Probably the visual-perception effect of two objects of roughly the same
brightness (or in this case darkness) bleeding towards or into one another.
A trick of perception. What I find curious is, at least to my knowledge,
the assertion that this effect was caused by the atmosphere of either Venus
or Earth was still in print fairly recently, despite the counter evidence of
Mercury transits at least for the Venus atmosphere case.
--
Sincerely,
--- Dave

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A man is a god in ruins.
--- Duke Ellington
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"Greg Hennessy" wrote in message
...
In article ,
Tom Van Flandern wrote:
They gave this example on NASA TV also, but it is a bit misleading
because the fingers effect is caused by diffraction (wave bending as a
sharp edge intervenes), whereas the black drop effect is caused by
variable refraction from moving air cells in Earth's atmosphere.

The black drop effect cannot be from an atmospheric effect, Pasachof
has observed the effect on the planet Mercury from the TRACE
spacecraft.