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June 6, 2012 (TSR) - The 2012 transit of Venus, when the planet Venus appeared as a small, dark disk moving across the face of the Sun, began at 22:09 UTC on 5 June 2012, and finished at 04:49 UTC on 6 June. A transit is similar to a solar eclipse by the Moon. While the diameter of Venus is more than 3 times that of the Moon, Venus appears smaller, and travels more slowly across the face of the Sun, because it is much farther away from Earth. Transits of Venus are among the rarest of predictable celestial phenomena and they occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. The previous transit was in June 2004, and the next pair of transits will occur in December, 2117 and December, 2125.
Venus transits are historically of great scientific importance as they were used to gain the first realistic estimates of the size of the Solar System. The June 2012 transit provided scientists with a number of other research opportunities, particularly in the refinement of techniques to be used in the search for exoplanets. These included,
1) Measurement of dips in a star’s brightness caused by a known planet transiting a known star (the Sun). This will help astronomers when searching for exoplanets. Unlike the 2004 Venus transit, the 2012 transit occurred during an active phase of the 11-year activity cycle of the Sun, and would have provided practice in detecting a planet’s signal around a “spotty” variable star.
2) Measurement of the apparent diameter of Venus during the transit, and comparison with its known diameter. This will have given information on how to estimate exoplanet sizes.
3) The number of locations documenting the event will provide much data via parallax that will generate more accurate measurements.
4) Observation of the atmosphere of Venus simultaneously from Earth-based telescopes and from the Venus Express spacecraft. This gave a better opportunity to understand the intermediate level of Venus’s atmosphere than is possible from either viewpoint alone, and should provide new information about the climate of the planet.
5) Spectrographic study of the atmosphere of Venus. The results of analysis of the well-understood atmosphere of Venus will be compared with studies of exoplanets with atmospheres that are unknown.
6) The Hubble Space Telescope used the Moon as a mirror to study the light reflected from Venus to determine the makeup of its atmosphere. This may provide another technique to study exoplanets.