black drop

An 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.

black drop

[¦blak ′dräp]

(astronomy)

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.

References in periodicals archive ?

He sought relief in something called the Kendal Black Drop, a quack cure-all which basically inflamed the problem since it contained even more opium mixed with vegetable acids, which left him with violent stomach pains.

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Black Drop, a film by Turner Prize-winning artist Simon Starling, looks at the rare planetary phenomenon of the transit of Venus and, in documenting early moving image technology, also touches on the Nasmyth photos.

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the existence of a ring of light round the planet and of one or more bright spots near the centre of the disk, and the phenomena of the black drop, which deserves special attention.

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This black drop effect, recorded by nearly all observers regardless of their location, would become the bane of the entire endeavor: it would hamper all efforts to time the transit's crucial moments.

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As this happens viewers may be able to see a phenomenon known as the black drop effect in which Venus appears to be linked to the sun by a dark neck.