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B. E. Schaefer (Univ. Texas Austin)
With the long-awaited transit of Venus soon approaching, I have been looking at the question of the physical cause of the notorious Black Drop effect. This effect has Venus' silhouette appear with a meniscus between the planet and the solar limb around the time of interior contacts. The Black Drop confused the timings of contacts and greatly reduced the accuracy of the measured Astronomical Unit. At least one book calls this the cause for the ultimate failure of the world’s first grand international science program. The majority of books claim that the Black Drop is caused by diffraction, illusion, or atmospheric refraction. However, all of these are easy to disprove. Diffraction redistributes light over a nano-arc-second angular scale and hence is negligible, while it cannot be illusion or a Venusian atmospheric phenomenon since the effect has been photographed during transits of Mercury. The correct explanation has been known at least since 1770 when J. J. de Lalande proposed that normal terrestrial atmospheric smearing would blur the image such that a meniscus appears. I have modeled this smearing and find that isophotal contours indeed show meniscuses. The atmospheric seeing should thus be correlated with the appearance of the Black Drop, as indeed it is. Visual perception also enters since the observer makes some judgement as to which isophotal contour to label as the edge, and the appearance of the bridge will depend on this choice. An additional source of smearing will be the ordinary diffraction within the telescope, and this accounts for why the transit observers in the 1700’s (with their small apertures) had much more trouble with the Black Drop than those during the 1800’s. Smearing can also arise due to the finite size of the eye’s pupil in the case of a thumb and forefinger held close together immediately in front of the eye, and this provides a Black Drop that can be easily seen or photographed at any time.