Around ingress, when the disk of Venus enters the sun’s disk, and around egress, when Venus leaves the sun, two interesting phenomena may occur: the illusive aureole effect and the notorious black drop effect. They have been reported by observers ever since the transit of 1761, when both ingress and egress were witnessed for the very first time. Their true origin, however, wasn’t reveiled until recent by using computer models and studying high resolution images from TRACE, a satellite intended for observing the sun.
Around the time of the interior contacts, when the limbs of Venus and the sun are about to merge, a fuzzy greyish zone or a diffuse shading between the outer limb of Venus and the dark sky off the suns edge may appear, fluctuating in size, shape and intensity. Sometimes the outline of Venus is even distorted into a teardrop shape. Other descriptions of the phenomenon vary from a dark region, a band, thread or ligament, connecting Venus to the dark sky when its entire disk is near the edge of the suns. In 1761, the first observers called it the black drop or gutta nigra according to its appearance. Although the distortion of Venus outline is just one of the many aspects of the effect, it is still referred to as black drop today. Because the effect wasnt anticipated for in the eighteenth century, the timings of the interior contacts as observed from the same location could then vary as much as a minute. In the nineteenth century however, astronomers practiced prior to the transit with an apparatus which artificially reproduced the ingress and egress circumstances. In this way, the accuracy of their contact timings was highly improved.
The last couple of years the black drop effect was studied intensively, both from optical theory and simulation experiments. Images of the transit of Mercury by the TRACE satelite were analysed and numerical models were employed to explain the peculiar effect. It was found from these investigations that the main causes of the black drop effect are image blurring (due to atmospheric seeing and telescopic diffraction) and solar limb darkening. This implies that the development of the black drop effect as seen by an earth bound observer mainly depends on the atmospheric conditions and the quality of his instrument. During the 2004 transit of Venus, amateur observers with telescopes of modest dimensions reported and even photographed the teardrop shape of Venus. Most likely, this classical black drop formed because the image was strongly blurred or overexposed. However, observers using large aperture telescopes saw just a hint of a greyish shade, or didnt perceive the generic black drop at all because they mentally sharpened the blurred image they saw.
To employ Halleys method of determining the suns distance successfully from observations of the transit of Venus, the times of first and second interior contact should be found as exactly as possible, preferably to a precision of one second. Understanding the causes of the black drop, the formation of the effect could recently be simulated by a numerical model. From such simulations, Sveshnikov et al. (1996) conclude that the half-sum of the apparent contact with the black drop and that of the drop breaking equals, approximately, the moment of geometrical contact. In other words, the true time of interior contact is halfway between the formation of the black drop and the breaking of the thread. Surprisingly, this conclusion is consistent with the nineteenth century recommendations given by Simon Newcomb to the American expeditions in 1874 and 1882.
For a long time it was thought that that the black drop would occasion an uncertainty in visually ascertaining the exact instant of actual interior contact, and thus bias observational measures of the geometric times of limb contacts. Now it turns out that the effect is the most readily timed event during the entire transit, and corresponds more precisely to the true instant of contact. Therefore, it should be appreciated that the black drop provides a timing advantage rather than a disadvantage for visual observers. Mike Begbie and David Pringle-Wood, observing from Harare (Zimbabwe), agree. They found the black drop a help, and not a hindrance to the timings. The sudden break between the black drop and a clear gap of sunlight Mike found very apparent, and would be pretty firm in saying that an error of ten seconds was too much... more like two seconds according to our timed recording. At the same time, ingress was a bit more difficult than egress, but the early morning seeing conditions compensated for this, so we reckon that overall, it was about even!
Any scatter of contact times for a single location might well be attributed to the personal equation of each observer, i.e. his systematic mis-observation of the phenomenon. Due to the personal equation, observations are normally recorded too late, but an analysis by Daniel Fischer of a small number of timings made at the 2004 transit of Venus showed that, although there was a pronounced cluster around the true time of contact, the majority of errant observers had spotted the interior contact at ingress far too early, while only some had seen it late. The results submitted to this website (which you may find in the Online Parallax Calculator) show that the number of early and late observations of the times of contact are about even. However, the majority of errant visual observers who timed second contact too early, saw third contact too late by about the same amount of time, and vice versa. For instance, I myself spotted ingress 9 seconds too early and egress 9 seconds late, and David Pringle-Wood saw ingress 10 seconds late and egress 12 seconds too early. This indicates that, although each single observer timed the same particular phase of the phenomenon at both ingress and egress, there might have been no common agreement on which of the phenomenons successive phases should be timed.