The black drop effectoo is a rare efect (optical illusion) produces when the disco of Venus is inside the solar disc.
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http://www.youtube.com/watch?v=9JPO6nfkr…
http://www.transitofvenus.nl/blackdrop.h…
http://www.transitofvenus.nl/blackdrop.h…
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My recollection is that it took 10 to 15 seconds during the 2004 transit. It probably varies depending on the seeing and the angle of ingress/egress.