Jay Pasachoff (Williams College) : Transits of Venus and Mercury: Exoplanet Analogs in Our Solar System

Since Johannes Kepler's predictions of transits of Mercury and Venus in 1631, and observations by Jeremiah Horrocks and William Crabtree of the 1639 transit of Venus, only 5 other transits of Venus have been observed: in 1761 and 1769, 1874 and 1882, and 2004.  Expeditions were sent all over the world for the 18th and 19th century transits to follow the methods of Halley and others to determine the Astronomical Unit, giving the size and scale of the solar system, arguably the most important problem in astronomy for centuries.  I will discuss how the infamous black-drop effect bedeviled astronomers in that quest for an accurate A.U., and how Glenn Schneider and I explained the effect through satellite observations of transits of Mercury, showing that it was not simply caused by the Cytherean atmosphere.  During the 2004 transit, we worked with Richard Willson of ACRIMsat to detect the 0.1% drop in the Total Solar Irradiance, showing the effect of solar limb darkening, positioning such observations of transits of Venus and of Mercury as analogs to exoplanet transits.  Our observations of the atmosphere of Venus with NASA's Transition Region and Coronal Explorer in 2004 led us to plan extensive observations of Venus's atmosphere and other phenomena during the June 5, 2012, transit of Venus, the last to be visible from Earth until 2117.  We used NASA's Solar Dynamics Observatory, Hinode, ACRIMsat, and other spacecraft, and ground-based solar telescopes at Sacramento Peak, Kitt Peak, Big Bear, and Haleakala to observe the transit, as well as participating in the Venus Twilight Experiment from our perch at 10,000 feet on Haleakala, with 9 matched coronagraphs distributed around the world. I will give preliminary reports on these observations during this talk.  I will discuss the attempt of David Ehrenreich, et al., to observe the transit of Venus in reflected light off the Moon during the 2012 transit, and our hopes for future observations of transits as seen from Jupiter and Saturn.