AAS 205th Meeting, 9-13 January 2005
Session 135 Finding and Measuring Exoplanets
Poster, Thursday, January 13, 2005, 9:20am-4:00pm, Exhibit Hall

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[135.11] The Effect of the Transit of Venus on ACRIM's Total Solar Irradiance Measurements: Implications for Transit Studies of Extrasolar Planets

J. M. Pasachoff (Williams Col.-Hopkins Obs.), G. Schneider (U. Ariz.-Steward Obs.), R. C. Willson (Columbia U.)

We used 131-s-cadence observations made with ACRIM3 on ACRIMsat on 8 June 2004 to follow the effect of the transit of Venus, which lasted about 6 hours, on the total solar irradiance (TSI). Venus's angular diameter, in transit, is approximately 1/30 the solar diameter, so it covered approximately 0.1% of the sun's surface. With our ACRIM3 data, we measure temporal changes in TSI with a one-sigma per sample (unbinned) certainty of approximately 100 milliwatts per square meter (0.007%). We found a diminution in TSI of approximately 1.4 watts per square meter (approximately 0.1%, closely corresponding to the geometrically occulted area of the photosphere) at mid-transit compared with a mean pre/post transit TSI of 1365.9 watts per square meter. The measured light curve is complex because of the parallactic motion of Venus induced by the satellite's polar orbit, but exhibits the characteristic signature of photospheric limb-darkening when orbit-driven variations are accounted for. Analysis of the limb darkening can reveal temporal structure with height in the photosphere and asymmetries can, in principle, be attributable to planetary atmospheres. Similar observations will increasingly be detected from exoplanet transits, so detailed analysis of the transit within our solar system will provide a useful analogue for interpreting the many more such transits expected to be discovered within the next decade.

JMP's and GS's transit of Venus observations were supported by a grant from the Committee for Research and Exploration of the National Geographic Society. NASA provides support for RCW at Columbia University under contract NNG04HZ42C.

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© 2004. The American Astronomical Society.