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black dropAn optical phenomenon seen near the beginning and end of transits of Venus and Mercury across the Sun's disk. It takes the form of an apparent elongation of the planet's silhouette against the solar surface, with a link being formed between the silhouette and the dark sky beyond the Sun's limb, as seen in photographs and telescopic projections. The effect makes the planet look rather like a teardrop. It appears to be caused by a combination of instability in the Earth's atmosphere (see seeing) and the resolving power and sensitivity of the observing instrument being used. The black drop effect was a major factor in reducing the accuracy of early attempts to determine the solar parallax by observing transits from different parts of the Earth in the 18th and 19th centuries. During the transit of Venus in 2004, a large number of astronomers failed to notice the black drop effect at all, probably because of technical improvements that have been made in modern telescopes.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
black drop[¦blak ′dräp]
As seen through a telescope, an apparent dark elongation of the image of Venus or Mercury when the planets' images are at the sun's limb.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.