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.