The black-drop effect explained

Abstract

The black-drop effect bedeviled attempts to determine the Astronomical Unit from the time of the transit of Venus of 1761, until dynamical determinations of the AU obviated the need for transit measurements. By studying the 1999 transit of Mercury, using observations taken from space with NASA's Transition Region and Coronal Explorer (TRACE), we have fully explained Mercury's black-drop effect, with contributions from not only the telescope's point-spread function but also the solar limb darkening. Since Mercury has no atmosphere, we have thus verified the previous understanding, often overlooked, that the black-drop effect does not necessarily correspond to the detection of an atmosphere. We continued our studies with observations of the 2004 transit of Venus with the TRACE spacecraft in orbit and with groundbased imagery from Thessaloniki, Greece. We report on preliminary reduction of those data; see http://www.transitofvenus.info for updated results. Such studies are expected to contribute to the understanding of transits of exoplanets. Though the determination of the Astronomical Unit from studies of transit of Venus has been undertaken only rarely, it was for centuries expected to be the best method. The recent 8 June 2004 transit of Venus provided an exceptionally rare opportunity to study such a transit and to determine how modern studies can explain the limitations of the historical observations.

Analysis of blackdrop effect in the transit of Venus on June 6, 2012

“…It seems that transit of Venus observations on June 6, 2012 in Biak showed pronounced blackdrop effect which was much worse than that observed in 2004 transit by Pasachoff (2004) because the 2004 transit was observed during a much better atmospheric condition compared with the 2012 transit. In a very clear sky, blackdrop effect will always be seen although not as great as in cloudy sky, as a result of smearing of light which come from the seeing and diffraction of the telescope.…”

“…The history of Venus transit can be traced back to the famous expedition of James Cook in 1761 [2]. In this expedition Cook confirmed the existence of the atmosphere of Venus, which was observed in previous transit in 1761.…”

“…Pasachoff and his team observed transit of Venus in 2004 [2]. They got a very clear sky and the blackdrop effect was seen, although not as pronounced as expected before.…”

The Hetu'u Global Network: Measuring the Distance to the Sun Using the June 5th/6th Transit of Venus

“…The Black Drop effect, an optical phenomenon that is known to limit these measurements (see Schneider, Pasachoff, & Golub 2004;Pasachoff, Schneider, & Golub 2005; Pasachoff 2012), was not directly observed, but would have also added to our uncertainty.…”

Using the 2016 Transit of Mercury to Find the Distance to the Sun

“…Two of us (JMP and GS) used spacecraft observations of a 1999 transit of Mercury to finally explain the true cause of the black-drop effect, which is a composite effect related to the finite resolution of the telescope and the extreme drop-off in brightness at the edge of the Sun, which, after all, is gaseous and so has no sharp edge. 9,10 In this article, we describe our use of simultaneous observations of the 2016 transit of Mercury made from two widely separated locations on Earth to determine the distance to the Sun in a way different from that suggested in 1715 by Halley. Using an internet link, teachers and students can make a similar derivation at the 2019 transit of Mercury also based on parallax and requiring only a set of measurements at one agreed-upon instant of time.…”