x-ray telescope on Hinode. The Atmospheric Imaging Assembly on Solar Dynamics Observatory also observed the transit. The European Space Agency's Solar and Heliospheric Observatory was so far toward the Sun from the Earth that no transit of Venus was visible for its instruments.
Much work remains to be done on the data from the three transits of Venus. But there won't be another transit of Venus visible from Earth until 2117, another transit of Venus visible from Jupiter until 2024, or another transit of Venus visible from Saturn until 2028. The collaborating scientists will soon apply for observing time with the Hubble Space Telescope for the 2014 transit of Earth that would be visible from Jupiter, since it would be exciting to detect Earth's atmosphere in an analogous way to the current observations of exoplanet atmospheres, and to provide therefore a test of current methods of observation and analysis.
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More information: 1. Pasachoff, Jay M., Glenn Schneider, Bryce A. Babcock, Muzhou Lu, Eric Edelman, Kevin Reardon, Thomas Widemann, Paolo Tanga, Ronald Dantowitz, Murray D. Silverstone, David Ehrenreich, Alfred Vidal-Madjar, Philip D. Nicholson, Richard C. Willson, Greg A. Kopp, Vasyl B. Yurchyhyn, Alphonse C. Sterling, Philip H. Scherrer, Jesper Schou, Leon Golub, Patrick McCauley, and Kathy Reeves, "Three 2012 Transits of Venus: From Earth, Jupiter, and Saturn," 221st AAS Meeting, Long Beach, CA, January 9, 315.06.
2. Edelman, Eric, Jay M. Pasachoff, Glenn Schneider, Bryce A. Babcock, Muzhou Lu, Kevin Reardon, Thomas Widemann, Paolo Tanga, and Ronald Dantowitz, 2013, "The 2012 Transit of Venus: A Closer Look at the Cytherean Aureole," 221st AAS Meeting, Long Beach, 353.04.