History of the Transit A transit of Venus

History of the Transit A transit of Venus is a rare event, occurring in pairs eight years apart separated by more than a century due to the tilt in Venus’s orbit compared to that of the Earth (Figure 1). Kepler accurately predicted the transit of Venus in 1631. However, it was not visible in Europe that year, and he incorrectly predicted that Venus would barely miss the sun in 1639. The young English scientist Jeremiah Horrocks discovered Kepler’s mistake. Horrocks and his friend William Crabtree were the only two people to observe the 1639 transit. They got a sense of the immensity of the solar system. Crabtree was so astounded by the event that he even neglected to take actual scientific data. The Transit of Venus Kayla Gaydosh, Bryn Mawr College ‘ 05 Keck Northeast Astronomy Consortium Fellow Advisor: Prof. Jay M. Pasachoff 8 June 2004 Figure 4: Expedition team at the birthplace of Aristotle. Figure 6: Me at Mt. Hollomon in Greece during the Transit. Greece Expedition Figure 2: Drawings by Thobern Bergman of the “black drop” effect. For centuries a transit of Venus was the best method available for the determination of the distance to the sun, also known as the Astronomical Unit. Transits of Venus can be used to determine the distance to Venus from its parallax, requiring observations to be taken at different parts of the world at the same time. This idea led to dozens of international expeditions for the 1761 transit. But Thobern Bergman in 1761 reported that the silhouette of Venus was joined to the dark background exterior to the sun (Figure 2). This dark "black drop" meant that observers were unable to determine the time of contact to better than 30 seconds or even 1 minute. The black-drop effect thus led to uncertainty in Venus’s contacts and thus the Astronomical Unit. The most famous of all the Venus transit expeditions was that of Captain Cook, who was sent by British Admiralty to Tahiti for the observations. His later wanderings around New Zealand the eastern coast of Australia are thus spin-offs of astronomical research. The observations by Cook and his astronomer, Charles Green, clearly show the black-drop effect. Cook also correctly mentioned the existence of an atmosphere around Venus (discovered at the 1761 transit) but incorrectly attributed the inaccuracy of the “black drop” effect to that atmosphere. Figure 3: Photo of 1882 transit from the U. S. Naval Observatory Library. The 1872 transit was observed in the Indian Ocean and Australia. During the 1882 transit, which was visible in the western hemisphere, the U. S. Naval Observatory took the image in Figure 3, which is one of the 11 surviving photographic plates from the observations they coordinated of that transit. The “black drop” effect was still a major problem during the 19 th century observations. The Williams College Transit of Venus Expedition Team traveled to the Aristotelian University of Thessaloniki, Greece. Ground-based observations were carried out at the University using their 20 -cm refractor with our Apogee and SBIG CCD cameras. Photos were taken with a Nikon F 5 camera (Figure 5). A team of students including myself was sent to Mt. Hollomon outside of Thessaloniki to observe the transit for weather insurance, but unfortunately it clouded out (Figure 6). Steven Souza was able to observe third and fourth contacts with the Carroll spar 5” refractor at Williams College. (b) Study its morphologic and photometric evolution over time prior to second contact and after third contact. (c) Confirm with space-based imagery the assessment that ground-based historical reports of “black drops” are due to the convolution of the instrumental and atmospheric PSF in conjunction with the limb darkening. (d) Investigate the detectability of “aureola” during photospheric transits. OBSERVATION TRACE imaged the transit at high temporal cadence during (and flanking) the planet's crossing of the solar limb and while on the heavily limb-darkened portion of the solar disk. All images were obtained in TRACE's "White Light" (WL) configuration, providing spectral sensitivity in the wavelength range from 1200 -9600 angstroms. During the transit, Venus's angular diameter was approximately 58. 2 arcseconds (12, 104 km at 0. 289 AU). IMAGES and MOVIES Figure 7: Ground-based image from Greece at approxmately second contact. Figure 8: Isophotes on a TRACE image with a limbdarkening function removed. Figure 9: Ground-based image from Greece of Venus during transit. Ground-based Observations The ground based images taken both in Greece and Williamstown have been partially reduced (Figures 7 & 9). These will give insight onto the properties of modern telescopes by inspection of the “black drop” effect. Analysis of final reduced images will give the characteristics of the point-spread function of telescopes. A qualitative analysis of these images shows that there is a decreased “black drop, ” but it is not entirely eliminated. The results of the 2004 expedition will provide better preparation for the 2012 Transit of Venus. Figure 10: Ingress (above) and egress (below) images in sequence taken by the TRACE spacecraft in white light showing the scattering of light in the Cytherian atmosphere and no black drop effect. The atmosphere can still be detected below (see arrow) in the second image from the right. (G. Schneider, Univ. of Arizona) Acknowledgments: - Glenn Schneider, Steward Observatory, University of Arizona, Tucson, AZ - Committee for Research and Exploration of the National Geographic Society as grant provider for the Williams College Expedition to Greece. The study of the Transit of Venus in 2004 is an alternative test for observing methods, strategies and techniques that can assist in the detection and characterization of extrasolar terrestrial-type planets as they transit their stars. Primary objectives of observations of our team, headed by Jay Pasachoff and Glenn Schneider, with NASA’s TRACE spacecraft: (a) Image the circum-Cytherian “aureola” (sunlight scattered by aerosols and refracted in the backlit Cytherian atmosphere) with high spatial resolution and image stability. Figure 5: Image courtesy of Jay M. Pasachoff, David Butts ‘ 06, Owen Westbrook ‘ 06, and Joseph Gangestad ‘ 06. Figure 1: Illustration of the orbits of Venus and Earth showing the possibility of a transit of Venus. Transition Region and Coronal Explorer (TRACE) Spacecraft Results To view/download the movies go to Glenn Schneider’s Website: http: //nicmosis. arizona. edu: 8000/ECLIPSE_WEB/TRANSIT_04/TRACE/TOV_TRACE. html or link to them from our site at http: //www. transitofvenus. info. Working with Glenn Schneider (University of Arizona’s Steward Observatory), we produced the first photometric results from the ingress and egress imaging sequence taken in TRACE's WL band pass (Figure 10). I assisted in the making of movies for both sequences with all frames aligned along the solar limb. Venus-centered movies were also made since the evolution of the morphological and photometric characteristics of the light scattered and refracted by the Cytherian atmosphere is better studied by “fixing” the position of Venus in each frame. APERTURE PHOTOMETRY Aperture photometry was executed using the IDP 3 (Schneider and Stobie 2002) image analysis software. For each image of Venus we measured three concentric, radially nonoverlapping rings, of which all three are centered on Venus (Figure 11). The central ring had a distance of 58. 63 pixels from the center of Venus, setting it 45 km above the “surface” of Venus (the optically thick cloud layer in the Cytherian atmosphere). Radially adjacent regions interior to and exterior to the central points were measured for proper background estimation and to estimate the uncertainties in the measures. Figure 11: Aperture photometry circles on a grid. Relevant data obtained for each aperture are the X/Y locations of the aperture center, the total number of pixels used to compute the mean value (pixels only partially contained in the aperture are weighted linearly by pixel area), the total flux, the maximum value of the flux in any pixel, the median flux in a pixel, and the one-sigma dispersion in flux. From this the background subtracted ring flux will be computed along with its uncertainty. The ring surface brightness will be characterized as functions of azimuth angle, the distance from the center of Venus, the distance from the point on the limb along a radius joining Venus and the heliocenter, and distance from the closest point on the solar limb. These data are currently ready for analysis and are the main objective in the weeks ahead.