Venus transit "black drop" effect due to what??

Pete Lawrence wrote:

On Wed, 09 Jun 2004 00:16:21 GMT, "Edward"
wrote:

I'm no expert (but I play one on the internet )

I was thinking it was a result of atmospheric refraction from Venus. Once
the third contact has been made, the sun doesn't "shine around the edge"
that is beyond the limb of the sun creating an extended dark drop. That of
course maybe wrong.

My problem with this Ed, is that there are images that show 2nd and
3rd contact with *no* black drop. If it was an effect happening
somewhere between the top of our atmosphere and the Sun, then I'd
expect a reasonable collaboration between images.

However, the effect is more than likely happening between the top of
our atmosphere and the observer's eye/camera.

Contrast seems to play a big part in seeing it but there are probably
other factors at play.

Here's a quick image set I threw together of my contacts...

http://www.pbl33.fast24.co.uk/black-drop.jpg

--
Pete Lawrence

Perhaps the black drop effect is a very localized event which depends on
the atmospheric conditions in the line of sight to the sun.


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